Mode of action of emulsifiers. Manufacture of emulsions

Transcription

1 Mode of action of emulsifiers Manufacture of emulsions

2 1 Introduction to emulsions Definitions Emulsion types Emulsifiers Structure and mode of action of emulsifiers Bancroft rule HLB value Kinetics Emulsifier/coemulsifier principle Properties of emulsions Stability Creaming and sedimentation Aggregation and flocculation stwald ripening Coalescence Droplet size distribution Rheology Electrical conductivity 13 2 Manufacture of emulsions Microemulsions Spontaneously emulsifying systems Self-emulsifying systems Mechanical emulsification Emulsification using other phase boundaries Phase inversion processes 17 3 Appendix Marker method Continuous emulsification in orifice systems High-throughput screening: automated testing and optimizing system Stabilization of oil/water emulsions with alcohol ethoxylates Poly dimethyl siloxane emulsions in water made with nonionic surfactants from BASF Amino modified silicon microemulsions made with nonionic surfactants from BASF, e.g. for textile softening Microemulsions 36

3 1 Introduction to emulsions 1.1 Definition An emulsion is a dispersion of two incompletely miscible liquids in one another. 1.2 Emulsion types Simple emulsions consist of a hydrophilic (aqueous) and a lipophilic (oily) phase. In the simplest case the two phases are water and oil. The internal or dispersed phase is dispersed in the external, continuous phase in the form of fine droplets. Depending on the nature of the droplet-forming phase, a distinction is made between oil-in-water (/W) and (W/) emulsions. In addition to simple emulsions, there are also multiple emulsions. An example of this type is the W//W double emulsion, which has an external phase of water and an internal phase consisting of a water-in-oil emulsion. 1.3 Emulsifiers Emulsions* are thermodynamically unstable. The droplets in the dispersed phase tend to coalesce into larger droplets, thus reducing the interfacial area between the two phases and leading to a thermodynamically more favorable, i.e. lower, energy state. The interfacial energy of an emulsion is given by: U A U = A interfacial energy interfacial tension between the two phases interfacial area This means that if the droplet size decreases and the total volume of the dispersed phase remains the same the interfacial energy of the emulsion will increase because the total interfacial area increases. But higher energy states generally have lower thermodynamic stability, so that the driving force for coalescence also increases (see also section 1.4.1) Structure and mode of action of emulsifiers Emulsifiers are surface-active substances whose molecules consist of a hydrophilic and a lipophilic part. Because of their amphiphilic properties, free emulsifier molecules accumulate at the interface between internal and external phases. A competing process also occurs in which emulsifier molecules aggregate into micelles. Above a certain concentration, known as the critical micellar concentration (CMC), the proportion of monomer emulsifier molecules remains constant. In practice, the best results are obtained when the emulsifier is applied in concentrations well above the CMC. The stabilizing properties of emulsifiers are based on various mechanisms, depending on the type of emulsifier: a) Electrostatic repulsion b) Steric repulsion I oil-in-water (/W) water-in-oil (W/) water-in-oilin-water (W//W) Electrostatic repulsion Different types of emulsions Emulsifiers are used to reduce the tendency to coalescence and stabilize the droplets. The interfacial tension of an /W interface is approx. 25 mn/m without emulsifier. Adding emulsifier lowers the interfacial tension to values typically around 3 5 mn/m. * To distinguish them from the thermodynamically stable microemulsions (cf. 2.1), emulsions are sometimes also called macroemulsions. By choosing suitable emulsifier systems, even lower interfacial tensions of below 1 mn/m are possible. Steric repulsion 3

4 In addition to repulsive forces, there are also attractive forces the London-van-der-Waals forces. Potential energy According to Derjaguin, Landau, Verwey and verbeek (DLV theory): U total = U el + U st U vdw Steric repulsion Electrostatic repulsion London-van der-waals attraction Sum of attractive and repulsive forces U total U el U st U vdw total potential energy electrostatic repulsion steric repulsion London-van-der-Waals forces Interdroplet distance The resulting potential energy is the sum of the electrostatic repulsion, steric repulsion and attraction by London-van-der-Waals forces. The figure below shows that below a certain distance, after the repulsive forces have been overcome, only attractive forces operate. At this point the droplets coalesce. Dependence of attractive and repulsive forces on interdroplet distance Bancroft rule Whether an emulsifier is better able to stabilize an /W or a W/ emulsion depends on which is larger, the hydrophilic or the lipophilic portion of the molecule. H H H H H H H H H H H H H H H H Emulsifiers with a larger hydrophilic portion are good /W emulsifiers, whereas those with a larger lipophilic portion are better able to stabilize W/ emulsions. Too large a hydrophilic or lipophilic part, on the other hand, leads to both low interfacial affinity of the emulsifier and poor packing at the interface. There is therefore an optimum size ratio between the hydrophobic and hydrophilic parts. 4 H H H H H H H H Lipophilic portion larger Hydrophilic and lipophilic Hydrophilic portion larger portions equal in size

5 1.3.3 HLB value In many cases, the HLB value (hydrophilic lipophilic balance) gives an indication of the type of emulsion for which emulsifiers are suitable. Strictly speaking, the Griffin formula applies only to nonionic ethoxylates, which are classified on a scale of 0 to 20. Emulsifiers with low HLB values tend to be lipophilic molecules, dissolving mainly in the oil phase of an emulsion and better able to stabilize W/ emulsions at room temperature. Emulsifiers with medium or large HLB values are hydrophilic. They are more soluble in water and are used preferentially for stabilizing /W emulsions at room temperature. For other emulsifiers, especially ionic ones, a method was developed by Davis in which the sum of experimentally determined increments is calculated. In practice, the procedure is as follows: 1) Choose an emulsion type (/W or W/) 2) Find the HLB value of the oil from tables 1 3) Select the HLB value of the emulsifier or emulsifier mixture to be the same as the HLB value of the oil 4) Corrections may be necessary if the temperature differs significantly from 25 C or there are salts in the aqueous phase. In the case of ethoxylates, for example, the HLB is chosen to be higher for each increase of 10 K in temperature or 5 wt% NaCl. According to Griffin s formula, the HLB value is calculated from the mass of the lipophilic portion as a fraction of the total mass of the molecule: M lipophilic M total lipophilic HLB = 20 (1 M lipophilic /M total ) mass of lipophilic portion total mass of molecule hydrophilic W/ emulsifiers HLB scale according to Griffin Mechanical energy /W emulsifiers Coalescence Kinetics In addition to thermodynamics, the kinetics of emulsions is very significant, especially in their manufacture. The critical step is the fragmentation of the internal phase. Large droplets are divided into smaller ones by introducing energy, e.g. by shearing. The newly created surface must be occupied by emulsifier molecules as rapidly as possible to protect the droplets and prevent them from coalescing. The rate at which an emulsifier molecule occupies or vacates a newly created interface depends on a number of factors: a) The rate at which emulsifier molecules are transferred from the continuous or the dispersed phase to the vicinity of the interface b) Penetration of the interface by emulsifier molecules c) The orientation of emulsifier molecules at the interface d) Distribution of emulsifier molecules over the interface (Marangoni flow) e) Removal of emulsifier molecules from the interface by thermal agitation f) Removal of emulsifier molecules from the interface by currents and eddies Under turbulent flow conditions, transfer of emulsifier through the continuous phase is faster than droplet breakup, irrespective of the emulsifier s diffusion coefficient. Similarly, Marangoni flow at the interface is usually more rapid than the creation of new interfaces. These two effects are therefore seldom rate-determining. Continuous phase + emulsifier + phase to be dispersed Deformation and breakup Slow emulsifier Fast emulsifier Stable droplets 5 Effect of adsorption rate on stabilization 1 e.g. Ullmann s Encyclopedia of Industrial Chemistry, 6 th edition, Wiley-VCH, Weinheim, 2000.

