Solubilization-emulsification mechanisms of detergency

Transcription

1 Colloids and Surfaces A: Physicochemical and Engineering Aspects, 74, (1993) Review Solubilization-emulsification mechanisms of detergency Clarence A. Miller a,* Kirk H. Raney b a Department of Chemical Engineering, Rice University, P.O. Box 1892, Houston, TX , USA b Shell Development Co., Westhollow Research Center, P.O. Box 1380, Houston, TX , USA (Received 14 November 1992; accepted 23 January 1993) Abstract The removal of oily soils from fabrics having high contents of polyester or other synthetic materials occurs largely by a solubilization-emulsification mechanism. A systematic investigation of this mechanism has been conducted during the past several years and is reviewed here. The research has utilized a variety of oily soils containing hydrocarbons, triglycerides, and long-chain alcohols and fatty acids and has included the determination of equilibrium phase behavior, the observation of dynamic behavior which occurs when surfactant-water mixtures contact oily soils, and measurement of soil removal from polyester-cotton fabrics. In most cases, pure surfactants and oils have been used for simplicity, but data showing the applicability of major conclusions to systems containing commercial surfactants are presented. Because typical anionic surfactants are too hydrophilic to achieve the desired phase behavior, the work has employed non-ionic surfactants and mixtures of non-ionics and anionics, One major conclusion is that maximum soil removal usually does not occur when the soil is solubilized into an ordinary micellar solution, but instead when it is incorporated into an intermediate phase such as a microemulsion or liquid crystal that develops during the washing process at the interface between the soil and washing bath. Indeed, for hydrocarbon and triglyceride soils, the washing bath is itself a dispersion of a surfactant-rich liquid or liquid crystalline phase in water for conditions of optimum detergency, i.e. the temperature of the surfactant solution is above - sometimes far above - its cloud point temperature. Key words: Detergency; Emulsification; Solubilization 1. General remarks on detergency Fabric detergency is a surprisingly complex process involving interactions among aqueous detergent solutions, soils, and fabric surfaces. This Process may occur in an industrial setting in which large volumes of similarly soiled fabrics are washed, or in a household setting in which small amounts of fabrics containing differing amounts of a wide variety of soils are washed. Because of their unique ability to adsorb at both fabric-water and soil-water interfaces, surfactants play an essential role in soil removal processes. To achieve the desired levels of surfactants in the washing solution, the * Corresponding author. concentration of surfactants (typically nonionic, anionic, or both) in most liquid and powder detergent formulations is in the range 10-40% by weight [1]. Several factors influence the effectiveness of surfactants in laundry detergents. The composition of a fabric is important in determining the mechanism by which soils are lifted from it. Cotton fabric contains rough and irregularly shaped hydrophilic fibers [1,2]. In contrast, synthetic polyester fabric contains uniform cylindrical fibers which, being of a more hydrophobic nature than cotton, are more tenaciously covered by oil. The differences in surface properties of the two materials are demonstrated by the much smaller contact angle in air formed by water on a cellulose film (32º)

2 170 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) than that by water on a polyester film (79º) [3]. In addition to fiber surface composition and morphology, the weave of the fabric can influence detergency, more loosely woven fabric typically being easier to clean. Not surprisingly, the amounts and types of soils present on fabric are key factors in determining the effectiveness of a detergent solution. Oily soils include such common soils as skin sebum, dirty motor oil, and vegetable oil. Clay is classified as a particulate soil. Scanning electron microscopy, both conventional and environmental, has proven quite useful for viewing the distribution of both oily and particulate soils within woven fabric samples [2,4]. While surfactants play the key role in removing oily and particulate soil, protease enzymes are commonly present in both powder and liquid laundry detergents to chemically break down polymeric protein soil stains such as blood, egg, and cocoa. Lipase enzymes are also now being used in detergents to hydrolyze triglyceride soils and thereby aid the surfactant in soil removal [5]. Bleaches, both peroxygen and chlorine-based, decolorize stains such as those from tea and wine by destroying the chromophores in the organic molecules adsorbed to the fiber surfaces [6]. Water hardness and temperature can profoundly influence detergent effectiveness. In hard water, e.g. 300 ppm hardness, calcium and magnesium ions may precipitate certain surfactants prior to their being able to act on the soil. The divalent ions also form complexes between soils and fabric which increase their attraction, making the soil more difficult to remove [1]. Builders such as zeolite, sodium tripolyphosphate (STPP) and sodium carbonate are used in powders to negate the effect of water hardness by either precipitating the divalent ions, as in the case of sodium carbonate, or sequestering the ions from the water, as in the cases of zeolite and STPP. The latter is particularly effective in this regard. Surfactant precipitation may occur in cold water for surfactants with high Krafft points [I]. Also, wash water temperature, in addition to changing the performance characteristics of the dissolved surfactants, determines the physical properties of oily soils left on the fabric. As the washing temperature is reduced, the viscosity of the soils increases or the soils may even solidify, making them more difficult to remove with the same level of agitation. Recent trends in washing habits around the world have made the proper choice of a surfactant system for a detergent formulation more critical than ever. Greater use of temperature-sensitive synthetic fabrics such as polyester or polyester-cotton blends as well as energy conservation have led to a world-wide trend to lower washing temperatures [7]. Phosphate limits or bans have resulted in the use of less effective builders or of no builders, so that surfactants are less protected from the negative impact of water hardness. Also, as a result of the effort to reduce the total amount of chemicals released to the environment as well as the volume of packaging materials, detergent manufacturers are formulating products with lower dosage requirements. As a result of these trends, the cleaning efficiency required from surfactant systems is steadily increasing. 2. Mechanisms for removal of oily soils The trends described above can have a particularly negative impact on the removal of Oily soils from synthetic fabrics. Commonly encountered examples of this troublesome soil-fabric combination are dirty motor oil, cooking oil, or sebum on 100% polyester or polyester-cotton blends. Many studies have been performed to visualize the mechanisms by which such oils are removed from synthetic fabrics [8-12]. Although in practical situations the oily soils are trapped in the interstices between fabric fibers and as thin films along the fiber surfaces, most work has focused on the removal of oil drops from flat surfaces or individual fibers with the assumption that similar removal mechanisms would be relevant to removal from fabric. In fact, good correlation between Oil removal from flat films and from chemically similar fabric has been reported [8]. Of relevance to this type of study is Young 5 equation, which relates the interfacial tension

3 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) between the surfactant solution and oil (γ ow ), oil and solid substrate (γ os ) and surfactant solution and solid substrate (gws) to the equilibrium contact angle q measured through the soil cos θ = γ ws γ os γ ow (1) For quite hydrophilic surfaces like cotton, gws is smaller than g0s, and a contact angle greater than go is commonly achieved. Anionic surfactants which adsorb on the fabric with their negatively charged head groups oriented toward the detergent solution are particularly effective in reducing gws. in this case, the roll-up mechanism is operative: the water preferentially wets the fabric, causing the oily stains to be entirely lifted off the fibers into the washing solution. This behavior, shown schematicaily in Fig. 1(b) for soil removal from a flat surface. is enhanced on cotton fabric due to swelling of the cotton fibers with water which increases the hydrophilicity of the fabric surfaces [9,13]. For low surface energy, i.e. hydrophobic, materials such as polyester, a contact angle of less than 90º is usually observed, and small portions of the oily soil may be removed by hydraulic currents at the soil-water interface, as shown in Fig. 1(a). In Fact, if the fabric surface is initially completely covered by oily soil, no location is available for the surfactant solution Fig. 1. Mechanisms of liquid soil removal: (a) emulsification; (b) roll-up. to reach the fiber surface and undercut the soil. Observation of this "necking" or emulsification mechanism has been made by many investigators for mineral oils and mineral oil-polar soil mixtures on hydrophobic flat films and fibers [8-12]. Removal in this manner is enhanced by low interfacial tension at the oil-water interface which allows the oil film to be deformed easily to form small emulsion droplets. Several factors have been studied with regard to their effect on the emulsification mechanism for the removal of mixtures of mineral oil and polar organic alcohols or acids from polyester [8-10]. Such model systems, depending on the ratio of the non-polar and polar constituents, can be considered to be representative of sebum soils from the skin. The rate of emulsification of mineral oil-oleic acid mixtures from polyester (Mylar) films was found to change as the oleic acid content was varied [8], Other factors such as electrolyte concentration and temperature were also found to have large effects on the rate of soil removal by this mechanism [8,9]. In some situations, emulsification of non-polar-polar soil mixtures without external agitation, i.e. spontaneous emulsification, has been observed [ 13,14]. Emulsification, roll-up, and other adhesion and detachment phenomena involving oily soils and solid surfaces are reviewed in the accompanying paper [ 15]. Another mechanism of oily soil removal involves the formation of intermediate phases at the detergent solution-oil interface [11,13,14,16]. Apart from the recent studies described below, this mechanism has most often been reported for the removal of soils containing large quantities of polar constituents. The growth of liquid crystal occurs in these systems due to interaction at the interface between the polar soil constituents and the adsorbed surfactants. After growing to a sufficient extent, the intermediate phase is broken off by agitation and emulsified into the aqueous solution allowing fresh contact of the remaining soil with the detergent solution. Direct solubilization of oily soils into surfactant micelles can also occur to a significant extent if a large excess of surfactant

4 172 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) relative to oil is present and if the surfactant is above its CMC. The solubilization of very small oil drops from polymer fibers has been visualized for a variety of non-polar oils representative of liquid laundry soils [17]. However, soil solubilization rates are often enhanced when surfactant-rich phases, either isotropic or liquid crystalline in nature, are present in the washing solution. Such phases exist, for instance, when non-ionic surfactants are above their cloud points. These phases can either solubilize oily soils directly or interact with soil to form intermediate surfactant-rich phases such as microemulsions containing large amounts of oil. Under favorable conditions, the intermediate phases can be emulsified into the washing bath. A detailed discussion of this mechanism of soil removal is the subject of this review. Clear evidence exists that solubilization and emulsification are major factors in removal of oily soils from hydrophobic, synthetic fabrics [18,19]. Unlike roll-up, in which the interaction of the fabric with the oily soil and water is most critical, the solubilization-emulsification mechanism occurs primarily at the soil-detergent solution interface and is therefore directly infiuenced by the phase behavior of the corresponding oil-water-surfactant system. For example, the formation of intermediate liquid crystalline phases in fatty acid-surfactant-water systems has been explained by the equilibrium phase behavior of those systems [ 16]. Also, spontaneous emulsification phenomena in oil-water-surfactant systems have been shown to be predictable from equilibrium phase behavior [20]. Therefore, an understanding of the phase behavior in these systems is needed to predict the effectiveness of and/or to optimize detergent solutions for specific soil compositions and washing conditions. Available information on phase behavior is reviewed in the next section. In subsequent sections, systematic studies of dynamic contacting between aqueous surfactant solutions and oils as well as detergency studies using the same systems are reviewed in order to further explain the role of the solubilizationemulsification mechanism in practical detergency processes. 3. Equilibrium phase behavior As indicated above, some knowledge of the equilibrium phase behavior of soil-water-surfactant systems is needed to understand solubilizationemulsification mechanisms of detergency. In this section, we review such phase behavior with emphasis given to model systems with well-defined components. Results are given for one- and twocomponent oils consisting of hydrocarbons, triglycerides, and/or long-chain alcohols or acids, representing soils such as lubricating oils, cooking oils and sebum. In many cases single-component specific alcohol ethoxylates are the surfactants. However, most features of the behavior described should be applicable to more complex systems as well, e.g. those containing multicomponent commercial surfactants Phase behavior of soil-free washing baths We begin with the fluids used for washing, i.e. rather dilute mixtures of surfactant and water with inorganic salts and/or various additives also present in some cases. As is well known, a typical hydrophilic surfactant above its Krafft temperature forms micelles in water at concentrations above its CMC. If the temperature or composition of a micellar solution is varied in such a way that the surfactant becomes less and less hydrophilic, the separation of another phase eventually results. If. for instance, the temperature of a micellar solution of an ethoxylated alcohol is increased, a second liquid phase begins to appear when the so-called cloud point temperature is reached. As Fig. 2 for the n-dodecyl pentaoxyethylene monoether (C12E5)-water system [21] shows, the cloud point is a function of surfactant concentration since clouding occurs when the coexistence curve forming the upper boundary of the aqueous surfactant solution L1 is crossed during heating. At temperatures well above the cloud point, the lamellar liquid crystalline phase La and yet another liquid phase L3, widely considered to consist of bilayers arranged in a sponge-like structure, are seen in this system for

