A Study of Performance Properties of Alkyl Poly(glucoside) and Sodium Dodecylsulfate in their Mixed Systems

J. Surface Sci. Technol., Vol 22, No. 1-2, pp. 75-88, 2006 2006 Indian Society for Surface Science and Technology, India A Study of Performance Properties of Alkyl Poly(glucoside) and Sodium Dodecylsulfate in their Mixed Systems R. M. SATAM and M. R. SAWANT* University Institute of Chemical Technology, Matunga, Mumbai 400019, India. Abstract The interaction parameters and performance properties such as solubilizing capacity and foaming behaviour of the aqueous Alkyl poly (glucoside) (APG) sodium dodecylsulfate (SDS) mixed surfactant system have been investigated. The critical micellar concentrations (cmcs) obtained by surface tension measurements revealed that there is a synergism between these two surfactants. Performance properties such as solubilizing capacity have been investigated by measuring the absorbance and the viscosity of solubilized polar oily materials like octanol, decanol, dodecanol. The solubilizing phenomenon exhibited by mixed surfactant systems showed better results than that of the individual surfactant system. The foaming capacity of Alkyl poly (glucoside) with Sodium dodecylsulfate showed highly stabilized foaming behaviour in both distilled water as well as in hard water (324 ppm). Keywords : Alkyl poly (glucosides), Sodium dodecylsulfate, Micellization, Mixed surfactant, Interaction parameter, Solubilization, Foaming. INTRODUCTION Surfactants are amphiphilic materials that have numerous functionality, which changes with its structural characteristics and are therefore employed to various industrial applications such as pharmaceuticals, polymerization processes, detergency, foods and enhanced oil recovery [1]. In many biological and industrial applications, mixed surfactants are employed because they form mixed micellar aggregates, which exhibit characteristic properties and are superior to those of the individual components [2,3]. Synergistic behaviour of mixed surfactant system can be exploited to reduce the total *Author for correspondance

76 Satam and Sawant amount of surfactant used in particular applications resulting in reduction of cost and environmental impact [4]. Alkyl poly (glucoside) has wide range of applications in areas such as cosmetics, pharmaceutical, paint, pesticide, etc. Surfactants used in all such applications should have good solubilizing power. The physico-chemical properties of an aqueous solution of mixed surfactants will change as the concentration increases and aggregates form. Mixture of surfactants forming insoluble aggregates and mixed micelles has assumed importance in industrial applications and laboratory surface chemical work of surfactants, because of the tendency to form aggregated structures substantially different from that in solutions having only pure surfactants [5]. In fact, some phenomena, which are not expected to take place in the single system, occur in aqueous solutions containing mixtures of surfactants [6-8]. Studies involved the investigation of various physiochemical properties such as surface tension, using tensiometry. We have used Rubingh s method [9] to calculate the mole fraction, interaction parameters and activity coefficients of surfactants in mixed micelles. Studies also deal with the solubilization capacity of mixed surfactant systems. Solubilization is a phenomenon in which a solution of a surfactant in concentrations beyond the critical micelle concentration dissolves a variety of organic compounds to yield a transparent solution, which results from the incorporation of the substances into the micelles [10]. Modes of incorporating the solubilizate into the micelles, which are closely related to the structure of micelles, are adsorption on the surface of the micelles, deep penetration of the palisade layer and dissolution in the hydrocarbon core. These methods have been described by Kolthoff [11], Riegleman [12] and Mulley [13] respectively. Effect of macro-monomer on the solubilization by ionic micelles have examined by A. B. Mandal [14,15]. To study the capacity of solubilization, normal higher alcohols were solubilized by individual, nonionic and anionic surfactants as well as mixtures of nonionic and anionic surfactants. Mechanism of solubilization was investigated by measuring the amount of solubilization by spectrophotometric and the relative viscosity measurements. The present investigation also deals with foamability and foam stability of surfactant solutions. A feature of many conventional nonionic surfactants, such as ethoxylates, is relatively low foaming capacity, alone or in combination with conventional anionic surfactants. The present method of evaluation and characterization of (i) the foam ability of solutions of different surfactants and (ii) the stability of the foams formed, by using simple and quick pneumatic test in which foam studies have been done by a well-controlled volume of gas introduced into a definite volume of solution.

