nonionic surfactant ... - Springer Link

Composition of Mixed Anionic/Nonionic Surfactant Micelles Halide Akbas¸*, Mehmet Is¸can, and Taliha Sidim Department of Chemistry, Faculty of Sciences and Letters, Trakya University, 22030 Edirne, Turkey

ABSTRACT: A proposed method of determining the composition of mixed micelles in equilibrium with monomer of known composition is described. The systems were sodium dodecyl sulfate-polyoxyethylene 23 lauryl ether (Brij 35) in water and in 0.1 M sodium chloride solution at 25°C. This technique applies the Gibbs-Duhem equation to the mixed micelle, which is treated as a pseudophase. This proposed methodology, which needs only critical micelle concentration data as a function of monomer composition, is applied to an anionic/nonionic surfactant pair. The calculated monomer–micelle equilibrium is found to be very similar to the much-used regular solution for nonideal systems. Paper no. S1066 in JSD 3, 77–80 (January 2000). KEY WORDS: Anionic/nonionic surfactant, critical micelle concentration, interaction parameter, mixed micelle, regular solution theory, surface tension.

Micelles composed of mixtures of surfactants with different structures (mixed micelles) are of great theoretical and industrial interest (1–4). There is increasing interest in understanding the structure and properties of mixed micelles, while the surfactants used in practical applications are rarely pure. Different techniques have been used to collect structural information on mixed micelle formation, and to obtain their critical micelle concentration (CMC) (5–8). CMC have been determined as a function of surface tension, density, conductivity, and pH for pure surfactant and mixed surfactants (9–11). Micelles formed from a solution of mixed surfactants generally have a different surfactant composition than a monomer (12). In modeling the equilibrium between monomer and these mixed micelles, the micelle is often considered as a pseudophase (a thermodynamic phase in equilibrium with the monomer). Micelles composed of mixtures of surfactants of similar structure are nearly ideal; i.e., ideal solution theory describes the thermodynamics of mixing in the micelle when it is considered as a pseudophase (13,14). However, mixed micelles containing ionic and nonionic or anionic and cationic surfactants show negative deviations from ideality (15,16). On the other hand, mixed micelles composed of mixtures of fluorocarbon and hydrocarbon surfactants exhibit positive deviations from ideality (17). There has been *To whom correspondence should be addressed. Copyright © 2000 by AOCS Press

a great deal of recent effort to model and understand these mixed surfactants. Rubingh (15) successfully explained this nonideal behavior by using the phase separation model of micellization and regular solution approximation. An equation was derived that describes the interaction between the two surfactants. Rosen and Shinoda (13) also derived equations for synergism in surface-tension reduction efficiency, mixed micelle formation, and surface tension reduction effectiveness in aqueous solution of mixed surfactants based on nonideal solution theory. A new method for calculating the composition of mixed micelles in equilibrium with monomer of known composition has been proposed by Nguyen et al. (12). This method applies the Gibbs-Duhem equation to the mixed micelle, which is treated as a pseudophase. Abuin and Lissi (18) investigated the competitive binding of counter-ions to mixed ionic/nonionic micelles by using sodium dodecyl sulfate (SDS) and Brij 35 surfactants. It is proposed that the factors involved in determining the affinity of different counterions for binding to the micelle surfaces are almost completely determined by the nature of the exchanging counterions and depend very little on the composition of micelles. On the other hand, the addition of a second surfactant that interacts with the first surfactant has been shown to change various fundamental interfacial properties (19,20). In this study, the composition of mixed micelles composed of SDS and Brij 35 is discussed to interpret the association of counter-ions with mixed ionic/nonionic micelles.

EXPERIMENTAL PROCEDURES SDS [CH3(CH2)11O SO3−Na+] from Merck (Darmstadt, Germany) was purified by repeated recrystallization from ethanol, followed by drying under a vacuum. The purity of SDS was confirmed by the absence of minima in surfacetension curves. Highly pure polyoxyethylene 23 lauryl ether (Brij 35) from Sigma (St. Louis, MO) was used as received. NaCl obtained from Merck was used without purification and was thermally dried before use. Distilled water was prepared by distillation from alkaline permanganate solution after having been passed through an ionexchange column. Conductivity of the distilled water was 1.10−6 Ω−1 cm−1, and the air/water surface tension was equal to 71–72 mN/m at 298 K. Surface tensions of aqueous solutions of single and mixed surfactants at various concentrations were deter-

Journal of Surfactants and Detergents, Vol. 3, No. 1 (January 2000)

77

78

H. AKBAS¸ ET AL.

mined using the DuNouy ring method. All measurements were done at 25°C. The surface tension of conductivity water was measured at 25°C before starting surface-tension measurements. After a change in surfactant concentration, the bulk solution was stirred for 10 min. Then the solution was placed inside a perspex cape for the surface to equilibrate for 10 min before measurements were taken. Although the new surface may not have fully equilibrated by this time, the time was kept constant as it was a general trend in the surface-tension measurement, because it is used to determine CMC rather than the the absolute surface tension. CMC values were determined at sharp break points in surface tension vs. logarithm of concentration curves at various mole fractions of Brij 35.