5 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 2. Phase diagram of C 12 E 5 -water system [21]. L 1, L 2, and L 3 denote isotropic liquids; Lα, H 1, and V 1 denote lamellar, hexagonal and viscous isotropic liquid crystalline phases, respectively. Reprinted with permission of the Royal Society of Chemistry. relatively low surfactant concentrations. As the particles of a liquid crystal do not coalesce as readily as liquid drops, the dispersions of La are frequently less turbid than those of L 3, a property which can be used to locate phase transition temperatures at which the La and L 3 phases form [22]. At the highest temperatures shown in Fig. 2 the surfactant-rich liquid phase L 2 coexists with water. For more hydrophilic surfactants such as n-dodecyl hexaoxyethylene monoether (C 12 E 6 ), clouding occurs at higher temperatures. Moreover, the Lα and L 3 phases do not appear at low surfactant concentrations; the La phase transforms continuously into L 2, and the cloud point curve is the only feature of this part of the phase diagram (see Fig.3). Phase diagrams for various binary nonionic surfactant-water systems are given by Mitchell et al. [23]. Temperature effects are weaker for ionic surfactants and generally act in the opposite direction. Since the Debye length, a measure of the electric double layer thickness, is proportional to (kt) 1/2, where kt is the characteristic free energy of random thermal Fig. 3. Phase diagram of C 12 E 6 -water system [23]. The symbols for the phases are as in Fig. 2 except that S is a solid phase and W is a water-rich liquid phase. Reprinted with permission of the Royal Society of Chemistry. motion, higher temperatures make ionic surfactant films more hydrophilic, with a greater tendency to curve toward an oil-in-water configuration. However, the addition of inorganic salts has the opposite effect, compressing electric double layers and causing ionic surfactant films to become less hydrophilic. In some cases, a- second liquid phase is ultimately formed as salinity increases, i.e. the behavior is similar to clouding of non-ionic surfactant solutions discussed above. This phenomenon was observed by McBain many years ago for aqueous soap solutions [23b]. Another example is shown along the upper boundary of Fig. 4 [24], with NaCl added to the sodium salt of a commercial ethoxylated sulfate based on a C 12 -C 13 alcohol and containing an average of three ethylene oxide groups (Neodol 23-3S). As Fig. 4 indicates, multiphase regions involving the lamellar liquid crystal La are observed at even higher salinities. In other systems, for instance the Aerosol OT-NaCl-water system, the first phase formed upon increasing the salinity is the lamellar liquid crystalline phase [25,26]. Indeed, such behavior is typical for anionic surfactant-short-chain alcohol systems investigated for possible use in

6 174 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 4. Phase behavior of mixtures of C 12 E 3, Neodol 23-3S, and NaCl brine, with the temperature and total surfactant concentration fixed at 30ºC and 5.4 wt.%, respectively [24]. B, NaCl brine; W(non), weight fraction of non-ionic surfactant in the surfactant mixture. Reprinted with permission from Dr. Dietrich Stemkopff Verlag. enhanced oil recovery, with the L 3 phase found at still higher salinities [27]. The addition of divalent cations (i.e. an increase in hardness) produces the same effects in these systems, although generally at lower electrolyte concentrations [27]. Whether a liquid phase or the liquid crystal forms upon the addition of salt apparently depends on the relative importance of reducing the effective electrical repulsion between nearby ions within a micelle and reducing it between micelles. If the former is the dominant effect, the micelle shape changes from spherical to cylindrical to planar as the effective surfactant head group area decreases, a sequence in accordance with well-understood surfactant packing considerations in micelles [28]. The large, bilayer sheets corresponding to the planar configuration with nearly equal head and tail areas arrange themselves into the lamellar phase. In contrast, if reducing the repulsion between relatively small micelles is the more important effect, the micelles eventually flocculate to form a "coacervate" or micelle-rich liquid phase. It appears from the available evidence that coacervation is the likely outcome of increasing salinity for anionic surfactants that are rather hydrophilic, while liquid crystal formation is probable for surfactants whose hydrophilic characteristics only slightly outweigh their lipophilic characteristics in the absence of salt. Figure 4 provides an example. The addition of less than 20% of the non-ionic surfactant n-dodecyl trioxyethylene monoether (C 12 E 3 ) reduces the hydrophilic nature of the surfactant mixture sufficiently such that the liquid crystal phase forms instead of a coacervate when the NaCl concentration is increased. Such behavior can be explained as follows. Hydrophilic surfactants such as Neodol 23-3S require large concentrations of salt for the surfactant aggregates to become planar. Repulsion between small, nonplanar micelles is apparently reduced sufficiently so as to produce coacervation before the salinity increases enough for planar micelles to form. For less hydrophilic surfactants, less salt is needed to produce planar aggregates, and formation of the lamellar phase occurs before coacervation. The cloud point phenomenon discussed above for non-ionic surfactants may also be viewed as coacervation of a micellar solution induced by making the surfactant less hydrophilic. In this case. raising the the temperature reduces the interaction between the ethylene oxide chains and water and thereby reduces the repulsion between micelles or even reverses it to an attraction. Of course, the effective area of the ethylene oxide head group is also reduced, and a change from spherical to cylindrical micelles, which would facilitate coacervation by an entropic mechanism, can occur before the cloud point is reached. Unlike the situation for anionic surfactants, however, there do not seem to be any reports of the lamellar liquid crystalline phase forming directly from micellar solutions Or ethoxylated alcohols before clouding occurs as the temperature is raised in dilute binary systems. The cloud point of a non-ionic surfactant natulrally depends on its structure, increasing

7 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) for longer ethylene oxide and shorter hydrocarbon chains. Shifting the point of attachment of the ethylene oxide chain from the end to the central portion of the hydrocarbon chain depresses the cloud point. The addition of many common salts, e.g. sodium and potassium chlorides and sulfates, lowers the cloud point although the effects are much smaller than for ionic surfactants. However, some salts cause the cloud point to increase. The former effect is generally considered to stem from reduced hydration of the ethylene oxide chains resulting from competition with the added ions for the available water molecules. The latter effect occurs for ions such as I -, SCN - and most multivalent cations Which break the structure of water. For some salts the anion and cation have opposite effects, with the stronger determining the direction of the cloud point shift. These effects have been recently discussed by Mackay [29]. Other additives also influence the cloud point and other phase boundaries in non-ionic surfactant-water systems. Long-chain alcohols make the system less hydrophilic, as Fig. 5 shows for the case of n-dodecanol added to mixtures of C 12 E 5 and water [30]. In this case, the cloud point is lowered by some 23ºC when the alcohol content is only 10 wt.% relative to the surfactant. The temperatures for the other phase transitions are lowered as well. For the binary surfactant-water system, the phase rule constrains the three-phase regions, e.g. W + L 1 + Lα, to a single temperature, but the addition of the alcohol provides an additional degree of freedom and allows these regions to span a finite temperature range, as Fig. 5 indicates. The additive can, of course, be another surfactant. It is well known that the addition of an ionic surfactant greatly increases the cloud point of an ethoxylated alcohol by adding an electrical repulsion between micelles and thereby inhibiting coacervation [31]. The opposite occurs when a second but more lipophilic non-ionic surfactant is added, e.g. C 12 E 3 to C 12 E 6, as shown in Fig. 6 [32]. This system is particularly interesting because, as noted previously, the Lα and L 3 phases do not occur in dilute mixtures of water and pure C 12 E 6. However, both these phases appear when only a few per cent of the more lipophilic surfactant has been added, due to some rather complex Fig. 5. Phase behavior resulting from the addition of small amounts of n-dodecanol to I Wt'% C 12 E 5 in water [30]; 30 denotes a three-phase region. Reprinted with permission from Academic Press. Fig. 6. Phase behavior of mixtures of C 12 E 6 and C1 2 E 3 in water. The total surfactant concentration is fixed at 1.0 wt.% [32].

8 176 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) phase behavior in the region indicated by the box. Although details are not given here, an interesting feature of this behavior is the existence of a four-phase region, which is constrained to a single temperature in this ternary system by the phase rule and which involves W, L 1, L 3 and La, phases [32]. Figure 6 also shows a transition from W + L 3 to W + L 2 at 58ºC for the C 12 E 3 -water system, which was clearly seen in this study using optical microscopy but does not appear in the existing phase diagram [23]. The addition of a non-ionic surfactant to an anionic surfactant makes the surfactant mixture more lipophilic and causes a shift from L1 to Lα, to L 3, as shown in Fig. 4. This sequence is, not surprisingly, consistent with that shown in Fig. 6 for increase in the C12E3 content of C 12 E 6 -C 12 E 3 mixtures at constant temperature. The same sequence is found when a lipophilic alcohol is added to an anionic [27] or a zwitterionic [33] surfactant system. Although cationic surfactants are rarely used for cleaning purposes, they are useful for neutralizing charge build-up on fabric surfaces and for fabric softening. As an important trend in detergent formulation is combining all ingredients into a single mixture in order to eliminate the need for the separate addition of bleaches, fabric softeners, etc. during the washing process, it seems useful to include some information on phase behavior with cationic surfactants present. Moreover, some recent work suggests that mixtures of cationic and nonionic surfactants may be useful in removing oily soils, though not by solubilization mechanisms [34]. Because anionic and cationic surfactants attract one another, surfactant aggregation occurs readily with substantial neutralization of charge. When either the anionic or the cationic surfactant is present in substantial excess, the result is mixed micelles but with a much lower CMC than for the individual surfactants. When the two surfactants are present in almost equal amounts, the formation of a solid or liquid crystalline phase can be expected if the hydrocarbon chain lengths are sufficiently long. In some cases, vesicles have been found to form spontaneously upon mixing aqueous solutions with intermediate ratios of anionic and cationic surfactants [35] Phase behavior of water-surfactant-hydrocarbon systems An important feature of the phase behavior of systems containing water, surfactants, and hydrocarbon soils is the existence of microemulsions, thermodynamically stable liquid phases containing substantial amounts of both water and oil. The formation of microemulsions requires that the surfactant-films which separate oil and water microdomains be rather flexible, and that the hydrophilic and lipophilic properties of the surfactant be roughly balanced. However, within conditions satisfying these overall constraints, the microstructure is quite sensitive to changes in the relative strength of hydrophilic and lipophilic interactions. In systems which contain comparable volumes of oil and water and which are dilute in surfactant but not so dilute as to preclude aggregation, packing considerations dictate that oil-in-water microemulsions coexist with excess oil for hydrophilic surfactants and water-in-oil microemulsions with excess water for lipophilic surfactants. Drop size' are of the order of 5-50 nm, increasing in size as the temperature, pressure, or system composition is changed to shift the surfactant closer to the condition of precise balance between hydrophilic and lipophilic properties. Very near this balance. the microemulsion becomes continuous in both phases and coexists with both excess water and excess oil. Interfacial tensions of this 'middlephase" microemulsion with both excess phases are frequently below mn m -1 - in some systems by an order of magnitude or more. The condition for which the hydrophilic and lipophilic properties are exactly balanced and the surfactant films have no spontaneous tendency 10 curve in either direction has been called the phase inversion temperature (PIT) or hydrophile-lipophile balance (HLB) temperature by Shinoda and Friberg [36] for the case of