6 This is not true of transfer through the dispersed phase. In the case of /W emulsions, hydrophobic, readily oil-soluble emulsifiers have the advantage here. They become concentrated in the oil droplets, where they diffuse rapidly to the phase interface. Little is known about the kinetics of transfer of emulsifier molecules to the interface from micelles in the two phases. Removal of small emulsifier molecules from the interface is dominated by thermal motion. In the case of polymers with molecular weights above approx. 100,000 daltons, eddies and currents play an increasingly important role. It has been found empirically that small emulsifier molecules stabilize newly generated interfaces more rapidly than large ones Emulsifier/ coemulsifier principle As described in the previous section, small emulsifier molecules often have an advantage when it comes to rapid stabilization of the internal phase during droplet fragmentation. However, small emulsifier molecules also have a disadvantage. Since they are generally less tightly adsorbed on to the interface than larger emulsifier molecules, especially polymers, they are more readily removed from the interface again by Brownian motion. Furthermore, their smaller molar mass often means they have smaller repulsive groups and therefore, according to the DLV theory, do not stabilize droplets so well against coalescence. Mechanical energy Dispersed phase Slow emulsifier Coalescence occurs before interface is occupied Good long-term stability Fast emulsifier Coalescence can occur because emulsifier molecules leave interface again Poor long-term stability Time scale 6 Different long-term stabilities of rapid and slow emulsifiers

7 It has proved advantageous to combine small and large emulsifier molecules. The rapid adsorption of the small molecules means they immediately occupy a newly created surface and then gradually make room for the slower but more tightly adsorbing large emulsifier molecules. In such combinations of two or more emulsifiers, the emulsifier(s) present in smaller quantities is (are) known as the coemulsifier(s). Intensity dn N d log M Coemulsifier Emulsifier log M Molar masses (M) of emulsifier and coemulsifier H H H H A further advantage of emulsifier/ coemulsifier mixtures is that higher packing densities can be achieved at the interface. Higher packing densities have the effect of increasing the rigidity and thickness of the emulsifier film. H H H H H H H H H H Without coemulsifier Higher packing density with surfactant alcohol as coemulsifier It has been found in practice that emulsifiers consisting of a mixture of smaller and larger molecules as a result of the synthesis process, for example ethoxylates with a broad E distribution, do not need coemulsifiers to be able to stabilize emulsions well. GPC of an ethoxylate with a broad molar mass distribution 7

8 1.4 Properties of emulsions Stability Emulsions have a much larger interfacial area between the two liquids than the corresponding unemulsified mixtures. Most emulsions are therefore thermodynamically unstable even in the presence of emulsifiers. nly in emulsions where the interfacial tension is extremely small can the thermal energy exceed the interfacial energy. Such emulsions are known as (thermodynamically stable) microemulsions (see 2.1) Creaming and sedimentation In addition to thermodynamic instability, there are other emulsion aging mechanisms of significance to the user, for example creaming and sedimentation. Creaming is reduced in emulsions containing small droplets, small density differences and a highviscosity continuous phase. It can be quantified by optical techniques or ultrasonic scattering. Very small droplets (< 100 nm) are in favorable cases also stabilized thermodynamically against creaming by Brownian motion. The thermodynamic stability of the emulsion can be derived from the Gibbs equation: G A S T G = G emulsified G unemulsified = A T S free energy interfacial tension between the two phases interfacial area entropy temperature Normally, G > 0, i.e. the emulsion is thermodynamically unstable. However, if the interfacial tension is very small, the greater disorder (entropy) of the emulsified state leads to a thermodynamically stable microemulsion ( G < 0). Stokes law gives the creaming rate for a droplet dispersed in a very dilute emulsion: d g v = gd2 18 droplet diameter acceleration of gravity difference in density between dispersed and continuous phases viscosity of continuous phase Analogous but more complex equations are used to describe creaming for dispersed droplets with finite surface viscosity, for distributions of droplet diameters and for more concentrated emulsions. But the parameters derived from the simple Stokes law apply here, too. 8 Density differences in emulsions lead to creaming (see figure) or sedimentation of droplets

9 The degree of thermodynamic stabilization is obtained from the Boltzmann distribution law: d 3 g h 6kT Height of liquid column h h k T d g height of liquid column Boltzmann constant temperature droplet diameter difference in density between dispersed and continuous phases acceleration of gravity Emulsion with droplets > 100 nm Emulsion with droplets <<100nm Droplet density N/V Emulsions with very fine droplets and small density differences can be entropy-stabilized against creaming Aggregation and flocculation Flocculation (or aggregation) refers to the formation of clusters of two or more emulsion droplets that behave like distinct particles but in which the identity of the individual droplets is retained. Such clusters can even be in dynamic equilibrium with single droplets, individual droplets leaving the cluster while new ones join it. Flocculation is therefore a process that is easily reversible, in contrast to coalescence, though flocculation often results in coalescence. Thermodynamically, flocculation is based on a secondary energy minimum, as described for example by the DLV theory (see and ). The kinetics of irreversible flocculation in monodisperse, unstirred emulsions can be described by the Smoluchowski equation: t() V () () N N 2 = W 4 d DTr = kt N 2 V 2 V W probability of a successful collision d droplet diameter D Tr diffusion constant of droplet viscosity of continuous phase N/V droplet density kt measure of thermal energy It should be noted that to a first approximation flocculation kinetics is independent of droplet size. 9 Attractive forces between droplets lead to aggregation or flocculation

10 stwald ripening Another phenomenon of emulsion aging is the growth of large droplets at the expense of smaller ones: stwald ripening. It arises from the fact that small droplets dissolve more readily in the continuous phase than large ones. The phenomenon was already described by Lord Kelvin in Kelvin s equation describes the effect of droplet size on the solubility of the dispersed phase: c(d) saturation concentration of the dispersed phase in the continuous phase for a droplet of diameter d interfacial tension RT measure of thermal energy molar volume of dispersed phase V Mol RTln c(d) = 4 V Mol + RTln c(d = ) d stwald ripening causes large droplets to grow at the expense of small ones In very dilute emulsions, the rate at which large droplets grow at the expense of small ones is given by the Lifshitz-Slezov-Wagner (LSW) equation. ( d) 3 = 64 D c (d = ) V Mol t 9 RT D=diffusion coefficient of the dispersed in the continuous phase 10 The third power of the mean diameter d increases in proportion to the interfacial tension and the solubility c of the dispersed in the continuous phase. This growth is accelerated by the Brownian motion of the droplets, the dispersed phase fraction and the presence of micelles in the continuous phase.