A Study of Performance Properties of APG and SDS in their Mixed Systems 77 EXPERIMENTAL Materials : Sodium dodecylsulfate (SDS) of a reagent grade was recrystallized from ethanol. Alkyl poly (glucoside) C 12/14 G 1.3 from Henkel KGaA, CFC Cospha, Germany, supplied as a 50 wt% solution in water, was further recrystallized by methanol to get a pure form. The octyl alcohol (C 8, bp 179.5 0 C), decyl alcohol (C 10, bp 228-232 0 C), dodecyl alcohol (C 12, bp 86.5 0 C) were the purest grade products. Doubly distilled water was used for preparing solutions. Conductivity of the doubly distilled water was 3.4 ms cm -1 and the surface tension was 71.8 mn/m at 30 0 C. Methods : Preparation of mixed surfactant solutions Into several 100 ml standard flasks, binary surfactant mixtures of nonionic surfactant APG 12/14 and an anionic surfactant (SDS) in water were taken at different mole fractions ranging from 0 to 1. The solutions were stirred for 5 h in a thermostated bath at 30 ± 0.2 0 C and kept overnight to establish equilibria. Surface Tension Measurements Surface tensions (g) of aqueous solutions of single and mixed surfactants at various concentrations were determined by a ring method using the Du-Nöuy tensiometer (S. C. Dey and Co., Kolkata, India). All measurements were done at room temperature, i.e., at 30 ± 0.2 0 C. The surface tension measurements have an accuracy of ±0.3%. Solubilization When oily materials are added to a surfactant solution, beyond the saturation point they are emulsified and the solution becomes turbid. The following procedure was used for the determination of the limits of solubilization. Into several 100 ml glass-stoppered flasks, 50 ml portions of a 1 10-2 M surfactants were placed; then varying quantites of polar oily materials were added to them and the mixtures were stirred for 5 h in a thermostat at 30 0 C, followed by settling until solubilization equilibria were established. After the equilibrium had been established, the turbidity of the solution was measured with an Electrophotocolorimeter, with a cell of 1 cm path length at 655 nm. The turbidity concentration obtained was extrapolated to zero in order to obtain the amount of solubilization. Viscosity Measurements The relative viscosities of the solutions solubilizing oily substances were measured at 30 0 C ± 0.2 with an Ubelohde viscometer with appropriate auxiliary equipment to keep atmospheric moisture off as far as possible. The flow times of water and surfactant solutions were used in calculating the relative viscosities. The times of flow were determined with a stopwatch of ±0.1 s precision; the overall fluctuations of the times measured were within ±0.1 to 0.2 s.

78 Satam and Sawant Foaming Apparatus used in the test was designed by K. Lunkenheimer and K. Malysa [16]. The solution (50 ml) was poured into the column in such a manner that no foam layer was formed. The gas from the syringe was introduced into the solution through sintered glass. Gas (50 ml at the experimental temperature and ambient pressure) was supplied manually within a period of 10 s, i.e., an average gas velocity of 18 L/h was used in most of the experiments. Then changes in foam height and level were determined as a function of time. The measurements were repeated 2 3 times for each solution concentration. The experiments were performed at room temperature (30 ± 0.2 0 C). RESULTS AND DISCUSSIONS Surface tension The critical micelle concentrations (CMC) of pure and mixed surfactants were determined by surface tension measurements at varying total surfactant concentration in aqueous solution at different mole fractions. The measured CMC values of individual surfactants, C 12/14 APG and SDS are listed in Table 1. The nature and strength of interaction between two surfactants can be determined by Rubingh s theory. According to Rubingh, the mixed CMC (C 12 ) for a binary surfactant system obtained by mixing two surfactants is given by the equation, 1 a 1 (1 a 1 ) = + (1) C 12 f 1 C 1 f 2 C 2 where a 1 is the mole fraction of surfactant 1 in the mixture, C 1 and C 2 are the CMC values of the surfactants 1 and 2 respectively; f 1 and f 2 are the activity coefficients of the surfactants 1 and 2, respectively. In case of ideal behaviour, f 1 = f 2 = 1; hence, the equation 1 reduces to the form 1 a 1 (1 a 1 ) = + (2) C 12 C 1 C 2 as proposed by Clint [17] for ideal binary mixtures of surfactants. The mixed CMC value (C 12 ) was calculated by using equation (2) for ideal behaviour. Fig.1 shows plot of CMC values vs. mole fractions (a 1 ) of the sodium dodecylsulfate for the anionic nonionic systems studied. The formation of micelle is the result of hydrophobic interaction. The CMC mix values thus obtained