9 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) non-ionic surfactants for which temperature is usually the variable of greatest interest. For ionic surfactants it is more common to speak of "optimal" conditions, e.g. optimal salinity [37]. Whatever one calls it, several criteria have been used to define the condition for balance in terms of readily measured experimental quantities. The most common criterion is equal volumetric solubilization in the microemulsion of the oil and water phases. The differences between the "optimal" conditions given by this and other criteria are small for practical purposes and will be ignored here. The effects of temperature and inorganic salts on making the surfactant more or less hydrophilic are basically the same as those described in the preceding section, and so are the effects of adding alcohols or additional surfactants, except that one additional factor must be considered - the relative solubilities of the surfactants and additives in the oil phase. It is the composition of the surfactant films separating oil and water domains that determines the microstructure of the microemulsion. In a mixture of two non-ionic surfactants the more lipophilic surfactant has a higher solubility in the oil phase and the surfactant films are thus more hydrophilic than the overall surfactant mixture. The magnitude of this effect for a given pair of surfactants depends on both the overall surfactant concentration and the water-to-oil ratio. Kunieda, Shinoda and co-workers have developed equations for predicting the dependence of the PIT on system composition for mixtures of two non-ionic surfactants [38] and for mixtures of an anionic and a non-ionic surfactant [39]. For instance, in the latter case the following relationship must be satisfied at the PIT is the mass fraction of oil in the oil-water mixture, and X is the total surfactant mass fraction in the system. The solubility of the anionic surfactant in the excess oil has been neglected. It is clear from this equation that a plot of Wn as a function of (X -1-1) at constant Row and temperature should yield a straight line from which values of Ssn and Son can be extracted. Figure 7 shows such plots for mixtures of C 12 E 3 and Neodol 23-3S at various temperatures along with the corresponding values of Ssn and Son. The oil phase is n-hexadecane and the aqueous phase is water containing 1 wt.% NaCl. As might be expected, nonionic surfactant solubility in the oil phase S on increases with increasing temperature. In contrast, the fraction S sn of nonionic surfactant in the films decreases. Since increasing temperature makes the nonionic surfactant less hydrophilic, it is reasonable that less of it would be required to achieve the W n = S sn + S on R ow [(1 - S sn )/(1 - S on )] (X -1) (2) where W n is the mass fraction of non-ionic surfactant in the overall mixture, S sn is the mass fraction of non-ionic surfactant in the surfactant films, S on is the mass fraction of non-ionic surfactant in the excess hydrocarbon phase, R ow Fig. 7. PIT results for the C12E3-Neodol 23-3S-1 wt.% NaCl brine-n-hexadecane system [24]; X is the total surfactant mass fraction in the system. Reprinted with permission from Dr. Dietrich Steinkopff Verlag.

10 178 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) balance between hydrophilic and lipophilic properties at high temperatures. The PIT is also influenced by the composition of the oil phase, being higher for hydrocarbons with longer chains. The reason is that their penetration into the hydrocarbon chain region of the surfactant films tends to make the films curve toward a water-in-oil configuration. Such penetration is less for longer-chain hydrocarbons [40], probably due primarily to an entropic effect [41] except for short-chain hydrocarbons where energy effects have recently been shown to be important as well [42]. Reed and Puerto [43] have developed a scheme relating optimal conditions to the molar volume of the oil and the solubilization at optimal conditions. The other factor mentioned above as being necessary for microemulsion formation is the existence of flexible films. Films are most rigid for surfactants having long, straight hydrocarbon chains. Flexibility can be increased by promoting less ordered packing in the hydrocarbon chain region of the films, e.g. by using branched-chain surfactants or mixtures of surfactants with different chain lengths, or by adding short-chain alcohols. Increasing the temperature also promotes flexibility. Too much flexibility can be undesirable, however, because it reduces the solubilization capacity of a microemulsion. However, when the surfactant films become too rigid, the lamellar liquid crystalline phase forms. Barakat et al. determined the conditions in several systems for which the lamellar phase formed instead of a middle-phase microemulsion because of film rigidity [44]. Hackett and Miller investigated the detailed phase behavior near the transition [45]. The liquid crystal can also form when the amount of oil or water present becomes too low [46] or when the surfactant concentration of a middle-phase microemulsion becomes too high [47]. Kunieda and Shinoda [47] have presented ternary diagrams at several temperatures ranging from below to above the PIT for the C 12 E 5 -water-n-tetradecane system. We discuss below the use of such diagrams in interpreting the dynamic behavior which occurs when surfactantwater mixtures contact oil Water-non-ionic surfactant- triglyceride systems Extensive studies have been made of microemulsions in hydrocarbon systems. Much less information is available for oils which are liquid triglycerides. An important difference is that triglycerides such as triolein which are of interest for detergency are of much higher molecular weight than simple straight-chain liquid hydrocarbons such as n-hexadecane. The higher molecular weight makes it much more difficult to incorporate such triglycerides into surfactant films, the result being a major reduction in solubilization in many systems. Figure 8 shows the general form of a water-nonionic surfactant-liquid triglyceride ternary diagram [48-50]. Deviations from this behavior occur at sufficiently high temperatures, a matter discussed further below. As indicated on the diagram, the phase designated D' is the same as that usually called L3 in binary surfactant-water systems, mentioned in section 3.1. While, like the middle-phase microemulsions discussed above, the D' phase coexists with both excess water and excess oil for suitable overall system compositions, it differs from the microemulsions in that it can Fig. 8. Schematic phase diagram for non-ionic surfactant-water-triolein system at low temperatures [48]. O denotes a triolein-rich phase.

11 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) solubilize only small amounts of the oil phase. Note that solubilization of triglyceride in the lamellar liquid crystalline phase is also low. Behavior of the W + D' + O three-phase triangle has been studied as a function of temperature for pure non-ionic surfactants and triolein. Figure 9 shows the results for C12E3 [48], a lipophilic surfactant that is already above its cloud point at 0ºC, well below the experimental temperature range. The surfactant content of the D' phase increases rapidly with temperature, the same as for the L3 phase in binary surfactant-water systems, but solubilization of triolein remains low. The solubility of surfactant in the triolein phase is substantial - more than 10% by volume even at the lowest temperature studied (30.5ºC) - and increases with temperature. For comparison, we note that the solubility of C12E3 in excess n-hexadecane in equilibrium with microemulsions at 30ºC is about 3% by volume (see Fig. 7). At about 40ºC the rate of increase with temperature of surfactant solubility in the triolein phase increases significantly. Simultaneously, water solubilization in this phase, previously rather low, rises dramatically. Such formation of water-in-oil microemulsions is one way the system can depart at high temperatures from the behavior shown in Fig. 8. Another way is illustrated in Fig. 10 by the Fig. 9. The W + D' + 0 region at several temperatures in the C12E3-water-triolein system [48]. Fig. 10. The W + D' + O and W + D + O regions at several temperatures in the C 12 E 5 -water-triolein system [48]. corresponding C 12 E 5 diagram [48]. Just above 64ºC a phase transformation occurs, and at higher temperatures the diagram shows a new W + D + O three-phase region instead of the W + D' + O region. The D phase is able to solubilize considerable triolein and is thus more favorable for detergency than D' if it forms as an intermediate phase during washing. According to Fig. 10, the composition of the D phase shifts to become richer in oil with increasing temperature in a manner similar to that seen for microemulsion systems [47] although the surfactant concentration of about 40% is well above that observed in typical microemulsions. Details of the phase behavior in the temperature region where the D phase first appears, including the existence of two four-phase regions at two closely spaced temperatures, are given for another system by Kunieda and Haishima [50]. As indicated above, the inability of the hydrocarbon chain region of the surfactant films to incorporate the large triglyceride molecules is the chief reason for the poor solubilization. Similar poor solubilization and phase behavior have been seen in systems containing the anionic surfactant Aerosol OT, hydrocarbons with chain lengths of twelve and above, and NaCl brine [26]. Recently, Binks [51] has investigated further the phase behavior of some of these systems. If the surfactant films were made more flexi-

12 180 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) ble, i.e. if chain packing in this region were made more disordered, one might expect solubilization to increase. Clearly in some systems such as water-c 12 E 5 -triolein (Fig. 10), increasing temperature provides sufficient disorder for the D phase to form. In other systems such as water-c 12 E 3 -triolein (Fig. 9), the D phase does not form at any temperature. One might expect that adding amphiphilic compounds with chain lengths different from the surfactant or with branched chains would promote less ordered packing and formation of the D phase. As Fig. 11 shows, the addition of tert-amyl alcohol (TAA) does, in fact, favor formation of the D instead of the D' phase in the water-c 12 E 4 -triolein system [52]. In the absence of TAA the D phase is seen only over a range of about 0.2ºC near 55ºC, and not at all for the somewhat lower temperatures of Fig. 12 [48]. For the purposes of improving detergency, the use of TAA to promote solubilization of long-chain liquid triglycerides at relatively low temperatures is not attractive because its high solubility in water requires that it be used at Fig. 12. Partial phase diagram of the C 12 E 4 -watertriolein-n-hexadecane system [48]. W. and 0. denote water-continuous and oil-continuous microemulsions. rather high concentrations. The same disadvantage applies to the use of hydrotopes, a possibility considered by Friberg and Rydhag [53]. Alander and Warnheim [54] managed to solubilize a mixture of medium-chain triglycerides (C 8 -C 10 ) in aqueous solutions of a 1 :2 mixture of sodium oleate and n-pentanol at 25ºC, but they had less success with long-chain triglycerides. Recent results suggest that the use of doublechain surfactants with varying chain lengths, e.g. secondary alcohol ethoxylate surfactants, instead of straight-chain surfactants may prove effective for forming the D phase and thereby solubilizing reasonable amounts of triolein at temperatures suitable for warm water washing [55]. Here, too, the basic idea is that solubilization should be improved for surfactant films with disordered packing in the hydrocarbon chain region. Fig. 11. Partial phase diagram of the C 12 E 4 -tertiary amyl alcohol-water-triolein system: surfactant content, 16 wt.%; equal volumes of water and triolein [52]; LC denotes the lamellar or Lα phase Mixtures of hydrocarbons and triglycerides Microemulsion formation is easier in mixed soils of hydrocarbon and triglyceride than for

13 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) pure triglyceride soils. In the mixed soil systems, hydrocarbon molecules presumably penetrate the surfactant films, allowing oil droplets or oil microdomains of other shapes to form. Triglyceride and hydrocarbon are jointly solubilized within the microdomains. Figure 12 shows phase behavior in the water-n-dodecyl tetraoxyethylene monoether (C 12 H 4 )-triolein-nhexadecane system for a particular overall surfactant concentration and equal volumes of oil and water phases [48]. The transition from the existence of a middle-phase microemulsion (D phase) in hydrocarbon-rich systems the D' phase in triolein-rich systems is clear. Note that the sequence of phases seen with increasing temperature over one temperature range for pure triolein is, omitting the excess oil phase, Lα, L 2 + D'. D', W + D'. The same sequence occurs in,he oil-free system at modest surfactant concentrations. That is, the sequence of phases with triolein present is the same as that when it is absent, except that an excess oil phase is present and the transition temperatures are somewhat lower. This behavior is to be expected when solubilization is low. At intermediate oil compositions, D phase formation can be promoted by increasing either the temperature or the hydrocarbon content of the oil. By making the surfactant less hydrophilic, increasing the temperature promotes a surfactant film configuration where the hydrocarbon chains diverge, and thereby facilitates the solubilization of triolein. The transition between the W + D' + O and W + D + 0 regions occurs by means of a narrow four-phase region W + D'+ D + O. Table I gives compositions of the four-phase region at one temperature. Clearly, solubilization of both hydrocarbon and triglyceride is greater in the D phase. Moreover, hydrocarbon is solubilized in preference to triolein in both phases. Similar phase behavior has been reported for another triglyceride [49]. Figure 13 shows interfacial tensions measured with the spinning drop apparatus at 30ºC for 1 wt.% C 12 E 4 with various mixtures of n-hexadecane and triolein [48]. For the pure hydrocarbon the tensions drops after 10 minutes Table 1 Compositions in volume fractions of four coexisting phases of the C12E4-water-triolein-n-hexadecane system at 39.2ºC [48] to about mn m -1, which is a reasonable value for a system near its PIT. In contrast, the tension reached after some 3 h is about 0.2 mn m -1 for pure triolein. The higher tension is expected in view of the low solubilization of triolein. Mixed oils have intermediate values of interfacial tension. Fig. 13. Interfacial tensions (IFT) at 30ºC for 1 Wt'% C 12 E 4 with various mixtures of triolein and n-hexadecane [48].