11 The main way of reducing stwald ripening is to ensure that the solubility of the dispersed in the continuous phase is as low as possible. To stabilize an emulsion in which the dispersed phase is too soluble, an inert, very poorly soluble auxiliary can be added in the case of water, for example, an extremely hydrophobic substance such as a hydrocarbon. Adding sufficient auxiliary allows a stable equilibrium to be established between the droplets. The emulsion can then be technically considered an emulsion of the auxiliary, the droplets of which contain the original internal phase in a dissolved state. The Higuchi-Misra relation describes the effect of using an auxiliary: Adding an auxiliary that is very poorly soluble in the continuous phase stabilizes an emulsion against stwald ripening if for every droplet i the effects of its diameter d i and the activity a i of the more soluble component in the auxiliary counterbalance one another until the above equation is fulfilled. V Mol RT d i, a i 4 V Mol + RTln a i = const d i interfacial tension molar volume of dispersed phase measure of thermal energy diameter of droplet i and associated activity of dispersed phase in auxiliary Koaleszenz Coalescence is the merging of two or more droplets into a single large droplet, driven by the reduction in interfacial area and hence interfacial energy. Below a distance of approx. 100 nm, two oil droplets in an aqueous solution experience a perceptible attractive force that increases as the droplets approach one another. It is suspected that even at these distances the water between the droplets adopts structures of lower entropy. The addition of a stabilizer such as an emulsifier super - imposes on this attractive force a repulsive force that derives from the spatial requirement or charge of the hydrophilic groups on the emulsifier. The balance of these forces is described by the theory of Derjaguin, Landau, Verwey and verbeek (DLV, see 1.3.1). An energy barrier keeps the aggre- Adding a very poorly soluble auxiliary can stop stwald ripening gated droplets at a distance and the height of this barrier is critical for the stability of the emulsion. Stabil Apart from stabilizers, emulsions can also be stabilized by restricting the mobility of the droplets by a continuous phase of very high viscosity. Such a high viscosity can, for example, be obtained by adding thickeners. Emulsions can also be stabilized hydrodynamically. In this case, strong turbulence reduces the contact time between two colliding droplets to the point where drainage of the liquid film of continuous phase separating the droplets is impeded and coalescence prevented. Koaleszenz 11 The merging of separate droplets into a single larger droplet is called coalescence

12 1.4.2 Particle size distribution Typical emulsions have droplet diameters ranging from 0.5 to 50 µm. The size and size distribution of the particles to a large extent determine the properties of emulsions, for example their stability and appearance. Thus emulsions tend to appear white or milky because of light scattering, unless the refractive indices of the dispersed and continuous phases happen to be thesame. Emulsions whose particles are much smaller than the wavelength of visible light appear opaque to clear. As a rule, emulsion droplets are present not in monodisperse form but as a relatively broad size distribution. nly by applying special techniques, such as membrane processes (see below) or micromixers, can a virtually monodisperse distribution be obtained. Various techniques are available for measuring the size distribution, for example laser diffraction methods Volume density % distribution q 3 (x) (µm -1 ) Cumulative volume distribution Q 3 (x) (%) Particle size (µm) Volume density % distribution q 3 (x) (µm -1 ) Cumulative volume distribution Q 3 (x) (%) Particle size (µm) Typical examples of broad and narrow particle size distributions (measured by laser diffraction)

13 1.4.3 Rheology The rheological behavior of emulsions is determined mainly by the fraction of the dispersed phase. Dilute emulsions are characterized by Newtonian flow. The viscosity of an emulsion depends on the viscosity of the continuous phase. This relationship is described by Einstein s equation: emulsion = cont. phase ( ) viscosity phase fraction of dispersed phase Electrical conductivity The specific conductance of emulsions is determined by the conductivity of the continuous phase (or phases in the case of bicontinuous emulsions). It is therefore a simple method for distinguishing W/ from /W emulsions. Moreover, the transition from /W to W/, or to a bicontinuous emulsion, is generally accompanied by an abrupt change in specific conductance, which is therefore an indicator of the formation of new phases or of emulsion inversion. The equation takes into account the contribution of momentum transfer due to the droplets, which are regarded as rigid spheres. The equation does not take into account further increases in viscosity due to deformed, deformable or aggregated droplets, droplet charges, polymers and thickener additions, or surfactant layers at the interface. In concentrated emulsions the viscosity increases more rapidly as a function of the phase fraction of the dispersed phase than described by Einstein, and site exchange similar to that encountered in diffusion phenomena in solids becomes the dominant factor. As the fraction of the internal phase increases, the emulsion starts to show non-newtonian flow behavior and the droplets are packed more and more closely together. At even higher internalphase concentrations, the droplets are deformed into polyhedra and rheologically the emulsion behaves like a foam. It exhibits a yield point and pronounced shear thinning caused by deformation and relaxation of the droplets. Such emulsions are known as gel or high-internalphase emulsions. 60 Specific conductance (µs/cm) 50 /W Bicontinuous W/ Phase fraction of oil (%) Discontinuities in specific conductance indicate fundamental changes in emulsion type. /W emulsions generally have good conductivity, bicontinuous emulsions are much poorer conductors, and W/ emulsions are practically nonconducting. 13

14 2 Manufacture of emulsions 2.1 Microemulsions Microemulsions are a special case because they are thermo -dynamically stable. By cleverly combining the incompletely miscible phase components and the emulsifier system or using very high surfactant concentrations, extremely low interfacial tensions are obtained, so that the emulsion is entropy-stabilized. Microemulsions are thermodynamically stable. The interfacial tension becomes so small approx. 1 to 100 nn/m that the increase in entropy on emulsification exceeds the surface energy and the emulsion forms spontaneously. G = G emulsified G unemulsified = A T S < 0 In addition, the emulsifiers must not be very soluble in either of the two liquid phases at the application temperature, so that they remain almost entirely at the interface and an emulsion forms spontaneously, creating new interface in proportion to the amount of emulsifier present. In practice, the three-phase region (Winsor III region) in the phase diagram is sought, in which water, oil and microemulsion coexist. The amount of emulsifier, via its inter - facial area requirement, determines the volume fraction of the micro - emulsion. 2.2 Spontaneously emulsifying systems Since the total energy G determined by the Gibbs equation is < 0, the microemulsion is thermodynamically stable. 0.1 µm Water il Even systems that do not ultimately form stable microemulsions can emulsify spontaneously. The necessary interfacial energy is supplied by the entropy of mixing. For example, a mixture of oil, surfactant and ethanol emulsifies spontaneously in water because the ethanol diffuses into the water. Another mechanism involves intermediate microemulsions that form as a result of high surfactant concentrations at the interface and break down in the continuous phase. Turbulence at the interface can in both cases lead to the formation of a (metastable) emulsion instead of two macroscopically separate water and oil phases. Electron micrograph of a frozen microemulsion Source: Science, Vol. 240, Microemulsions, Manfred Kahlweit. Copyright 1988, AAAS Emulsifiable concentrate 2 phases Mole fraction of water increases Temperature 3 phases (microemulsion, oil, (water) 2 phases 1 phase (Microemulsion) Entropy of mixing leads to formation of interface 14 (wt. % surfactant) Typical phase diagram of a microemulsion When emulsifiable concentrates are added to water, chaotic turbulence at the interface leads to the formation of an emulsion instead of two macroscopic phases.