A Study of Performance Properties of APG and SDS in their Mixed Systems 79 TABLE 1. CMC (C 12 ), X 1, b, and Average b for SDS + Glucoside APG (C 12/14 ) systems with varying mole fractions (a 1 ) of SDS in Water at 30ºC. SDS (a 1 ) C 12 10 3 / M X 1 b Average b f 1 f 2 1.0 6.8 0.8 0.7 0.379 3.95 0.217 0.567 0.6 0.54 0.288 3.54 0.166 0.745 3.94 0.4 0.44 0.219 3.50 0.118 0.845 0.2 0.32 0.206 4.8 0.048 0.815 0.0 0.4 Fig. 1. Variation of critical micelle concentration (CMC) with mole fraction (a 1 ) of the SDS for the SDS APG (C 12/14 ) systems at 30 0 C. ------- ideal values; experimental values. experimentally, were found to be lower than the values calculated from the above equation, which indicate non-ideality as indicated by Fig. 1. Fig. 2 shows the plot of X 1 vs. a 1 for an SDS + APG system which reveals that the contribution of anionic surfactant is significant up to a certain mole fraction whereas, at higher mole fractions the mixed micelles become rich in nonionic content. The nature and strength of the interaction between the two surfactants in mixed systems are determined by calculating the values of the b parameter from the plot

80 Satam and Sawant Fig. 2. Plot of mole fractions of APG in mixed micelles (X 1 ) vs. mole fractions of SDS in bulk (a 1 ) at 30 0 C. ------- ideal values; experimental values. of surface tension g vs. the concentrations of aqueous solutions of the individual surfactants and their mixtures. This is indicative of some interaction between the two ionic nonionic surfactant molecules; by considering the phase separation model for micellization, Rubingh derived the relation, (X 1 ) 2 ln[(a 1 C 12 /X 1 C 1 )] = 1 (3) (1 X 1 ) 2 ln[(1 a 1 )C 12 /(1 X 1 )C 1 ] where X 1 is the mole fraction of surfactant 1 (APG) in the mixed micelle. Equation (3) was solved iteratively to obtain the value of X 1. ln[c 12 /C 1 X 1 ] b = (4) (1 X 1 ) 2 The b values calculated for all ionic nonionic surfactant systems studied are listed in Table 1. b values calculated for SDS + APG system are negative and indicate considerable attractive interaction between the ionic and the nonionic surfactants. Activity coefficients were calculated as defined by Rosen and Hua et al. [18] in equations 5 and 6, f 1 = exp[b (1 X 1 ) 2 ] (5) f 2 = exp[b X 2 1 ] (6) where f 1 and f 2 are the activity coefficients of nonionic and anionic surfactants respectively, in the mixed micelles. Equations 5 and 6 show that the activity