14 182 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Water-surfactant-polar soil systems Hydrocarbons and triglycerides are frequently referred to as non-polar soils, while long-chain fatty acids and alcohols are termed polar soils. In this section we consider only the case of pure polar soils. Ekwall has studied the phase behavior of many anionic surfactant-water-polar soil systems [56]. Figure 14 is a partial ternary diagram at 50ºC for the water-sodium octylsulfonate-n-hexanol system, which has been investigated in recent years by Kunieda and Nakamura [57]. A matter of interest for later sections of this paper is that the region of coexistence of L 1 and L 2 phases near the water-hexanol axis is bounded by a three-phase triangle involving these phases and the lamellar liquid crystal L a. In other cases, including the water-sodium octanoate-n-decanol system that was studied extensively by Ekwall's group, careful examination of the dilute region reveals that the D' (or L 3 ) phase replaces Lα, in the three-phase triangle which terminates the L 1 -L 2 region [58]. In this system a second three-phase triangle D'-Lα-L 2 also exists, the arrangement being similar to that shown in Fig. 8 for triglyceride systems. In almost all diagrams of this type involving relatively long-chain compounds, the lamellar phase and its associated multiphase regions are prominent although other liquid crystalline phases may be present as well at high surfactant concentrations [56]. Kunieda and Nakamura [57] showed that the addition of NaCl to the system of Fig. 14 caused the D' phase to appear and the dilute portion of the phase diagram to resemble Fig. 8. The higher salinity is likely to make the surfactant-alcohol bilayers more flexible and enables them to assume the locally saddleshaped configuration necessary for formation of the sponge-like microstructure of the D' phase for slightly lipophilic conditions. Apparently only the lamellar structure is possible for more rigid bilayers. When the surfactant is non-ionic, the same behavior, i.e. the appearance of the D' phase and a shift from a diagram resembling Fig. 14 to one resembling Fig. 8, can be effected by increasing the temperature, as Kunieda and Miyajima [591 showed for the water-n-dodecyl octaoxyethylene monoether (C 12 E 8 )-n-decanol system. Here. too. the ability to form the D' phase at temperatures above about 14ºC is probably the result of increased flexibility of the surfactant-alcohol bilayers Mixtures of non-polar and polar soils Fig. 14. Phase behavior of the sodium octylsulfonate-water-nhexanol system in the dilute region at 50ºC [57]. Reprinted with permission of the American Chemical Society. As mentioned in section 3.2, the addition of rather lipophilic amphiphilic compounds reduce, the PIT of non-ionic surfactant-water-hydrocarbon systems. Long-chain alcohols and (undissociated) fatty acids ate of this type, as Fig. 15 shows for the addition of oleyl alcohol to systems containing water, n-hexadecane, and several non-ionic -surfactants [12]. Note that 5% oleyl alcohol in the system by oil phase reduces the PIT in the C 12 E 6 about 35ºC. Similar results were found by the same authors for oleic acid. When enough polar soil is present that the system is above its PIT but the surfactant is

15 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 15. PIT values for non-ionic surfactant-water-nhexadecane-oleyl alcohol systems [12]. Reprinted with permission of the American Oil Chemists' Society. below its cloud point, one might expect that phase behavior in the dilute region would be similar to that described in the preceding section, i.e. the L 1 -L 2 region would terminate in a three-phase region involving either the D' or the La phase. Experiments at 30ºC with C 12 E 7 and oils having ratios of n-hexadecane to oleyl alcohol of 3/1 and 1/1, respectively, showed that such behavior did, in fact, occur, with the third phase being the lamellar liquid crystal Lα [60]. However, in contrast to the situations shown in Fig. 8 and 14 where the oil phase solubilizes modest amounts of water, L 2 phases in these systems extended to compositions containing up to 75-80% water which were in equilibrium with aqueous micellar solutions. Evidently, the presence of hydrocarbon and of the double bond in the alcohol chain makes the surfactant films sufficiently flexible so that the L 2 phase can invert continuously and become water continuous, ultimately reaching compositions comparable to those of the D' phase in systems such as that shown in Fig. 8. The relevance of this behavior to detergency is discussed later. When the long-chain alcohol is mixed with a liquid triglyceride instead of with a hydrocarbon, multiphase regions containing the D' phase are prominent, as is shown in Fig. 16 [61] for the C 12 E 6 -water-triolein-oleyl alcohol system. The sequence of phases observed with increasing temperature in Fig. 16 for oils having oleyl alcohol contents exceeding about 20% is the same as was found for the water-non-ionic surfactant-triolein systems discussed in section 3.3, as may be seen, for instance, along the right-hand boundary of Fig. 12 for water-c 12 E 4 - Fig. 16. Partial phase diagram of C12E6-water-triolein-oleyl alcohol system with 10 wt.% surfactant, 45 wt.% water, and 45 wt.% mixed oil [61]. The symbol IV denotes the four-phase region W + D'+ D + O.

16 184 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) triolein. The D phase is seen, however, for surfactant concentrations well above that of Fig. 16. Note that the amounts of oleyl alcohol needed to depress the temperatures of the various phase boundaries are much greater than are shown in Fig. 15 for hydrocarbon systems. A likely explanation is that much of the oleyl alcohol is dissolved in the bulk triglyceride phase, leaving relatively little alcohol in the surfactant films which, to a large extent, determine the basic phase behavior by controlling the aggregate shape. 4. Diffusion path analysis Being of rather short duration and typically involving small quantities of soils, detergency processes are strongly influenced by dynamic, diffusional phenomena which occur on a microscopic scale. Oily soil removal, in particular, depends on phase transitions which occur at the oil-washing solution interface. The preceding section described equilibrium phase behavior in both water-surfactant systems, representing the washing solution, and oil-water-surfactant systems, As demonstrated below, such equilibrium phase behavior can be combined with the theory of diffusion processes to interpret certain dynamic behavior such as intermediate phase formation and spontaneous emulsification that occurs during detergent processes. A mathematical technique called diffusion path analysis has been used with success in predicting certain dynamic phenomena in multicomponent solid and liquid systems when two phases not in equilibrium are brought into contact with one another [62-64]. Essentially, a time-invariant path of compositions can be plotted across an equilibrium phase diagram by-solving component transport equations with certain assumptions and boundary conditions. First, convection in the system from any source is assumed to be negligible. Second, the two phases are assumed to be semiinfinite in extent. This assumption simplifies the mathematical analysis and is probably reasonable at least for short times after contacting. Third, diffusion of each species is assumed to be dependent only on its own concentration gradient with a uniform diffusion coefficient in each phase. Also, local equilibrium is assumed at all interfaces which form; this means that the compositions at the interfaces are defined by equilibrium tie lines. Diffusion path analysis is most conveniently applied to three-component, i.e. ternary, systems. In this situation, the phase diagram can be represented in the form of a two-dimensional triangle as, for instance, in Fig. 8, with two-phase regions shown as regions of varying shape containing equilibrium tie lines and three-phase regions represented as triangles in which the compositions of the equilibrium phases are shown as the vertices. The analysis in this case consists of solving in each phase the following transport equations for two of the species ( wi/ t) = Di( 2 wi/ x 2 ) i = 1,2 (3) where x is the distance from the initial surface of contact, t is time and wi and D i are the mass fraction and diffusivity of species i, respectively. The value of W3 for the third species is found by invoking Σw i = 1. The semi-infinite phase assumption allows transformation of the above equations to ordinary differential equations in the similarity variable η i = [x/(4d i t) 1/2 ]. Integration yields the following error function solutions for the diffusion path segment in each phase w i = A i + B i erf η I i = 1,2 (4) where A i and B i are constants which are evaluated from the boundary conditions. Since η i varies from - to + at each value of time, the set of compositions given by Eq. (4) is independent of time although the position x of a specific composition does vary with time. It is often convenient to plot the compositions or "diffusion path" directly on the equilibrium phase diagram. In evaluating A i and B i for phases in contact. iteration on a tie line is performed until the individual species mass balances at the interface are satisfied. In addition to obtaining the path of compositions that forms between the initial

17 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) phases, one may also calculate the relative velocities of all interfaces, and therefore the growth rates of intermediate phases. More detailed descriptions of the mathematical analysis and the specific error function solutions can be found elsewhere for a ternary system forming a single interface [63] or two or more interfaces [65]. The utility of diffusion path theory in ternary liquid systems was first shown for predicting the occurrence of spontaneous emulsification in alcohol-water-oil systems [63]. Specifically, when an alcohol-oil mixture denoted d in Fig. 17 is brought into contact with water, spontaneous emulsification of oil drops in the water phase is observed. In this situation, the construction of a diffusion path between the initial compositions shows the formation of an interface with equilibrium compositions b and c connected by the tie line represented by the broken line. Spontaneous emulsification in the aqueous phase can be explained by the passage of that path segment from b to W through the corner of the two-phase envelope, thereby predicting the formation of small drops of oil-alcohol mixture below the interface. Experiments showed that, in the absence of interfacial turbulence, interfacial displacement Fig. 17. Schematic diffusion path in alcohol(a)- water(w)-oil(o) system showing supersaturation leading to spontaneous emulsification. is proportional to the square root of time, as predicted by the theory [65,66]. This diffusion mechanism of spontaneous emulsification is distinct from other modes of spontaneous emulsification in which interfacial instability results in the mechanical dispersion of one phase in another [67]. Diffusion path analysis was later applied to oil-water-surfactant systems [20,64,68]. In these cases, the use of pseudoternary phase diagrams was required. For example, commercial surfactants are almost always complex mixtures containing numerous species of surfactants. Rather than solving the diffusion equations for each species, one can sometimes combine all surfactant components together and treat them as a pseudocomponent. Mixtures of hydrocarbons can also be considered as pseudocomponents. Although diffusion path studies are typically performed when single-phase systems are originally present, the ability to calculate diffusion paths in which one of the initial compositions is a stable dispersion of one phase in another, e.g. a liquid crystalline dispersion, has also been demonstrated [64]. 5. Dynamic contacting studies Direct observation of the dynamic phenomena that occur when non-equilibrated liquid phases are brought into contact can be made in various ways. On a macroscopic scale, a liquid can be gently placed on top of another liquid in a tube, and rather large-scale phenomena can be observed. This simple technique was used in the early studies of spontaneous emulsification in oil-water-alcohol systems [63] and has been used with surfactant systems to monitor the formation of microemulsion and liquid crystalline phases between oil and surfactant solutions [68-71]. In these cases, the oil is gently layered on top of the aqueous phase, and dynamic phenomena are observed without magnification. Crossed polarizers aid in the identification of birefringent liquid crystalline phases. A shortcoming of this technique is the inability to observe events which occur immediately after the contacting of the two