15 2.3 Self-emulsifying systems In the case of microemulsions and spontaneously emulsifying systems, emulsions are formed without energy being supplied from the outside, but with self-emulsifying systems this is not strictly true. However, only a small power input is required to turn self-emulsifying systems into emulsions, for example from a slowly turning stirrer or simply by shaking. Important for this process are small dynamic interfacial tensions, so that even a small power input will result in a sufficiently high critical Weber number to produce small droplets. Generally speaking, oil-soluble surfactants with a low HLB value, or mixtures containing such surfactants, tend to be used for spontaneously or self-emulsifying systems. 2.4 Mechanical emulsification In mechanical emulsification processes involving high power densities, very low interfacial tension is helpful but not essential. High-turbulence zones, laminar shear flows and cavitation induce emulsion formation even without an emulsifier. Much more important in such processes is good stabilization of the resulting emulsion, since the outlet of dispersion machines is often characterized by turbulent flow conditions and a high droplet collision rate. The Weber number is defined as the quotient of the external and internal forces acting on a droplet. It describes the emulsion process in terms of laminar forces. We = v/ l c d 2 v l laminar shearing field c viscosity of continuous phase d droplet diameter interfacial tension between phases The external forces are transmitted by the viscosity of the continuous phase, while the internal restoring forces are caused by the interfacial tension. The critical Weber number We cr is obtained by substituting into the above equation the diameter of the smallest droplet that can just still be (or can no longer be) broken up. Critical Weber numbers can also be calculated from the type of laminar flow (extensional or shear flow) and from the viscosities of the continuous and dispersed phases. The diameter of the smallest attainable droplet is then derived from the above equation. The Reynolds number relates inertial and viscous forces to one another: v c l c Re = v l c c flow rate viscosity of continuous phase characteristic dimension (see below) density of continuous phase If the dimensions of the stirrer or other emulsifying apparatus are sub - stituted for l, then there is laminar flow up to Re 10 3 and turbulent flow above Re Between these values is a tran -sition range. Substituting the droplet diameter for l allows the Reynolds number in the vicinity of a droplet to be calculated. Below Re Tr 1 droplet breakup (Re Tr ) is caused mainly by laminar forces; above Re Tr 1 mainly by inertial forces. 15

16 A large number of emulsifying machine types are available on the market. Typical are rotorstator systems such as toothed disc dispersing machines, reaction mixing pumps and colloid mills. ther machines consist of pumps combined with static mixers, nozzles or orifices, for example the very efficient high-pressure homogenizers. thers again use ultrasonic sonotrodes. Significant criteria for selecting an emulsifier for such a mechanical emulsification process are that it rapidly occupies the newly formed interface, is not carried away again by eddies, and protects droplets from coalescence in the turbulent discharge zone of the emulsifying apparatus. Low-molecular fast emulsifiers are generally superior in this regard to polymeric emulsifiers (see 1.3.4). Fine emulsion High-pressure homogenizer Crude emulsion Rotor-Stator-System High-pressure homogenizers and rotor-stator systems are commonly used to manufacture emulsions Droplet formation at membrane pores Mechanical energy Continuous phase + emulsifier Membrane Phase to be dispersed Fast emulsifier Very slow emulsifier Mechanical energy Membranes are very energy-efficient emulsifying apparatuses. However, they are mostly too expensive for wide-scale industrial application. 2.5 Emulsification using other phase boundaries Another method of producing emulsions is to use other phase boundaries, for example the solid-liquid interfaces of membranes and micro - mixers. Here the large interface of what is to be the dispersed phase is generated by appropriate differential pressure across a membrane that is not wettable by emulsifier. The process is very energy-efficient. Nature makes use of it, for example, in producing milk. A gas-liquid interface, such as in aerosol and condensation processes, can also be used to produce emulsions. 16

17 2.6 Phase inversion processes Temperature A simple way to obtain very fine emulsions with very little energy expenditure is the phase inversion method. It is particularly simple to apply with ethylene oxide based surfactants. Three basic cases are distinguished: PIT /W Case 1 W/ Case 3 1) In the first, the surfactant is mixed with, for example, oil and water and heated to above, or just below, the phase inversion temperature (PIT). The mixture is then emulsified and the emulsion is stabilized by cooling. This approach takes advantage of the low interfacial tension at the PIT, and only a small amount of energy is required to produce a fine emulsion. Emulsification above the PIT and subsequent cooling is the more efficient method, but care must be taken to ensure that the emulsion does not break during inversion. The risk of this occurring is smaller if the mixture is heated to just below the PIT*, which assumes, however, that the PIT is known exactly. The phase inversion method produces fine emulsions with little energy expenditure. It is illustrated here for ethylene oxide based emulsifiers. Case 1 involves the inversion of a W/ emulsion into an /W emulsion by cooling. Case 2 is isothermic phase inversion, and Case 3 is a combination of the other two cases. 2) In the second case, the surfactant is mixed with, for example, oil and very little water and emulsified. Then, without altering the temperature, water is added to induce phase inversion. (Such inversion of an emulsion containing a very large fraction of the future internal phase shows hysteretic behavior when the emulsion is rediluted with oil and can be described by catastrophe theory; thus it is called catastrophic phase inversion. Inversion of small regions of the emulsion ultimately leads to inversion of the emulsion as a whole.) Fine emulsions can be produced very simply by this method, but it requires an emulsifier that tends to be soluble in oil rather than water. Such emulsifiers usually have small hydrophilic groups and are therefore, according to the DLV theory, poorer stabilizers. Case 2 il fraction of the emulsion 3) The third case is a combination of the first two. A water-in-oil emulsion or microemulsion, for example, is produced at high temperature and high surfactant concentration and then inverted by adding cold water. This is a simple way to produce very fine, stable emulsions. These phase-inversion techniques can also be used with ionic surfactants, but it should be remembered that, because of the expan sion of their Debye ion clouds, ionic surfactants become more hydrophobic at low temperatures and more hydrophilic at high temperatures the exact opposite of ethylene oxide based surfactants. 17 * It is usual to emulsify approx 10 15K below the PIT.

18 3 Appendix 3.1 Marker method A technique used at BASF to determine the stabilizing properties of emulsifiers is the marker method*, based on the Danner dyeing method**. In this technique, the dispersed phase is marked with marker substances in such a way that after emulsion coalesced and non-coalesced droplets can be distinguished and determined quantitatively. Here we describe how the method is used to analyze /W emulsions with oil-soluble dyes as marker substances. First, two coarsely dispersed raw emulsions are prepared with identical formulations but a different-colored oil phase in each (one blue and one yellow). The two emulsions are mixed by slowly stirring them together. green) are formed by coalescence, i.e. the droplets combine because they are not sufficiently stabilized by the emulsifier. The higher the proportion of droplets of mixed color, the more poorly the emulsifier has stabilized them against coalescence. If only a few green droplets or none at all are found in the sample, it can be concluded that the emulsion is well stabilized by the emulsifier. The emulsion is evaluated quantitatively by digital image analysis, which allows the area fraction of each droplet color to be determined. Quantitative determination of the stabilizing properties under actual emulsification conditions is therefore possible. The emulsion mixture is then subjected to a coalescence experiment. This may, for example, be storage, temperature change, shearing, centrifuging or addition of chemicals. The emulsion is subsequently examined under a microscope and the proportion of droplets of mixed color determined. Droplets containing the mixed color (in this case * DE ** Dr. Thomas Danner, Tropfenkoaleszenz in Emulsionen, dissertation at the University of Karlsruhe, GCA-Verlag, Herdecke 2001 Crude emulsion A Fine emulsification Crude emulsion B Principle of the marker method 1:1 mixture Micrograph showing coalesced droplts 18

19 Advantages of the marker method: Coalescence can be isolated from other effects such as stwald ripening and flocculation. Coalescence can be accurately measured at an early stage, reducing development time. Coalescence can be measured under shearing/stirring conditions. riginal image Droplet separation Color evaluation Source: GCT, Dr. Thomas Danner Evaluation of the micrographs Left image sequence: no coalescence, two signals, one for each of the yellow and blue color areas Right image sequence: 100% coalescence, only one signal is obtained for the green color area 19