A Study of Performance Properties of APG and SDS in their Mixed Systems 81 coefficients are directly related to both interaction parameter of mixing, b and micellar mole fraction of the anionic surfactant. Increases in the attractive or repulsive forces between various molecules in the mixed systems change the activity coefficients of the components. Activity coefficients are less than unity, owing to interaction between anionic and nonionic surfactants (Table 1). The activity coefficients tend to unity only when the mole fraction of any one of the components approaches unity. The activity coefficient values are nearer to unity indicating that both these components are very near to their respective standard states. In a binary system, the main nonideal behaviour attributed to the activity coefficients of the constituents, is due to the change in the electrostatic repulsion energy of surfactant molecules. Solubilising Power of surfactants The change in solubilizing property of different surfactants in mixed micellar solution is studied to check the synergistic effect of mixed surfactant systems on the solubilising power of individual surfactants. Fig. 3 shows the absorbance of varying molar concentrations of APG with SDS solutions at different mole concentrations with increasing amounts of dissolved octanol. The rapid increase in I 0 /I began when emulsion droplets first appeared. The point of the interaction of the curves representing the optical density with horizontal axis is considered to give the solubility of octanol in mixed surfactant solutions. As is evident from Fig. 3, 4 and 5, the solubilizing power of mixed surfactants is higher than that of the individual surfactants. The arrows in the figures indicate the limit of solubilization. This result is considered to be due to the mixed micellar synergistic interaction between SDS and APG surfactant. The solubility and solubilizing power decreases with an increase in the number of carbon atoms in the molecules of oily materials. This result is obtained due to the decrease in the magnitude of the dipole moment with the increase in the number of carbon atoms in the molecule of the oily material. Limit of solubilization of individual and mixed surfactants are listed in Table 2. Relative Viscosity The above results were confirmed by viscosity data; Fig. 6, 7 and 8 show the relative viscosities of solutions, which were produced by solubilizing oily materials using SDS and APG12/14 mixture of a given concentration (1X10-2 mol/l). For a given concentration of oily material, relative viscosity of the mixed surfactant solutions showed far different values than that of the individual surfactant solutions. For a given concentration of oily materials, the relative viscosity increases with an increase in number of carbon atoms in the molecules of oily materials in mixed surfactant solutions. The viscosity pattern in Fig 6 is more or less similar to that in Fig 7, but one can observe unexpected decrease in viscosity with increasing concentration of oil. Regarding the reasons for change in behaviour, the key probably lies in a change in the site of solubilization at higher oil content. This result is

82 Satam and Sawant Fig. 3. Absorbance vs concentration of octanol in SDS, APG 12/14 and mixtures of the two in different mole ratios, APG : SDS = 4:1, 3:2, 2:3 and 1:4. t SDS, n APG12/14, s 4:1, 6 3:2, Q 2:3, l 1:4. Fig. 4. Absorbance vs concentration of decanol in SDS, APG 12/14 and mixtures of the two in different mole ratios, APG : SDS = 4:1, 3:2, 2:3 and 1:4. t SDS, n APG12/14, s 4:1, 6 3:2, Q 2:3, l 1:4.

A Study of Performance Properties of APG and SDS in their Mixed Systems 83 Fig. 5. Absorbance vs concentration of dodecanol in SDS, APG 12/14 and mixtures of the two in different mole ratios, APG : SDS = 4:1, 3:2, 2:3 and 1:4. t SDS, n APG12/14, s 4:1, 6 3:2, Q 2:3, l 1:4. considered to be due to synergistic effect of the mixed surfactant systems, which increases the solubilizing property of surfactants. Foaming property Fig. 9 presents the height of foam column formed by individual as well as mixed surfactant system in distilled as well as hard water. The concentration of the surfactants used was 1 10 2 mol/l each. In the mixture the surfactants were present in 1 : 1 mol ratio. This shows how foam stability of mixed TABLE 2. Capacity of solubilization of individual and mixed surfactant systems at 30ºC. Fatty Limit of solubilization (mol dm 3 ) in individual and mixed srufactant systems alcohol APG12/14 SDS APG:SDS(4:1)* APG:SDS(3:2)* APG:SDS(2:3)* APG:SDS(1:4)* C8 0.70 10 2 1.87 10 2 0.86 10 2 1.93 10 2 1.91 10 2 1.91 10 2 C10 0.30 10 2 0.86 10 2 0.73 10 2 0.83 10 2 0.88 10 2 0.92 10 2 C12 0.48 10 3 0.74 10 3 0.51 10 3 0.58 10 3 0.73 10 3 0.76 10 3 *Ratios within brackets indicate mole ratios.