18 186 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) phases. Also, the experimental scale increases the possibility of convection occurring due to the formation of adverse density gradients. Nevertheless, intriguing phenomena including oscillations in the interfacial position and abrupt changes in the interfacial velocity have been observed by Friberg and co-workers [70,71] in systems in which the lamellar liquid crystal is present, although generally at rather long times (a few days) after initial contact. For various reasons, diffusion path theory is not applicable for describing such phenomena, as these workers have pointed out. To facilitate the observation of dynamic phenomena at short times after contact, a microscopic technique was developed. A key aspect of the technique is the use of rectangular glass capillaries, having a path width 200 gm, tohold the sample [68]. Figure 18 shows a diagram of the sample cell that is 50 min in length. After the aqueous phase is imbibed about half way into the capillary, that end is sealed by a resin curable by ultraviolet light. The oil phase is then injected into the other end by use of a syringe. Initially, the resulting diffusion phenomena were observed using a conventional microscope with the cell in a horizontal configuration. However, due to the density difference between the two phases, overriding of the oil over the surfactant solution occurred, causing distortion of the interface and complicating the interpretation of the results [68]. A vertical-stage microscope was then designed that allowed the cell to be placed in a vertical configuration in a controlled temperature environment. Details of the microscope and contacting technique are given elsewhere [20]. In this configuration in a stable region of contact between the two initial phases can be maintained, allowing easy viewing. The vertical-configuration microscope was subsequently improved and equipped with a video imaging system [48,72]. The use of video taping and image analysis allows detailed review of phenomena which may be missed initially in real time, and improved determination of, for example, the velocities of interfaces and rates of formation of intermediate phases. The vertical-contacting technique was first used to study the dynamic contacting in water-anionic surfactant-oil systems representative of those used in enhanced oil recovery processes [20]. Intermediate phase formation and spontaneous emulsification experimentally observed in a brine-petroleum sulfonate-hydrocarbon system were found to be predictable from calculated diffusion paths based on relevant phase diagrams [64]. Widely varying but predictable phenomena were found as the salinity of the aqueous phase was varied. 6. Diffusional phenomena in detergent systems 6.1. Water - alcohol ethoxylate - hydrocarbon systems Fig. 18. Rectangular glass capillary cell used in vertical-stage contacting experiments. Studies of diffusional phenomena in systems having direct relevance to detergency processes

19 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) have recently been performed. Experiments were designed to investigate the effects of changes in temperature on the dynamic phenomena which occur when aqueous solutions of pure non-ionic surfactants contact hydrocarbons such as tetradecane and hexadecane [18,72]. These oils can be considered to be models of non-polar soils such as lubricating oils. The dynamic contacting phenomena, at least immediately after contact, are representative of those which occur when a detergent solution contacts an oily soil on a synthetic fabric surface. The following is a summary of the observed behavior interpreted through the use of schematic diffusion paths. Detailed phase behavior in such systems has been reported previously and was used in construction of the diffusion paths [47]. With C 12 E 5 as the non-ionic surfactant at a 1 wt.% level in water, quite different phenomena were observed below, above, and well above the cloud point when tetradecane or hexadecane was carefully layered on top of the aqueous solution. Below the cloud point temperature of 31ºC, very slow solubilization of oil into the one-phase micellar solution was observed. An interesting phenomenon was observed at 20ºC involving a "volcanolike" instability which caused flow of the aqueous solution to the oil interface. This flow column, which is believed to have resulted from an adverse density gradient within the aqueous phase, is shown in Fig. 19 [72]. The upper tip of the column was observed to oscillate, probably due to gradients in interfacial tension along the oil-water interface (Marangoni flow). Of more importance to a detergency process, the schematic diffusion path shown in Fig. 20(a) explains why no intermediate phase formed between the water and oil. Also, due to the low solubility of oil in the dilute aqueous surfactant solution in this region of the ternary phase diagram, it predicts the quite slow solubilization of oil into the surfactant solution. At temperatures just below the cloud point temperature, an intermediate phase depleted in surfactant did form between the micellar solution and the oil. The schematic diffusion path in this case is shown in Fig. 20(b). Once again, instabilities in the aqueous Fig. 19. "Volcano" instability in C 12 E 5 -water-ntetradecane system at 20ºC. The image is out of focus to allow observation of refractive index variations [72]. Reprinted with permission of Academic Press. Fig. 20. (a) Diffusion path well below the cloud point showing no intermediate phase formation; (b) diffusion path slightly below the cloud point showing the formation of intermediate phase W [72]. Reprinted with permission of Academic Press. ous phase occurred, in this case due to a density difference between the original micellar solution and the intermediate phase, causing flow of surfactant solution to the oil interface. Nevertheless, at all temperatures studied below the cloud point, only very slow solubilization of oil into the surfactant solution was observed. At 35ºC, which is just above the cloud point, a much different behavior was observed. The surfactant-rich L1 phase separated to the top of the aqueous phase prior to contacting by hexadecane.

20 188 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Upon addition of the oil, the drops of the L 1 phase rapidly solubilized the hydrocarbon to form an oil-in-water microemulsion containing an appreciable quantity of hydrocarbon. After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase. This behavior is shown schematically in Fig. 21. Unlike the experiments carried out below the cloud point temperature, an appreciable solubilization of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the phase inversion temperatures of the hydrocarbons with C 12 E 5 (45ºC for tetradecane and 50ºC for hexadecane), two intermediate phases formed when the initial dispersion of L1 drops in the water contacted the oil. One was the lamellar liquid crystalline phase Lα (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, appreciable solubilization of hydrocarbon into the two intermediate phases, shown 4 min after contacting in Fig. 22, was observed. A diagram of the phenomena observed is shown in Fig. 23. A similar progression of phases was found at 35ºC using n-decane as the hydrocarbon. At this temperature, which is near the phase inversion Fig. 22. Video frame showing intermediate phases 4 min after contact in C 12 E 5 -water-n-tetradecane system at 45ºC near the PIT [72]. Reprinted with permission of Academic Press. Fig. 23. Schematic diagram showing the conversion of L 1 phase into a middle-phase microemulsion and a liquid crystal dispersion, for the experiment depicted in Fig. 22 [72]. Reprinted with permission of Academic Press. Fig. 21. Schematic diagram showing conversion of the L 1 phase into an oil-in-water microemulsion at temperatures above the cloud point [72]. The symbol me denotes a microemulsion. Reprinted with permission of Academic Press. temperature of the water-c 12 E 5 -decane system, the existence of a two-phase dispersion of Lα and water below the middle-phase microemulsion was clearly evident. To compare the rate of tetradecane solubilization at temperatures below and near the phase inversion temperature, verticalcontacting experiments were performed between the L 1 phase and oil at 40 and 48ºC. In both cases, the surfactant-rich phase was the equilibrium phase that separated from a 10% aqueous solution at that temperature. At 40ºC a liquid crystalline phase and an oil-in-water

21 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) micro emulsion formed between the L 1 and oil phases. At 48 º C, a similar phase progression was observed, with a middle-phase microemulsion forming in place of the oil-in-water microemulsion. As shown in Fig. 24, plots of the position of the oil-micro emulsion interface versus the square root of time were straight lines in both cases indicating diffusion-controlled mass transfer. In Fig. 24, an arbitrary constant has been added to each position for ease of comparison, i.e. x = 0 is not the initial contact position. At any given time, the relative slopes of the two lines are indicative of the relative rates of oil solubilization. In this case, the rate of solubilization near the PIT is 2.2 times greater than at 40ºC. Well above the phase inversion temperatures for both hexadecane and tetradecane, 1 wt.% C 12 E 5 exists as a dispersion of liquid crystal Lα in water. Rapid movement of the liquid crystal to the oil occurred upon contacting, causing extensive spontaneous emulsification of water in the oil phase. Eventually a layer depleted in liquid crystal formed near the oil interface. Figure 25 shows the spontaneous emulsification that occurred. Interpretation of the phenomena Fig. 25. Video frame showing spontaneous emulsification observed 18 min after initial contact in the C 12 E 5 -water-n-hexadecane system at 60ºC [72]. Reprinted with permission of Academic Press. by diffusion path analysis indicated that the oil was being converted into a waterin-oil microemulsion at this high temperature; this means that very little solubilization of oil into the aqueous phase was taking place. The spontaneous emulsification occurred due to the passage of the oil-phase diffusion path segment across the twophase water-micro-emulsion region, as shown in Fig. 26. Experiments similar to those described above were also performed using C 12 E 4 as the surfactant [18]. This surfactant is more hydrophobic than C 12 E 5 and therefore has a lower Fig. 24. Variation of oil- microemulsion interface with time at 40ºC and 48ºC following contact of the L 1 phase with oil for the C 12 E 5 -water-n-tetradecane system. Fig. 26. Schematic diffusion path for the experiment depicted in Fig. 25. Point d represents the composition of the initial water-surfactant mixture; HC, S, and tie denote hydrocarbon, surfactant, and microemulsion. The last is oil-continuous in this case. Some multiphase regions are not shown [72]. Reprinted with permission of Academic Press.

22 190 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) cloud point (7ºC) and PIT with hexadecane (30ºC). Contacting experiments were performed below, at, and above the PIT. In contrast to C 12 E 5, the structure of the 1 wt% C 12 E 4 solution was a lamellar liquid crystalline dispersion in water at all the temperatures studied. With regard to the intermediate phases which formed at the different temperatures, the following results were obtained. Below the PIT, an oil-in-water microemulsion formed between the lamellar liquid crystalline dispersion and oil. At the PIT, the behavior was dependent upon the concentration of liquid crystalline material at the initial oil-surfactant solution interface. At the low concentration of liquid crystal present at a 1% surfactant level, no intermediate phase formation was observed when the aqueous dispersion was contacted with hexadecane. To provide a higher level of liquid crystal at the initial point of contact, La drops were allowed to cream to the air-water interface prior to the contacting studies. These drops converted to concentrated La domains upon heating to 30ºC. With this initial aqueous phase structure, the formation of a middle-phase microemulsion layer was observed upon contacting with oil. Finally, if a pure lamellar liquid crystalline phase containing 25% surfactant was contacted with oil, swelling of the liquid crystalline phase was observed, as shown in Fig. 27. An interpretation of these results was made by comparing the qualitative diffusion paths, shown in Fig. 28, resulting from the different aqueous starting compositions [18]. The differences in behavior for the dilute (C), concentrated (B) and pure liquid crystal (A) cases were attributed to the relatively high solubility of C 12 E 4 in the oil phase at this temperature, which influenced whether the diffusion path passed above or below the various three-phase regions that are present on the phase diagram. At temperatures of 40-50ºC, which are above the PIT, behavior similar to that for C12E5 was observed. Dissolution of the lamellar liquid crystalline phase into the oil resulted in the formation of a water-in-oil microemulsion and spontaneous emulsification of water in the oil phase. Fig. 27. Video frame showing swelling of the lamellar liquid crystalline phase 43 min after initial contact for the C 12 E 4 -water-n-hexadecane system at the PIT of 30ºC [18], Reprinted with permission of Academic Press. Dynamic contacting studies were also performed with hydrophobic additives combined with C 12 E 5 [30]. C 12 E 3 and n-dodecanol were added to C 12 E 5 in proportions to yield cloud points near that of C 12 E 4 (approximately 7ºC). This study was conducted Fig. 28. Schematic diffusion paths representing different beha- vior observed at different surfactant concentrations for the C 12 E 4 -water-n-hexadecane system at the PIT of 30ºC [18] The symbol (mp µe) denotes a middle-phase microemulsion. Reprinted with permission of Academic Press.