20 3.2 Continuous emulsification in orifice systems An emulsification technique whose results can be readily evaluated by the marker method (see separate appendix) is emulsification in orifice systems, as used for example in high-pressure homogenizers. In a laboratory test, an autoclave is filled with two differently colored crude emulsions with a particle size of µm. While being slowly stirred, the crude emulsion with different colored droplets is forced through an orifice by gas pressure. Before entering the orifice, the crude emulsion passes through a region of laminar extensional flow, which deforms the emulsion droplets. Within the orifice is a small zone where laminar shear flow predominates. Here the droplets are further deformed and some of them break up. Most of the droplets, however, break up in the turbulent outflow of the orifice, where, depending on the power input, cavitation may occur and produce shock waves. During the emulsification process, the droplets collide with one another. Where droplets are created by division of larger droplets, the emulsifier must rapidly occupy the newly formed interface or they will coalesce. Details of these processes are discussed in numerous monographs on emulsification. Laminar extensional flow rifice Turbulence Autoclave with orifice in outlet High pressure, low flow rate Emulsification zones in flow through an orifice Low pressure, high flow rate Laminar Extensional flow Shearing, turbulence Cavitation To obtain a measure of the perfor - mance of the emulsifier in terms of coalescence, the coalescence probability of the droplets is determined from the measured proportion of green droplets. The lower the coalescence probability, the better the emulsifier stabilizes the emulsion against coalescence in turbulent regions and other shearing zones. Extension of droplets and occupation of newly formed surface by emulsifier 20

21 We recommend the emulsifiers listed below for preparing oil-in-water emulsions in laminar flow under high shear with a high power input (> W/m 3 ). These conditions are typically present in screen mills, rotor-stator colloid mills and highpressure homogenisers. The droplets that are formed in this type of apparatus are usually small enough, but the emulsifier molecules have to be able to migrate quickly to the newly formed interfaces and they have to be capable of preventing the oil droplets from coalescing in the zones of turbulent flow in the outlet ports of the machinery. f course, the size of the droplets also depends on the type of emulsification process and the on the power that is introduced into the system. Depending on the type of process that is involved, much smaller or larger droplets can be obtained if required, but the ranking of the emulsifiers in the diagrams still applies. Because of the similarity of these processes, these recommendations also apply without restriction to membrane processes, and they also apply to a certain extent to ultrasonic processes and turbulent emulsification processes with a high power input, such as in high-speed stirrers, toothed-ring dissolvers or in large rotor-stator-type homo - genisers. Good emulsifier Stable emulsion No coalescence Poor emulsifier Unstable emulsion Coalescence Difference between emulsifiers with good and poor stabilizing properties Emulsifiers recommended for process with a high power input Small droplets with a low tendency to coalesce can be prepared with the following emulsifiers: Paraffin oil Emulan P, AT 9, NP 3070 Naphthenic mineral oil Aromatic mineral oil Triglycerides Silicone oil Emulan T range, NP 3070, EL Emulan T range, U, C, G Emulan NP 3070 Emulphor FAS 30 Emulan EL Emulan T range, EL Emulan NP

22 3.3 High-throughput screening: automated testing and optimizing system An emulsifier selection process that is rational and above all reproducible is the high-throughput screening method (HTS). This fully automated process, consisting essentially of a robot with metering, emulsifying and analysis units, makes it possible to vary a large number of different parameters (concentration, temperature, emulsifier type, composition of formulation, etc.) in very extensive screening tests. The emulsification step can, for example, be carried out using an ultrasonic probe or a rotor-stator system (both laminar and turbulent flow). The stabilizing effect of emulsifiers can be evaluated by transmittance measurements or by automatic measurement of the particle size distribution using laser diffraction. ther significant variables, such as the viscosity and creaming properties of the emulsion obtained, can also be determined fully automatically. Pipetting station for emulsifiers HTS apparatus with robot 22

23 3.4 Stabilization of oil/water emulsions with alcohol ethoxylates Introduction The HLB concept gives some indication of which hydrophile of the emulsifier is suitable for an existing emulsification task. n the other hand, which emulsifier class is best suited in the case of optimization of the HLB value cannot be given by the concept, since it oversimplifies the relevant physical-chemical effects. Ultimately, this is only possible empirically. Simple, industrial-grade alcohol ethoxylates are especially suited for a systematic study of the stability of oil-water emulsions. They are simple to produce from a great variety of H active compounds. Their HLB value is easily variable for any hydrophobic part and they react only slightly to impurities in both the oil and aqueous phases, such as ions, for example. Why can one surfactant class stabilize while another cannot? H H?? ther alcohol ethoxylates C 10 Guerbet alcohol ethoxylate (Lutensol XP) C 10 Guerbet alcohol ethoxylate containing small quantities of higher alkylene oxides (Lutensol XL) Nonylphenol ethoxylate (Lutensol AP) Ethoxylates of linear alcohols n-decanol ethoxylate C 12 C 14 coconut fatty alcohol ethoxylate (Lutensol A..N) C 13 C 15 xo alcohol ethoxylate (Lutensol A) C 16 C 18 Tallow fatty alcohol ethoxylate (Lutensol AT) Emulsion preparation and methodology The stability against coalescence at room temperature of /W emulsions was measured with two model oils: Thin fluid paraffin oil with a visco - sity of 30 mpa s (23 C) and an Mn of about 300 as a model of hydrophobic hydrocarbons Macroemulsions were prepared with a low power input. Quantity ratios: 30% oil, 1% surfactant, 69% water Emulsification procedure: Propeller agitator, P/V=10 4 W/m 3, 23 C, 15 minutes Ethoxylates of oxo alcohols from the oligomerization of higher olefins iso C 10 xo alcohol ethoxylate (Lutensol N) iso C 13 xo alcohol ethoxylate, medium branched (Lutensol T) Sunflower seed oil with a viscosity of 56 mpa s (23 C) and an acid number of maximum 0.15 mg KH/g as a model of native triglycerides Demineralized water was used as the aqueous phase Then the emulsions were stored at 23 C, the stability index S determined by the marker method and plotted for each ethoxylate class as a function of the HLB value. iso C 13 xo alcohol ethoxylate, highly branched (Lutensol TDA) iso C 17 xo alcohol ethoxylate, highly branched Eleven classes of alcohol ethoxylates served as emulsifiers. For each class, a set of ethoxylates was prepared in steps of 1 2 HLB units and measured. 23

24 Visualized process for calculation of the stability index S From time to time the average green content is determined with 30 different microscopic images From repetitions it follows that (S) A 2 bar is found in each diagram. t in month A hyperbola was adapted to the measured data by using the least squares method S = - log (r x month) is repre-sented as a function of the HLB 200 % / (100 % green %) S = - log (r * month) r = /month HLB value (= 20* wt.% E) Values of S = 1.5 correspond to roughly 10% coalesced droplets within a half-year, values of S = -1.5 to about 10% coalesced droplets in five hours. Emulsions of paraffin oil in water ethoxylates of linear alcohols In the case of emulsions of paraffin oil in water stabilized by ethoxylates of linear alcohols, one recognizes, on the one hand, that the suitable HLB range depends on the chain length of the alcohol: C 10 Alcohol ethoxylates: HLB = approx. 13 C 12 C 14 to C 16 C 18 alcohol ethoxylates: HLB= approx n the other hand, the resistance to coalescence increase in the order less suitable well suited Stability index C 20 C 22 x 7.3 E Lutensol A..N, A n-decanol ethoxylate 10 % of the drops have coalesced after Lutensol AT HLB value Months Weeks Days Hours Minutes C 10 C 12 C 14 = C 13 C 15 C 16 C 18 In each case by about one order of magnitude. A comparison with C 20 C 22 x 7.3 E (HLB = 10) shows that an additional increase in resistance to coalescence by chains longer than C 18 is not possible; therefore, among the ethoxylates of linear alcohols C 16 C 18 ethoxylates such as Emulan AT 9 or Lutensol AT 11 are optimal. 24