84 Satam and Sawant Fig. 6. Relative viscosity vs. octanol concentration in APG, SDS and mixtures of the two in various mole ratios (APG : SDS). t SDS, n APG12/14, s 4:1, 6 3:2, Q 2:3, l 1:4. Fig. 7. Relative viscosity vs. decanol concentration in APG, SDS and mixtures of the two in various mole ratios (APG : SDS). t SDS, n APG12/14, s 4:1, 6 3:2, Q 2:3, l 1:4. surfactant is superior to that of the individual surfactant. Foam stability in hard water is another important parameter; Fig. 10 shows that the stability of the foam in SDS improves in hard water on addition of APG to it. This study has been done by studying the parameter called the R 5 parameter, which can be determined as the ratio of foam height at a time of five min after foaming, h 5, to the initial height h 0 of

A Study of Performance Properties of APG and SDS in their Mixed Systems 85 Fig. 8. Relative viscosity vs. dodecanol concentration in APG, SDS and mixtures of the two in various mole ratios (APG : SDS). t SDS, n APG12/14, s 4:1, 6 3:2, Q 2:3, l 1:4. Fig. 9. Height of foam formed on injecting 50 ml gas within 10 s in distilled (o) as well as hard water (n).

86 Satam and Sawant Fig. 10. R 5 parameter values to calculate % foam stability as a function of surfactant in distilled (o) as well as hard water (n). Fig. 11. Foam stability after 20 min as a function of surfactant in distilled (o) as well as hard water (n).

A Study of Performance Properties of APG and SDS in their Mixed Systems 87 the foam formed after having introduced a definite amount of air into a definite volume of solution (50 ml of air into 25 ml of solution). R 5 = (h 5 /h 0 ) 100 (7) The same parameter applied to study the foam stability after 20 min, Fig. 11 implies that stability of foam found in APG and mixed surfactant is higher than that in SDS in hard water. CONCLUSION Solubilizing property of SDS for polar oily materials showed better performance in presence of APG in mixed surfactant system. The nature and strength of interaction between two surfactants have been studied by using the phase separation model for mixed micelization, the interaction parameter b and the compositions in the mixed micelles are evaluated. These studies revealed very superior synergism of surfactant monomers in the mixed micelle. This effect has also been seen on foaming property of surfactants. SDS which is a relatively better foaming agent in plane water, was found to be poorer in hard water. But in mixed surfactant system, its performance in hard water was better. The synergistic effect of surfactants in mixed systems facilitated often better performance than pure surfactants. REFERENCES 1. M. Rodgers, C. Rodgers and R. Palepu; J. Surf. Sci. Technol., 20, 33 (2004). 2. J. F. Scamehorn, in Phenomena in Mixed Surfactant Systems;Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society : Washington D. C., 324 (1986). 3. M. Abe and K. Ogino, Mixed Surfactant Systems; Surfactant Science Series; Marcel Dekker : New York, 46 (1992). 4. D. Blakschtein and A. Shiloach, Langmuir, 14, 1618 (1998). 5. A. B. Mandal and S. Ray, Indian J. Chem, 19A, 620 (1980). 6. H. Lange and K. H. Beck, Colloid Polym. 251, 424 (1973). 7. Moroi Y., N. Nishikido and R. Matsuura., J. Colloid Interface Science, 60, 344 (1977). 8. Y. Moroi, H. Askisada, M. Sato and R. Matsuura, J. Colloid Interface Sci., 61, 233 (1977). 9. D. N. Rubingh, in Solution Chemistry of Surfactants, Ed. K. L. Mittal, Plenum Press, New York, 3, 37 (1979).

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