23 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) ducted to determine whether formation of intermediate microemulsion phases containing a high proportion of oil could be obtained at temperatures lower than those for C 12 E 5 alone. However, despite exhibiting phase behavior in the absence of oil similar to that of C 12 E 4, the C 12 E 5 -C 12 E 3 mixture behaved in a way intermediate to the behavior seen with C 12 E 4 and C 12 E 5 upon being contacted with hexadecane. Also, the microemulsion phases formed at various temperatures when the C 12 E 5 -dodecanol system contacted oil were essentially unchanged from those seen in the C 12 E 5 system without any additive present. For example, rather than forming a middle-phase microemulsion with hexadecane at 30ºC, the two systems formed oil-in-water micro-emulsions. The contacting temperature had to be increased to 40ºC in the case of the C 12 E 5 -C 12 E 3 System and 50ºC in the case of the C 12 E 5 -dodecanol system before middlephase microemulsion formation was observed. These differences between the two systems were attributed to differences in partitioning of the additive and the more water-soluble C 12 E 5 between the oil and the microemulsion phases. The observed differences between the two additive systems would be less if smaller quantities of oil relative to the surfactant solution had been present Water-alcohol ethoxylate-triglyceride (+ hydrocarbon) systems Triolein is a pure triglyceride suitable for use as a model for kitchen soils such as vegetable oils. Dynamic contacting studies similar to those described above for hydrocarbons were performed with triolein using aqueous solutions containing the three alcohol ethoxylates C 12 E 3, C 12 E 4 and C 12 E 5 [48]. As described in the phase behavior section, ternary triolein - water-nonionic surfactant systems exhibit different phase behavior than those containing straight-chain hydrocarbons. Specifically, the large size of the triolein molecules inhibits solubilization and formation of microemulsion phases. At low temperatures, schematic phase behavior like that shown in Fig. 8 is observed in which two three-phase regions are present in the ternary diagram. At these temperatures, the surfactantwater mixture is a dispersion of the liquid crystal La in water. When this dispersion is contacted with triolein, a water layer forms between the liquid crystal and the oil, and extensive spontaneous emulsification occurs in the oil phase [48]. This behavior can be explained in terms of a diffusion path which passes below the bottom three-phase region in Fig. 8. Spontaneous emulsification occurs in the oil phase due to passage of that diffusion path segment across the two-phase water-oil region. In general, insufficient surfactant is available at the interface to form an intermediate L 3 or D' phase due to the high solubility of non-ionic surfactants in triolein at these conditions. At still higher temperatures where C 12 E 3 and C 12 E 4 are in the form of aqueous dispersions of L 3 (greater than 30ºC for C 12 E 3 and 55ºC for C 12 E 4 ), similar behavior occurs except that even more vigorous spontaneous emulsification is observed. C 12 E 5 exhibited behavior at approximately 65ºC quite comparable to that observed with hydrocarbon systems near the PIT. In fact, as shown in Fig. 10, the D phase in that system contains almost equal volumes of triolein and water at 65ºC. When a concentrated lamellar liquid crystalline dispersion contacted triolein at that temperature, the D phase formed, first as lenses along the water-oil interface, then as a continuous layer. A schematic diffusion path corresponding to this behavior is shown in Fig. 29. The dynamic contacting of C 12 E 4. solutions with oil mixtures containing varying proportions of triolein and hexadecane was also studied [48]. For 3/1 hexadecane-triolein mixtures, behavior comparable to that found with the pure hydrocarbon system was obtained. Similarly, 3/1 triolein-hexadecane systems behaved in the contacting experiments like the pure triglyceride system. However, contacting of the concentrated liquid crystalline dispersion at 38ºC with a 1/1 mixture of triolein and hexadecane resulted in the formation of transient middle-phase microemulsion droplets. The failure to form a

24 192 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 29. Diffusion path corresponding to observed behavior for the C 12 E 5 -water-triolein system at 64.5ºC [48]. complete microemulsion layer during the course of the experiment probably resulted from the high solubility of the surfactant in the oil mixture. The latter experiment was repeated with tertiary amyl alcohol (TAA) added to the concentrated La dispersion at three different TAA/C12E4 ratios: 0.05, 0.10, and 0.20 by weight [52]. All three experiments showed the rapid formation of an intermediate phase, presumably the D or microemulsion phase. The higher the ratio of TAA to surfactant, the faster was the formation of the intermediate phase as a continuous layer. As indicated in the previous paragraph, the formation of a continuous middle-phase microemulsion layer did not occur in the absence of TAA. 7. Oil drop-contacting experiments As discussed in the preceding section, contacting experiments using vertically oriented cells, and their interpretation using diffusion path theory, have provided a fundamental understanding of the conditions for the occurrence of intermediate phase formation and spontaneous emulsification for a variety of systems containing water, pure surfactants, and non-polar oils. However, when either surfactant mixtures, e.g. the C 12 E 5 -additive systems discussed above, or commercial surfactants that are themselves complex mixtures, are used, the conditions for which intermediate phases form during a vertical cell-contacting experiment are frequently rather different from those expected during washing experiments in the same system at the same temperature. The reason is differential partitioning of different surfactant species into the oil phase. As explained previously in section 3, the extent of differential partitioning depends on both the overall surfactant concentration in the aqueous phase and the water-to-oil ratio (see Eq. (2)). In particular, the surfactant remaining after some partitioning into the oil has occurred, the factor which largely controls whether an intermediate phase will form, is less hydrophilic for a washing experiment in which the water-to-oil ratio is very large than for a contacting experiment in which the volumes of oil and water are comparable. It should be noted that this problem does not arise for systems containing mixtures of anionic surfactants and hydrocarbon oils, because none of the individual surfactant species has appreciable solubility in the hydrocarbon phase. When the oil consists of one or more polar components or of both polar and non-polar components, there is another limitation of the vertical cell-contacting technique that is significant even when pure surfactants are used. Basically, both experiment and diffusion-path theory yield information on the behavior of the system during the early stages of contact when the oil phase remains at its initial composition except in a region near the surface of contact. However, when the volume of the oil phase is small, the diffusion of polar material out of and/or diffusion of surfactant into the oil can cause the composition of the entire oil phase to vary with time, i.e. no location exists where the oil has its initial composition. This effect is especially important when inverse micelles or other aggregates form in the oil that have mixed films of surfactant and polar compounds. As a result, situations occur in which an intermediate phase does not form on initial contact but de-

25 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 30. Schematic illustration of contacting experiment in which a small oil drop is injected into an aqueous surfactant solution. velops later in the experiment when the oil composition becomes suitable. Examples of such behavior are discussed below. An oil drop-contacting technique was developed in which the water-to-oil ratio is large, as in practical washing situations. As shown in Fig. 30, a drop of oil, usually some mm in diameter, is injected into a horizontal rectangular glass cell by means of a very thin hypodermic needle. The cell, which is 400 Rm thick, is inside a thermal stage modified to enable the drop to be observed by videomicroscopy from the moment of injection [24,60]. Since the drop must be viewed through the surfactant solution in which it is immersed, this technique works best when the surfactant solution is below its cloud point temperature. However, some experiments have been successfully carried out in which the initial surfactant solution was a dispersion of the lamellar liquid crystal in water, as discussed below. It is noteworthy that no similar limitation exists for the vertical cell technique, and indeed almost all of the experiments described above for non-polar oils were conducted above the cloud point temperature Experiments with surfactant mixtures and nonpolar oils Mixtures of anionic and non-ionic surfactants are now almost universally used in liquid detergents for laundry applications since they are more effective than anionics alone for washing synthetic fabrics at low temperatures. The oil dropcontacting technique was used to determine whether an intermediate microemulsion phase would form near the PIT for these mixed surfactant systems with hydrocarbon soils [24] in a manner similar to that described above for pure non-ionic surfactants with the vertical cell technique. The pure non-ionic surfactant C 12 E 3 and the commercial anionic surfactant Neodol 23-3S were used in this study, i.e. the same combination as discussed above in the phase behavior section. The use of a commercial mixture rather than a pure anionic surfactant had minimal effect on the differential partitioning since all the individual anionic species in the mixture had very low solubilities in n-hexadecane, the hydrocarbon used. Because the volume of the oil drop injected was small, it dissolved little non-ionic surfactant, and the relevant PIT was that for which the surfactant composition Ssn in the films within the microemulsion phase was the same as the overall surfactant composition in the system. Data on Ssn for this system when the aqueous phase contains 1 wt.% NaCl are given in Fig. 7. As may be seen from Fig. 4, the initial washing bath, i.e. the oilfree mixture of the surfactant and a 1 wt.% NaCl solution, forms a dispersion of the lamellar liquid crystal in brine for surfactant compositions equal to the relevant values of S sn. Contacting experiments were conducted for a surfactant mixture containing 78 wt.% of the nonionic surfactant [24]. According to Fig. 4, the relevant PIT is 30ºC. At 25ºC, no intermediate phase was observed and the drop diameter did not decrease appreciably with time. Thus, solubilization of hydrocarbon by the liquid crystalline phase was very slow at this temperature below the PIT. At 30ºC, an intermediate microemulsion phase was observed. Its volume continued to increase until the oil phase disappeared. At 40ºC, the drop diameter increased with time, the expected behavior above the PIT as the oil phase takes up surfactant and water. The liquid crystalline particles surrounding the drop made it difficult to discern whether spontaneous emulsification

26 194 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) in the oil occurred under these conditions, as would be expected based on observations above the PIT described above for the vertical cell experiments. The conclusion reached from these experiments is that mixed surfactant systems behave in basically the same manner as pure surfactant systems for hydrocarbon oils, provided that the PIT used to interpret the behavior is as defined above. Valuable results were also obtained by the oil drop technique for the C 12 E 4 -water-triolein-taa system. As discussed in Section 3, the addition of TAA reduced the temperature at which the D phase formed in this system and also promoted formation of the D instead of the D' phase. The latter has considerably less ability to solubilize triolein than the former. When a drop of pure triolein was injected into an alcohol-free mixture of C 12 E 4 and water at 50ºC, spontaneous emulsification was observed within the drop as well as the growth of small myelinic figures at the drop surface [52]. The drop volume decreased slowly with time, an indication that some triolein was being solubilized into the liquid crystalline particles present initially. This behavior is, as expected, the same as was seen with the vertical cell technique for the same system and temperature. In contrast, with TAA added to the surfactantwater mixture in an amount equal to 20 wt.% of the surfactant, an intermediate liquid phase, presumably the D phase, formed when a triolein drop was injected at the same temperature. As shown in Fig. 31, the drop shrank considerably during the first few minutes of the experiment as triolein was solubilized into the D phase. After about 5 min, the triolein drop had disappeared completely. The contacting experiments thus confirmed the conclusion reached from the phase behavior results that TAA promoted the formation of the D phase, which is capable of solubilizing considerable triolein Experiments with pure long-chain alcohols It can be shown using diffusion path theory that when the initial surfactant concentration in Fig. 31. Video frames showing the dynamic behavior of a drop of triolein contacted with 5 Wt-% C 12 E 4 with added TAA at 50-C (TAA/C12E7=0.20). Obtained from Ref. 52. an aqueous micellar solution is sufficiently low, no intermediate phases form upon initial contact of this solution with a long-chain alcohol. However, an intermediate phase does form when the surfactant mass fraction exceeds a