25 Emulsions of paraffin oil in water ethoxylates of branched alcohols Among the ethoxylates of branched oxo alcohols one finds HLB ranges similar to those among the linear alcohol ethoxylates iso C 10 -Alcohol ethoxylates, Lutensol N: HLB = approx. 14 iso C 13 -Alcohol ethoxylates, medium branched, Lutensol T HLB = approx iso C 13 -Alcohol ethoxylates, highly branched, Lutensol TDA HLB = approx iso C 17 Alcohol ethoxylates, highly branched HLB = approx Stability index iso C 17 Ethoxylate 7 Lutensol TDA Lutensol T Lutensol N 10 % of the drops have coalesced after HLB value Months Weeks Days Hours Minutes ne can also recognize a somewhat better stabilization than with the ethoxylates of linear alcohols. Therefore, Lutensol T, for example, stabilizes just as well as ethoxylates of branched or linear alcohols with 16 to 18 C atoms in the hydrophobic part. Surprisingly, the mediumbranched Lutensol T types offer a broader HLB window than the highly branched Lutensol TDA brands, which is an advantage, since with Lutensol T one has to make fewer compromises with regard to other target values such as foam or formulability. As opposed to the linear alcohol ethoxylates, among which C 16 C 18 ethoxylates clearly stabilize better than C 12 C 14 ethoxylates, in the case of the branched alcohols, the longer chains, such as in the analogous iso C 17 alcohol ethoxylates, do not provide any significant extra advantage. Emulsions of paraffin oil in water direct comparison of C 10 ethoxylates In the direct comparison of the C 10 alcohol ethoxylates linear, Guerbet and iso C 10 oxo alcohol one finds ideal HLB values around 13 14, but above all, the following surprising order in the coalescence stabili zation: less suitable well suited Lutensol N Lutensol XP = linear decanol ethoxylate Lutensol XL Stability index Lutensol XL Lutensol XP Lutensol N 10 % of the drops have coalesced after Decanol ethoxylate HLB value Months Weeks Days Hours Minutes 25

26 Emulsions of paraffin oil in water direct comparison of C 12 -C 15 ethoxylates In the direct comparison of the C 12 to C 15 alcohol ethoxylates one recognizes once more the above mentioned order with slight advantages on the part of Lutensol T due to the larger HLB window: less suitable well suited Lutensol A..N = Lutensol A Lutensol TDA = Lutensol T Stability index Lutensol TDA Lutensol A..N, A 10 % of the drops have coalesced after Lutensol T HLB value Months Weeks Days Hours Minutes 26 Emulsions of paraffin oil in water substitution of nonylphenol ethoxylate In the direct comparison of fatty alcohol ethoxylate vs. nonylphenol ethoxylate, C 16 to C 18 ethoxylates can be excluded because of their totally different behavior in terms of surfactant properties and formulation behavior, and Lutensol TDA because of its poor biodegradability. Therefore one can consider Lutensol T and Lutensol XL to be optimal substitutes for nonyl - phenol ethoxylate for stabilizing paraffin oil emulsions. Summary: Paraffin oil in water In conclusion, we can say that among the ethoxylates tested, the types Lutensol T, Emulan/Lutensol AT and Lutensol XL are the best suited for stabilizing emulsions of paraffin oil in water. Stability index Lutensol T 7 Lutensol XL 10 % of the drops have coalesced after Nonylphenol ethoxylate HLB value Months Weeks Days Hours Minutes

27 Emulsions of sunflower oil in water ethoxylates of linear alcohols In the case of emulsions of sunflower oil in water stabilized by ethoxylates of linear alcohols, one finds a totally different picture than in the case of paraffin oil. Ethoxylates of linear decanols cannot stabilize the emulsion at all, only from C 12 on does one recognize a slight stabilization around HLB = 11. But good stabilization is only achieved by the ethoxylates of the long-chained C 16 C 18 fatty alcohols. C 16 C 18 -Alcohol ethoxylates, Emulan/Lutensol AT: HLB = approx Stability index C 20 C 22 x 7.3 E n-decanol ethoxylate Lutensol AT 10 % of the drops have coalesced after Lutensol A..N, A HLB value Months Weeks Days Hours Minutes A comparison with C 20 C 22 x 7.3 E (HLB = 10) shows that the stabilization of triglyceride in water with ethoxylates of linear alcohols clearly decreases again beyond C 18. This is due to the incompatibility between the waxlike C 20 C 22 alcohol and the triglyceride. Emulsions of sunflower oil in water ethoxylates of branched oxo alcohols Now one finds a very surprising picture with sunflower oil and ethoxylates of branched alcohols. The order C 10 C 13 C 17, anticipated from the ethoxylates of linear alcohols is found only at a significantly higher level, although the medium branched iso C 13 ethoxylates Lutensol T display highly reproducibly a very sharp maximum in the stabilization around HLB = 11, which corresponds approxmately to Lutensol T 6. Here the ethoxylates exceed highly branched iso C 13 ethoxylates by more than an order of magnitude in stabilization, so that the following order now exists: less suitable well suited Lutensol N Lutensol TDA iso C 17 ethoxylate Lutensol T Stability index 2 2 Lutensol T Lutensol TDA 7 Lutensol N 10 % of the drops have coalesced after iso C 17 Ethoxylate HLB value Months Weeks Days Hours Minutes 27

28 Emulsions of sunflower oil in water direct comparison of C10 ethoxylates In the direct comparison of the C 10 ethoxylates, one recognizes an unambiguous order in the stabilizing effect less suitable well suited Linear C 10 ethoxylate Lutensol N Lutensol XL = Lutensol XP where only the surfactants based on C 10 Guerbet alcohol act well against coalescence. Stability index Lutensol XP 7 Lutensol N n-decanol ethoxylate Lutensol XL 10 % of the drops have coalesced after HLB value Months Weeks Days Hours Minutes Emulsions of sunflower oil in water direct comparison of C 12 -C 15 ethoxylates In the direct comparison of the C 12 -C 15 alcohol ethoxylates the great differences in performance are again especially striking. less suitable well suited Lutensol A..N, Lutensol A Lutensol TDA Lutensol T 28 Stability index Lutensol TDA 7 Lutensol T 10 % of the drops have coalesced after Lutensol A..N, A HLB value Months Weeks Days Hours Minutes

29 Emulsions of sunflower oil in water nonylphenol substitution Compared with nonylphenol ethoxylate, one will note that the latter like Lutensol T, but with a slightly higher HLB displays a surprisingly good stabilization and is exceeded only by Lutensol T or long-chained ethoxylates such as Emulan AT or Lutensol AT. Stability index Lutensol AT 7 Lutensol T 10 % of the drops have coalesced after Nonylphenol ethoxylate HLB value Months Weeks Days Hours Minutes Effect of the degree of branching on the emulsion stabilization If the maximally attainable stability index with optimal HLB value for a group of commercial and experimental ethoxylates is plotted against the number of branches per C atom in the hydrophobic part, then one finds a surprising correlation for the group of C 10 and C 12 to C 15 alcohol ethoxylates*. It is further favorable that the ethoxylates of these medium-branched alcohols are usually readily biode - gradable and lowly aquatoxic. These products are therefore the best choice if one is searching for short or medium chained non-ionic surfactants that are to stabilize emulsions well. 2 Lutensol T The optimal window contains ethoxylates of alcohols with about 0.15 branches per C atom in the hydrophobic part, such as the commercial ethoxylates Lutensol T or Lutensol XP. The reason for this window is not clear; obviously an optimum is achieved here in terms of packing and flexibility in the interfacial film C 12 C 15 Ethoxylates of paraffin oil Lutensol XP C 10 Ethoxylates of paraffin oil C 12 C 15 Ethoxylates of sunflower oil -2 C 10 Ethoxylates of sunflower oil * Ethoxylates of long-chained alcohols with C atoms in the hydrophobic part stabilize against coalescence so well that no correlation with the degree of branching is recognizable Number of branches per C atom in the hydrophobic part 29

30 3.5 Poly dimethyl siloxane emulsions in water made with nonionic surfactants from BASF Three principal methods are used to produce poly dimethyl siloxane (PDMS) emulsions: Microemulsions can only be made if the silicon oil is of very low viscosity e.g. short chains or cyclic oligomers or by using siliconbased emulsifiers. Here, simple mixing both the oil and water phases together with a well-tuned emulsifier package leads to the microemulsions; a special emulsification machine is not required. High-energy emulsification is used to emulsify normal (viscous) PDMS oils. For oils with viscosities up to ca. 500 mpa s rotor-stator machines can be used, whilst highpressure homogenizers can emulsify oils with viscosities up to ca. 100,000 mpa s. Emulsification of viscous oils at elevated temperatures as done with e.g. hydrocarbons is usually not helpful, because of the low temperature dependence of the PDMS viscosities. Emulsion polymerization is another common process to manufacture PDMS emulsions. For target oil viscosities beyond 100,000 mpa s emulsion polymerization is the only feasible process. Standard process to emulsify PDMS Charge your vessel with 40 parts of water Upon stirring add 10 parts of emulsifier Then add 50 parts of the PDMS oil, mix thoroughly Subject the mixture to the emulsification procedure Fine-tune the emulsifier content and HLB value for optimum stability. The following charts illustrate the stability of aqueous PDMS emulsions against coalescence for various alcohol ethoxilates as a function of emulsifier chemistry and HLB value as determined by the Marker method. Performance of alcohol ethoxilates Coalescence stability at room temperature 1% of the droplets coalesced after month weeks days hours minutes Lutensol AT, T and XL offer best coalescence stabilization. Lutensol XL is preferred, as it forms small droplets at rather low power input and stabilizes across a wide HLB range HLB value 30% oil, 1% emulsifier, 69% water, storage at 23 C 30 Lutensol /Emulan AT Lutensol XL Lutensol XP Lutensol N Lutensol T APE (for comparison)

31 Performance of alcohol ethoxilates Coalescence stability at elevated temperature 1% of the droplets coalesced after month weeks days hours minutes HLB value 30% oil, 1% emulsifier, 69% water, storage at 70 C At elevated temperatures the longer chain ethoxilates Lutensol AT and T offer significant better stability than the medium chain nonionics Lutensol XL and XP. Lutensol /Emulan AT Lutensol XL Lutensol XP Lutensol N Lutensol T APE (for comparison) Conclusion For maximum emulsion stability we recommend the emulsifier Emulan AF. Lutensol T 5 to 7 is the second best robust alter native. For emulsions that are not subjected to elevated temperatures Lutensol XL 50 works perfectly. Also, Lutensol XP 50 suits nicely here. Performance of nonionic emulsifiers in emulsification of PDMS Low Low Small Low Ecology coalescence creaming droplets odor Emulan AF ++ + o Lutensol T o o o ++ Lutensol XL 50 + o + +* ++ Lutensol XP 50 o o + o ++ Emulan AT 9 ++ o Lutensol AT o APE (for comparison) * for extremely low odor choose Lutensol XA, the narrow range versions of Lutensol XL In addition to perfect coalescence stabilization, the special nonionic emulsifier Emulan AF offers good resistance against creaming plus a low odor. 31

32 3.6 Amino modified silicon microemulsions made with nonionic surfactants from BASF, e.g. for textile softening Amino modified silicones are perfectly suited for textile softening The ammonium group exhibits good affinity to the fabric, thus the wash permanence is good.... offers the opportunity to make silicon oil microemulsion concentrates with perfect shelf stability.... offers the opportunity to make silicon oil microemulsion concentrates which upon dilution form fine nanoemulsions. Those are able to penetrate the fiber bundle very uniformly. Making the microemulsion concentrate The literature cites a large number of methods for making amino modified silicon oil microemulsions. Those include high- and low-energy input, cold, hot and sometimes even multi-step processes, with cofeed of water, emulsifier or acids in between. All these processes are optimized to overcome gel formation and to achieve stable emulsions with small droplet sizes. However, the simplest processes can be sketched roughly as follows: Method 1 dissolve the nonionic surfactant with water. add the amino silicon oil and emulsify as finely as possible upon vigorous stirring add acid to charge the amino oil. The microemulsion will form automatically. Method 2 mix the non-aqueous nonionic surfactant with the silicon oil and stir upon stirring add acetic acid to charge the amino oil upon vigorous stirring add water and emulsify In both cases elevated temperatures help to overcome the formation of gels or gel particles. Typical start values for the final microemulsion are: 25% silicon oil, 12% nonionic emulsifier (HLB = ca ), water to 100%, adjust ph to 4.5. Subsequently, optimize oil and emulsifier content plus HLB. 32

33 Ranking of the nonionics microemulsion Definitions: Efficiency is the required minimum amount of nonionic emulsifier to manufacture a microemulsion with phase stability between 5 and 50 C. Tolerance window is the width of the HLB window of the nonionic emulsifier leading to a microemulsion with phase stability between 5 and 50 C. Gelling tendency is the degree of formation of gel lumps during microemulsion manufacture, which are difficult to dissolve. High oil content suitability is the possibility to form microemulsions with both high oil content and low viscosity. Clear, nonviscous microemulsion concentrate Gel formation due to choice of less suitable emulsifier Examples: Microemulsions with different types of amino oils are checked in a temperature interval of 5 to 50 C to determine both the efficiency and the tolerance window of the nonionic emulsifiers. Recipe: 25% Silicon il 1.0 Pas/ 0.6 mmol N/g x% Lutensol T 75-x% Water ph = 4.2 Please note that the microemulsion phase diagram is highly dependend on the oil type and needs to be optimized for every formulation. Lutensol T % Surfactant C 5 23 C Microemulsion Tolerance window Efficiency o/w Emulsion HLB Silicon oil A: Silicon oil B: 1.0 Pa s, 0.3 mmol N/g 1.0 Pa s, 0.6 mmol N/g NPE solid/gel solid/gel Lutensol T ca. 40,000 mpa s ca. 9,000 mpa s Lutensol XL ca. 6,000 mpa s ca. 5,000 mpa s Lutensol XP ca. 2,000 mpa s ca mpa s High oil content suitability Microemulsions with high oil content have been optimized with respect to both emulsifier content and HLB value and their viscosity have been determined. Systems with low viscosity offer the possibility to process viscous amino oils or manufacture micro emulsions of high oil concentration. Experimental conditions: 40% oil, ca. 20% surfactant, HLB ca. 10. Brookfield Spindle 4 6, 30 rpm, 23 C. 33