27 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) critical value ws* given by the following equation [73] w s * = w sb F 1 (w oc ) + w sc F 2 (w oc ) (5) Here w sb and w sc are the surfactant mass fractions at the L 1 and L 2 ends of the limiting tie line forming one boundary of the two-phase region for these phases, and w oc is the oil mass fraction at the L 2 end. The quantity w sb is often called the "limiting association concentration" or LAC [56]. The functions F 1 and F 2 are defined as follows where D o and D s are the diffusivities of the oil and the surfactant in the L2 phase, D's is the diffusivity of the surfactant in the L1 phase and hoc is the (constant) value of the similarity parameter [x/(4d.t) 1/2 ] at the L2 end of the limiting tie line (see discussion of diffusion path analysis above). Vertical cell-contacting experiments gave results in reasonable agreement with Eq. (3) for the sodium octanoate-n-decanol-water system [73]. One might expect that detergency would be improved when the intermediate phase - in this case the lamellar liquid crystal - is formed since more of the alcohol is solubilized. Indeed, Kielman and van Steen [11] observed such behavior in the potassium octanoate-n-decanolwater system. The focus of this section is the time-dependent behavior of the system if a drop of alcohol is injected into a solution whose surfactant concentration is below the critical value. The question to be answered is whether an intermediate phase will form at some time after initial contact. A series of such experiments was performed for the C 12 E 5 -water-oleyl alcohol system at 30ºC [74]. The L 1 -L 2 coexistence curve and some interfacial tensions between these two phases are shown in Fig. 32. Note that the coexistence curve terminates at a point F corresponding to a water content of about 70 wt.%, which is one vertex of the L 1 -Lα-L 2 three-phase triangle. Video frames taken at various times during an experiment in which the surfactant concentration was 1 wt.% are shown in Fig. 33. Note that the drop, initially some 70 mm in diameter, swells as it takes up water and surfactant. After about 23 min, the lamellar (La) phase begins to develop as myelinic figures which grow into the aqueous solution. Eventually, nearly all the alcohol is converted to liquid crystal. The order of magnitude of the time required for diffusion within the drop is the ratio of the square of its radius to the diffusion coefficient. As this time is much less than that of the experiment, a quasi-steady state scheme may be used to model drop behavior. Basically, this means that drop composition may be viewed as TIE -LINE INTERFACIAL TENSION (dyne/cm) Fig. 32. Partial ternary phase diagram for the C 12 E 5 -water- alcohol system at 30ºC showing the L 1 -L 2 coexistence curve and the limiting tie line EF. The oil drop composition varies as indicated by the arrows. The interfacial tensions are between pre-equilibrated phases for the four tie lines shown [74]. Reprinted with permission of Plenum Press.

28 196 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 33. Video frames showing dynamic behavior following contact of 1.0 wt.% solution of C 12 E 5 with a drop of pure oleyl alcohol at 30ºC [74]: (a) alcohol drop about 2 min after injection; (b) the same, about 20 min later; (c) initial formation of lamellar phase about I min later; (d) growth of myclinic figures into the surrounding aqueous phase about 3 min later; (e) almost complete conversion of alcohol into liquid crystal about 8 min later. Reprinted with permission of Plenum Press. moving along the coexistence curve in the direction of the arrows in Fig. 32. Since the long-chain alcohol has low solubility in the aqueous phase, the drop volume increases during this process as its contents of water and surfactant increase. The result is swellingas

29 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) observed during the experiment. When the drop composition reaches point F at the end of the coexistence curve, the driving force for diffusion of the surfactant and water into the drop remains. gut because the drop cannot remain in the L 2 region, according to the phase diagram, the lamellar phase begins to form. A theory has been developed which predicts that the time t from the start of the experiment until the liquid crystal starts to form is given by the following expression [60] t = K s (R 0 2 /D s w s ) (6) Here R 0 is the initial radius of the drop, D s and are the diffusivity and bulk concentration of the surfactant in the aqueous solution and K s is a constant that depends only on the shape of the coexistence curve and the location of point F. The predicted proportionality between t and Ro 2 has been confirmed by experiment (see Fig. 34), as has the inverse relationship between t and the bulk surfactant concentration w s. With a value of for K. calculated using the phase behavior of Fig. 32, Eq. (6) and the measured values of t were used to estimate D s. A value of about 4 x m 2 s -1 was obtained, which is reasonable for a micellar solution. A similar equation has been developed for the case of a uniform layer of oil on a flat solid surface immersed in a stirred aqueous surfactant solution [74] t = K p (h 0 d/d s w s ) (7) where ho is the initial thickness of the oil layer and d is the thickness of the diffusion boundary layer adjacent to the oil. Like K s in Eq. (6), K p depends only on the shape and terminal point of the coexistence curve between the L 1 and L 2 phases. For the C 12 E 8 -water-n-decanol system at temperatures above 14ºC, the three-phase triangle bounding the L 1 -L 2 region has D' instead of Lα as the additional phase [59]. When a contacting experiment was conducted at 27ºC in this system, with a drop initially about 60 µm in diameter, a liquid intermediate phase developed after about 14 min and surrounded the initial alcohol drop [55]. As the intermediate phase grew during the next 8 min to a diameter of about 83 mm, the alcohol drop shrank slightly to a diameter of about 83 µm. Presumably, this growth process could be analyzed using a "shrinking core" model with the quasi-steady state approximation. However, as the available data on phase behavior [59] do not include coexistence curves for the L 1 -D' and L 2 -D' two-phase regions, it is not currently possible to make quantitative comparisons between predictions of the analysis and the experimental results Experiments with mixtures of hydrocarbons and long-chain alcohols Fig. 34. Plot of the square root of the time t required to initiate liquid crystal formation as a function of initial drop size for the system of Fig. 32. The surfactant concentration in the aqueous phase is 1.0 wt.% [74]. Reprinted with permission of Plenum Press. More interesting for detergency applications than pure alcohols are mixtures of polar and nonpolar oils, which are representative of sebum-like soils. Here we discuss a series of experiments in which drops of various mixtures of n-hexadecane and oleyl alcohol, typically

30 198 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) containing at least 50 wt.% hydrocarbon, were injected into dilute aqueous solutions of pure non-ionic surfactants at temperatures below their cloud points [60]. PIT measurements for these systems were given previously (Fig. 15). A general pattern of behavior was observed. At temperatures below the PIT, as given by Fig. 15, no intermediate phase was seen at any time during the experiment and the drop volume decreased very slowly with time owing to solubilization of the alcohol and hydrocarbon into the micellar solution. Above the PIT, the behavior was similar to that described in the preceding section for the C 12 E 5 -water-oleyl alcohol system. That is, the drop would swell, and, at a particular time, the lamellar liquid crystalline phase could be seen growing as myelinic figures. However, the myelinic figures were shorter, smaller in diameter and more numerous, and they grew faster than in the pure alcohol system. The differences can be seen by comparing Fig. 35 for a drop initially containing 85 wt.% hydrocarbon and 15 wt.% alcohol immersed in an aqueous solution containing 0.05 Wt-% C12E8, with Fig. 33 for the pure alcohol system. The low surfactant concentration for the mixed oil experiment is typical of those used in household laundry processes. Coexistence curves for the L 1 -L 2 region were determined at 30ºC for systems containing n-dodecyl heptaoxyethylene monoether (C 12 E 7 ) and oils with 50% and 75% n-hexadecane, respectively [60]. In both cases the curves extended to water contents of wt.%. Contacting experiments for various initial drop sizes and surfactant concentrations in the latter system confirmed that the dynamic behavior was consistent with Eq. (6). Here, too, liquid crystalline intermediate phases were seen for drops in contact with solutions containing only 0.05 wt.% surfactant. It was also noted that the rate of swelling of the drops increased markedly as the limiting tie line was approached. Since the coexistence curves exhibited nearly constant alcohol-tosurfactant ratios near the limiting tie lines, this behavior was expected as little surfactant must diffuse into the drop for it to experience a substantial increase in volume. The basic analysis leading to Eq. (6) confirmed quantitatively that rapid swelling should occur for these conditions. Values of the parameter K s were found from the phase behavior data to be and for the two systems. That is, for a given initial drop size and surfactant concentration, formation of the liquid crystal occurred more rapidly in the system containing less alcohol, presumably because less surfactant had to diffuse into the drop to balance hydrophilic and lipophilic properties of the surfactant films -to the extent that conditions favorable for formation of the lamellar phase were created. Indeed, a general result of the contacting experiments with various surfactants and oils was that more time was required for liquid crystal formation when the system was made less hydrophilic by increasing temperature or the alcohol content of the drop or by reducing the ethylene oxide chain length of the surfactant [60]. One may ask why the intermediate phase formed during the contacting experiments should be the lamellar liquid crystal instead of a microemulsion, in cases where the drops contained substantial amounts of hydrocarbon. After all, diffusion of surfactant into the drop should cause the surfactantalcohol films within it to become more hydrophilic, as indicated above. Eventually, one might expect film composition to approach that corresponding to the PIT at the experimental temperature and thereby cause formation of a middle-phase microemulsion. The answer is that the drop itself becomes a microemulsion as it takes up water and surfactant. The occurrence of low interfacial tensions supports this conclusion. For instance. a tension of about 0.03 mn m -1 was measured by the spinning drop technique about 50 min after the start of the experiment for an oil phase initially containing 75% n-hexadecane and 25% oleyl alcohol and an aqueous solution containing 0.1 wt% C 12 E 7 at 30-C [60]. The coexistence curve for this system indicates that the water-to-hydrocarbon ratio during an oil drop contacting experiment varies

31 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 35. Video frames showing dynamic behavior following the contact of 0.05 Wt'% C12E8 solution with a drop of 5.67/1 n-hexadecane-oleyl alcohol at 50ºC [60]: (a) oil drop about 13 min after injection; (b) the same about 8 min later; (c) growth of myelinic figures less than 6 s later; (d) further growth of myelinic figures into the aqueous phase 12 s later. Reprinted with permission of the American Chemical Society. from its initial value of zero to about 5 when the drop composition eventually reaches the end of the coexistence curve (corresponding to point F of Fig. 32). No intermediate phase forms until point F is reached because the hydrocarbon content does not exceed the solubilization limit of the microemulsion. However, at point F, where the hydrocarbon-to-amphiphile ratio has dropped to about 1.5, the microemulsion structure apparently cannot be sustained and an intermediate lamellar phase begins to form. That it would form at such ratios of the various components and when the composition of the surfactant-alcohol films is approximately balanced between the hydrophilic and lipophilic properties is generally consistent with phase behavior results for other systems reported by Ghosh and Miller [46]. Indeed, lamellar phases with even higher oil contents have been reported near the PIT of the C 12 E 4 -water-n-hexadecane system [75].