34 Low gelling High oil content Efficiency & tendency* suitability tolerance window Lutensol T ++ o + Lutensol XL Lutensol XP Lutensol N Conclusion: ranking of the nonionic surfactants with respect to microemulsion formation Please note that the table exhibits the average trend. From oil to oil slight variations may occur. For comparison: Nonyl phenol ethoxilate o + Fatty alcohol ethoxilate o * Microemulsions are checked for the degree of intermediate formation of gel particles and their dissolution kinetics. Ranking of the nonionics diluted microemulsion under application conditions Application The microemulsion concentrate is diluted to ca. 5 g/l silicon oil to be applied on the fabric. Upon heating, addition of salts, bases or shear stress this diluted emulsion must be stable, otherwise staining of the fabric or silicon residues in the machinery are possible. Emulsifiers with additional high-speed wetting power help to deaerate the fabric during the impregnation step. Stability against high temperatures The diluted microemulsion is subjected to high temperatures and stored or sheared. Instable emulsions lead to fogging, creaming or coalescence. Silicon il 1.0 Pas/0.6 mmol N/g with APE with Lutensol T storage at ph 10 Stability against high ph The diluted microemulsion is subjected to high ph and stored or sheared. Instable emulsions lead to fogging, creaming or coalescence. Performance under application conditions Stability of the diluted emulsion high temperature MgCI 2 ** high ph Lutensol T Please note that the table exhibits the average trend. From oil to oil slight variations may occur. Lutensol XL Lutensol N ++ o o Lutensol XP ++ For comparison: Nonyl phenol ethoxilate + + to ++ + to ++ Fatty alcohol ethoxilate **see next page = Registered trademark of BASF group

35 Stability against MgCl 2 The diluted microemulsion is subjected to MgCl 2 and stored or sheared. Instable emulsions lead to fogging, creaming or coalescence. Silicon il 1.0 Pas/0.6 mmol N/g with APE with Lutensol T storage in 1 w% MgCI 2 Conclusion amino modified silicon oil microemulsions For the manufacture of amino modified silicon oil microemulsions for textile softening we recommend Lutensol T and XL types. Both offer easy formation of the micro - emulsion. Lutensol T additionally offers optimum robustness in the application plus good wetting and penetration. Lutensol XL offers highspeed wetting and thus very uniform penetration, the formulation of microemulsions of low viscosity plus good robustness under application conditions. Microemulsion Robustness Wetting, during application deaeration Lutensol T Lutensol XL Lutensol N Lutensol XP ++ o ++ For comparison: Nonyl phenol ethoxilate o + to ++ o Fatty alcohol ethoxilate ++ o Please note that the table exhibits the average trend. From oil to oil slight variations may occur. 35

36 3.7 Microemulsions In the microemulsion, oil and water phases are still molecularly separated by the surfactant film; When an oil phase, an aqueous phase and one or more surfactants are mixed, then microemulsions can be formed under certain conditions: The hydrophobic parts of the surfactants are miscible with the oil phase The hydrophilic parts of the surfactants are miscible with the aqueous phase The surfactants are scarcely miscible with the oil or water phases on the molecular level i.e. when the cmc* in oil and water phase are exceeded, the surfactants are behaving amphiphobically. Hydrophilic and hydrophobic part of the surfactant or surfactant mixture are approximately of equal size; the surfactant film therefore prefers planar geometries The Krafft temperature of the surfactants is exceeded frequently bicontinuous structures are formed, but lamellar and other structures are also known. Macroscopically one observes a swelling of the surfactant phase by oil and water phase; one therefore says that the surfactant has solubilized the oil and water, and one reports the solubilization parameters S and S W which describe the ratio of the volumes of oil and water phase, respectively, to the pure surfactant in the microemulsion. These values in efficient systems may amount to more than S = 10 and correlate with the interfacial tensions in the system. For high solubilization parameters S micro - emulsions appear opaque due to the Tyndall effect; they are never - theless thermodynamically stable. *cmc = critical micelle concentration 0.1 µm Water il Electron microphotograph of a frozen microemulsion, Source: Science vol. 240, Microemulsions, Manfred Kahlweit. Copyright 1988, AAAS [caption poorly legible] 36

37 Substantial differences between microemulsions and conventional (macro-) emulsions Microemulsions are thermo - dynamically stable phases that form spontaneously after the mixing of all components without an input of energy. In microemulsions ultra-low interfacial tensions occur between the oil and water phases. Microemulsions react most strongly to a change in intensive magnitudes in the system, e.g., the temperature, the composition of the oil and water phases as well as the surfactant mixture. When the range of existence of micro - emulsions is exceeded, they often transform into macroemulsions, and this is an effective means of producing the latter. Microemulsions, as opposed to conventional emulsions, separate excess oil and water phases out relatively quickly. Microemulsions have maximal stability at the phase inversion point of the system. Microemulsions are very dynamic; boundary surfaces form and disappear, usually within micro - seconds. Examples of applications of microemulsions Stable formulation of two nonmiscible liquid phases. Formulation for production of macroemulsions via spontaneous emulsification. Creation of a large contact area between two non-miscible phases, e.g. to facilitate material transfers during dissolving and cleaning processes, reactions or extractions. Generation of ultra-low interfacial tension between two phases, e.g., to mobilize the one in the other. Simultaneous transporting of two non-miscible phases through a porous medium, a fabric or through narrow capillaries. For use in the following areas: Analytics Soil sanitation Fuels and propellants Chemical reactions Extraction Corrosion protection Cosmetics Varnish Leather industry Metal working Foods Nanotechnology il field Plant protection Pharmaceuticals Cleaning Textile industry Environmental protection 37

38 Practical rules for the preparation of micro emulsions The microemulsion is a stable phase in the oil-phase, water-phase and surfactant phase space. The practical exploitation of this phase space, however, is frequently made difficult by kinetic inhibition of coalescence by other, surfactant-richer phases, often of higher viscosity, and precipitations of particularly ionic surfactants. It has been shown empirically that the simplest way to produce micro - emulsions is with ethoxylates of medium-chain branched alcohols. The Lutensol N brands are parti - cularly useful for this, but also the Lutensol XA, Lutensol XL or Lutensol XP brands. It is best to start with a 45% oil phase, 45% water phase and 10% Lutensol; as the starting point for water and alkanes an HLB around 9 11 is suitable. An addition of co-surfactants is also helpful; here branched, short-chain alcohols with 4 6 carbon atoms are suitable. Then the temperature is varied until a single-phase region the micro - emulsion or a three phase region the microemulsion with additional oil or water phase is established. Here it is essential to distinguish between the real three-phase region and creamed, partially coalesced emulsion, which may have a similar appearance. ne has now reached the phase inversion. By varying the HLB value of the surfactant or surfactant mixture, now the range of existence of the microemulsion can be adjusted to the desired temperature. A 10 Kelvin shift downward in the case of ethoxylates corresponds to an HLB value lower by about units and vice versa. The volume of the microemulsion phase can finally be expanded by increasing the surfactant content in the formulation until the one-phase region is reached. If the optimal HLB value of the surfactant mixture is determined with the medium-chain ethoxylates Lutensol XA, Lutensol XL, Lutensol XP or Lutensol N, these may be successively replaced with longerchain surfactants, such as, e.g., Lutensol T, Emulan A, Emulan AT 9 or Lutensol AT in order to optimize the properties of the micro emulsion. Example of a microemulsion In a 1 liter upright cylinder at 23 C one adds: 436.4g H g NaCl 50g Lutensol T 6 (HLB = 11) 5g sec-butanol g n-dodecane Then the components are thoroughly mixed and allowed to stand over - night. Three phases are formed: A dodecane phase on top, a salt solution at the bottom and an opaque microemulsion in the middle. By varying the temperature, one can observe how the ratios of dodecane and salt solution in the microemulsion vary. For example, if one touches the upright cylinder with the hand in the region of the microemulsion phase, the latter will break up suddenly into a w/o emulsion due to the warming effect. The microemulsion can also be removed and a stable, finely divided /W emulsion produced by injection into the tenfold quantity of water. Isotherm 23 C Berührung mit der Hand Bereiche mit w/o Emulsion 38