32 200 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) When the alcohol content of the oil is low and the temperature is near or only slightly above the PIT, only a small amount of surfactant need diffuse into the drop in order for hydrophilic and lipophilic properties of the surfactant-alcohol films formed there to be almost balanced. In this case, the hydrocarbon solubilization limit is exceeded and drops of oil form within the original drop, which has become a middle-phase microemulsion. Such behavior was observed, for example, when a drop containing 90 wt.% n-hexadecane was injected into a solution containing 0.05 wt.% C 12 E 7 at 40ºC, which is near the PIT for an oil of this composition. As the oil content of the microemulsion decreased during the experiment owing to this spontaneous emulsification, and as the surfactant content continued to increase, a point was eventually reached when the hydrocarbon-to-amphiphile ratio was too low to sustain the microemulsion structure and the lamellar phase again developed as myelinic figures [76]. Finally, it is noteworthy that when drops of oil containing 75% n-hexadecane and 25% oleyl alcohol were contacted with a surfactant solution containing 0.05 Wt-% C 12 E 7 at temperatures above 40ºC, it appeared that the first intermediate phase formed was a liquid, presumably D' [76]. That is the system apparently experienced between 30. and 400 the transition discussed in the phase behavior section above which D' replaces La as the third phase in the three-phase region bounding the region of coexistence between La and L2. Detailed phase behavior at 40º was not determined, however. The intermediate D' phase in the contacting experiment was later converted to liquid crystal (myelinic figures) Experiments with mixtures of triolein and longchain alcohols Drops containing various mixtures of triolein and oleyl alcohol were injected into 0.1 wt.% solutions of C 12 E 6 at 40ºC, which is about 10º below the cloud point temperature of dilute solutions of this surfactant [61]. As discussed in section 3 and shown in Fig. 16, the sequence of phases seen with increasing temperature in this system is similar to that found with pure triolein, the various transition temperatures being lower for higher oleyl alcohol contents. For drops containing less than 25 wt.% oleyl alcohol, a scarcely perceptible change in drop size with time was observed, an indication that the oil was being slowly solubilized into the surfactant solution. As indicated above, similar behavior was seen for hydrocarbon- alcohol drops below the PIT. Figure 16 confirms that the system is quite hydrophilic under these conditions. A liquid intermediate phase, presumably D', formed after a few minutes for a drop containing 50 wt.% oleyl alcohol (Fig. 36(b)). Careful study of the image at various depths of focus revealed that this phase did not surround the original drop but instead formed a drop in contact with the original drop but larger in diameter, as the figure shows. Later, myelinic figures began to grow outward into the aqueous solution from the D'phase (Fig. 36(c)). Ultimately, what was left of the original oil drop - its volume was only about a third of the initial value - became detached from the intermediate liquid phase and showed no further changes with time. Although this system has four components, with the result that its phase behavior cannot be adequately described by a triangular diagram similar to Fig. 32, an explanation can be given for the dynamic behavior observed. As in the hydrocarbon-alcohol systems, surfactant diffuses into the drop, and the surfactant-alcohol films which form there become more hydrophilic with time. Eventually, the boundary of the L 1 -L 2 coexistence region is reached, and an intermediate D' phase forms as surfactant continues to diffuse into the drop. As this new phase solubilizes little triolein, the alcohol-to-triolein ratio in the original drop decreases greatly. However, the D' phase is itself in contact with the surfactant solution and accordingly experiences an increase in surfactant content with time. When the hydrophilic and lipophilic properties of its surfactant - alcohol films are balanced, the D'

33 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 36. Video frames from an experiment in which a drop containing equal amounts of triolein and oleyl alcohol was injected into a 0.1 Wt'% C 12 E 6 solution at 40ºC [61]. (a) Shortly after injection; (b) approximately 9 min later (the intermediate phase has formed); (c) about 5 min later (the intermediate phase has grown, myelinic figures are starting to form); (d) about I min later (a drop of unsolubilized oil separates from the intermediate phase); (e) the drop is never completely solubilized. Reprinted with permission of Marcel Dekker publishers.

34 202 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) phase, whose films are known to be slightly lipophilic, cannot persist, and the lamellar phase forms as myelinic figures. Since the triolein-rich drop does not dissolve, it may well be that the system enters a four-phase region at the time the myelinic figures develop. For a drop containing 75% oleyl alcohol behavior was more complex. Two intermediate liquid phases formed at various times during the experiment and greater solubilization was observed [61]. Ultimately, the liquid crystalline phase was seen as well. When the drop is pure oleyl alcohol, no D' phase is seen, and the first intermediate phase is the lamellar liquid crystal. It develops as many short myelinic figures, the appearance being more similar to that of Fig. 35 for hydrocarbon-oleyl alcohol drops than to that of Fig. 33 for the C 12 E 5 -water-oleyl alcohol system. 8. Fabric detergency test methods Laboratory evaluations of laundry detergency can range from use of an actual washing machine with a capacity of several gallons to use of a Terg-O-Tometer mini-laundry machine. The Terg-0-Tometer is a washing machine which replicates on a small scale the cleaning action of an agitatortype washing machine [77]. The capacity of a single Terg-O-Tometer pot is approximately 11. The temperature can be controlled quite accurately through the use of a water bath surrounding a bank of four or six pots. The small size of a Terg-O-Tometer allows rapid comparative studies of detergent formulations under the same washing conditions. Also, formulations containing expensive ingredients, e.g. pure alcohol ethoxylates, or experimental surfactants of limited quantity, can be evaluated at reasonable costs. For these reasons, the use of Terg-O-Tometer machines in detergent research laboratories has become quite common. The measurement of soil removal from fabrics can be performed in several ways [78]. The most straightforward technique involves visual inspection for determination of cleanliness. More sophisticated techniques involve the use of spectrophotometry, chemical analyses, and radiotracer methods. As with visual inspection, the measurement of light reflectance from fabric does not quantify actual soil removal but depends on factors such as the distribution of soil on the fabric, and the particle sizes. Analyses of weight gain or loss through the chemical extraction of soils from the fabric as well as radiotracer methods both yield measurements of actual soil removal and do not depend on the change of appearance of the fabric. Although not representative of real-world visualization of cleanliness, both techniques provide complementary information to reflectance measurements and are more useful for mechanistic studies. Specific advantages of the radiotracer detergency method are given elsewhere [79,80]. This method makes use of mildly radiolabeled soils for highly sensitive quantitative determination of soil removal by simple radiochernical techniques. Several radioisotopes may be used as labels. Generally, polar oily soils such as alcohols and acids are tagged with small amounts of carbon-14 labeled material. The use of two labels in a soil such as artificial sebum, which contains both polar and non-polar components, allows the removal of both types of soil to be monitored simultaneously. The recipe for such a labeled artificial sebum has been given elsewhere [80]. Radiolabelled particulate and protein soils have also been developed and utilized [80]. The Shell Development Company has utilized a radiotracer detergency method for many years to evaluate laundry detergents and to study the fundamental nature of oily soil removal [81,82]. In the case of oily soils, a 4-in square fabric swatch is soiled with 28 mg of radiolabeled oil. The soil is applied to the fabric in a toluene solution, which is allowed to air dry. The swatch is washed under controlled conditions in a Terg-0-Tometer. Aliquots of wash water are removed for radioactive counting, with the level of soil remaining on the swatch determined by difference. In addition to the sebum oil described above, oily soils which have been commonly utilized include tritium-labelled hexadecane (cetane), 1/1 hexadecane-squalane

35 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) (a C 30 branched hydrocarbon) and triolein. Also, hydrocarbon-fatty acid (or alcohol) soils of varying composition have been studied in which both carbon- 14 and tritium labels are utilized to allow the discrimination of removal of both components. The following are results of fundamental detergency studies using some of these soils, which were performed to allow correlation to the phase behavior and dynamic contacting studies described above. 9. Fabric detergency - non-polar hydrocarbon soils Results of radiotracer detergency studies of removal of hexadecane from 65/35 permanent press polyester-cotton fabric are shown in Fig. 37 [18]. This soil can be considered a model for non-polar hydrocarbon-based soils such as lubricating oils. The washing solutions contained 0.05 wt.% surfactant, resulting in a fabric-to-soil weight ratio of Fig. 37. Removal of n-hexadecane from 65/35 polyester-cotton fabric using 0.05 wt.% aqueous solutions of C 12 E 4 and C 12 E 5 [18]. The arrows show the cloud point temperatures of the surfactants. Reprinted with permission of Academic Press. 40/1 and a surfactant-to-soil ratio of 9/1. Triethanolamine (50 ppm) was included as a buffer, and no water hardness was present. The washing time in the Terg-0-Tometer was 10 min. Of particular interest in these studies was the effect of temperature and non-ionic alcohol ethoxylate surfactant structure on the levels of soil removal. As shown in Fig. 37, a strong dependence on temperature was observed with the highest levels of soil removal occurring almost 200C above the cloud point of the washing solution, a temperature regime in which the washing solution structure is a liquid crystalline dispersion. In fact, the optimum detergency temperature (ODT) in each case occurred very near the PIT of the water-surfactant-hexadecane system. Although not shown in Fig. 37, very poor detergency occurred in the temperature range of interest with C 12 E 3 as the surfactant. Also, when a 1/1 by weight mixture of hexadecane and squalane was used as the soil, a similar peak in performance was found for each surfactant near its PIT for the mixed soil. These PITs were approximately 7ºC higher than the corresponding values for hexadecane alone, i.e. 37 and 58ºC versus 30 and 52ºC. The equivalence of the PIT and ODT in this type of detergency system has been confirmed by the work of Schambil and Schwuger [22] as well as Solans et al. [83]. Also, the same correspondence has been found for the removal of hydrocarbon by the same surfactant from 100% polyester fabric [84]. The high levels of soil removed near the PIT can be attributed to the ultralow interfacial tensions achieved near that temperature, and to the high rates of oily soil solubilization into middle-phase microemulsions, as was visualized in the dynamic contacting studies described previously. When the detergent properties of mixtures of C 12 E 5 with the hydrophobic additives C 12 E 3 and n-dodecanol (C 12 E 0 ) were studied, optimum detergency was found at temperatures lower than the ODT for C 12 E 5 alone but somewhat higher than the ODT for C12E4. Detergency data for the two additive systems, which exhibited the same cloud point as C 12 E 4, are

36 204 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) Fig. 38. Removal of n-hexadecane from 65/35 polyester-cotton fabric using 0.05 wt.% aqueous solutions of a 90/10 blend of C 12 E 5 and n-dodecanol [30]. Reprinted with permission of Academic Press. shown in Figs. 38 and 39 [30]. The optimum temperatures correspond quite closely to the phase inversion temperatures extrapolated to the low soil-to-surfactant ratios used in the detergency studies. In this regard, C 12 E 3 was somewhat more effective than dodecanol in lowering the optimum detergency temperature. Of interest here is that the detergency results differed from the behavior observed in the verticalcontacting studies in which, as discussed above, somewhat higher temperatures were required for fastest oil solubilization due to partitioning of the additive into the oil. Practical detergency applications utilize commercial alcohol ethoxylate surfactants which contain a broad range of species having varying hydrophobe lengths and levels of ethylene oxide, Detergency studies were performed in the manner described above for the single ethoxylate and ethoxylate-additive systems using commercial ethoxylates based on a blend of predominantly normal C 12 -C 13 alcohols and containing an average of 3, 4 and 5 mol of ethylene oxide, respectively [85]. These materials are denoted N23-3, N23-4 and N23-5. Table 2 compares the ODTs for these systems to those of the corresponding specific alcohol ethoxylate systems having the same average structure. The ODTs of the commercial materials and their molar-average equivalents are the same in all three cases. The ODTs of the commercial systems, in fact, match the PITs for those systems with hexadecane extrapolated to very low levels of oil. The PIT data are shown in Fig. 40. Shown for comparison are the PIT vs. soil-surfactant ratio plots for C 12 E 4 and C 12 E 5. These are flat, indicative of the fact that the PITs for ternary specific alcohol ethoxylatewater-hydrocarbon systems are independent of the oil-surfactant ratio. Bercovici and Krussman Table 2 Correlation of PIT to optimum detergency temperature ODT Fig. 39. Removal of n-hexadecane from 65/35 polyester-cotton fabric using 0.05 wt.% aqueous solutions of a 60/40 blend of C 12 E 5 and C 12 E 3 [30]. Reprinted with permission of Academic Press. Surfactant PIT (ºC) ODT (ºC) Specific ethoxylate C 12 E C 12 E C 12 E 3 < 20 < 20 Broad-range Ethoxylate N a 50 N a 30 N23-3 < 20 a < 20 The data shown are for cetane removal from 65/35 polyester-cotton with 0.05% surfactant [85]. a Evaluated at cetane-surfactant ratio 0.