RESULTS AND DISCUSSION Mixed surfactant CMC results from surface-tension measurements are listed with the pure surfactant CMC data in Table 1. CMC data for SDS/Brij 35 systems without electrolyte and with 0.1 M NaCl are shown in Figures 1 and 2. Equations for curves in Figures 1 and 2, respectively, are as follows, and were found using linear regression analysis of the data (Eqs. 1,2): CMCM = 1496.68 − 671.697 YA − 21010.8 YA2 − 61881.6 YA3 − 64760 YA4 + 2398.2 YA5

[1]

CMCM = 598.591 − 3411.2 YA + 11746.8 YA2 − 19360.32 YA3 + 4968.7 YA4 − 197.24 YA5

[2]

where YA is the mole fraction of Brij 35 as monomer and CMCM is the mixture CMC. Since the monomer is considered to be in equilibrium with micelles, the chemical potential of each surfactant in the monomer phase is equal to that component’s chemical potential in the micelle. If the Gibbs-Duhem equation is applied to this pseudophase at constant pressure and temperature, Equation 3 holds: YA − XA d ln (CMCM) —————— = —— —— dYA YAYB

FIG. 1. Critical micelle concentration (CMC) data for the Brij 35 (polyoxyethylene 23 lauryl ether)/sodium dodecyl sulfate (SDS) system without electrolyte.

where XA is the mole fraction of Brij 35 in the micelle. According to Equation 3, the slope of ln (CMCM) vs. YA can be used to calculate the value of XA at a given YA. That is, the micellar composition at CMC can be found at any monomer composition from CMC data alone. By using values in Table 1 and Equation 3, the resulting monomer–micelle equilibrium compositions are shown in Figures 3 and 4. Figures 3 and 4 show how the mole frac-

[3]

TABLE 1 Mixed Critical Micelle Concentration (CMC) Results for SDS/Brij 35 Systems at 25°C With and Without 0.1 M NaCl Mole fraction of Brij 35 0.00 0.20 0.40 0.50 0.60 0.80 1.00 a

Mixed CMC values (mM) Without electrolyte 3

1.0 · 10 4.0 · 102 2.2 · 102 2.0 · 102 2.5 · 102 3.0 · 102 5.0 · 102

With 0.1 M NaCl 2.0 · 102 8.5 · 102 7.0 · 10 9.0 · 10 9.5 · 10 1.1 · 102 1.4 · 102

SDS, sodium dodecylsulfate; Brij 35, polyoxyethylene 23 lauryl etter.

FIG. 2. CMC data for the Brij 35/SDS system with 0.1 M NaCl. See Figure 1 for abbreviations.


COMPOSITION OF MIXED MICELLES

79

to Rubingh, if the surfactants are mixed, the mixed CMC (C*) is given by Equation 4: 1 α1 (1 − α1) — = —— + ——— C* f1C1 f2C2

[4]

where α is the mole fraction of surfactant 1 in the total mixed solute, f1 and f2 are activity coefficients of surfactants 1 and 2, respectively, and C1 and C2 are the CMC of surfactants 1 and 2, respectively. In the case of ideal behavior, f1 = f2 = 1. Hence Equation 4 can be written as Equation 5: 1 α1 (1 − α1) — = —— + ——— C* C1 C2

FIG. 3. Monomer/micelle equilibrium compositions for Brij 35/SDS system without electrolyte. See Figure 1 for abbreviation.

[5]

Mixed CMC (C*) values calculated using the above equation for ideal behavior are also plotted against the mole fraction of Brij 35 in Figure 1. It is seen that experimental mixed CMC values are lower than those obtained by assuming ideal behavior. Rubingh used the phase separation model of micellization and regular solution approximation to explain this nonideal behavior and derived Equation 6: X12 ln [C*α1/C1X1] —————————————— =1 (1 − X1)2 ln [C*(1 − α1)/(1 − X1)]

[6]

where X1 is the mole fraction of surfactant 1 in the mixed micelle. Equation 6 can be solved iteratively to obtain the value of X1, from which the interaction parameter, β, can be evaluated using Equation 7: ln [C*α1/C1X1] β = ————— —— (1 − X1)2

FIG. 4. Monomer/micelle equilibrium compositions for Brij 35/SDS system with 0.1 M NaCl. See Figure 1 for abbreviation.

tion of Brij 35 as monomer changes with increasing mole of Brij 35 in the micelle. It can be seen that the two fractions are equal, so that XA = XB. Prediction from ideal solution theory shown in the figures was determined from the Rubingh theory (12). Since SDS/Brij 35 systems contain dissimilar surfactants, they exhibit a large negative deviation from ideality. According

[7]

β is an indication of the degree of interaction between the two surfactants. It is found for the SDS/Brij 35 systems that β values obtained at various mole fractions remain nearly constant, i.e., β = −9.72. Negative β values in this binary surfactant system suggest that a strong attractive interaction between the two surfactants exists. Unlike interactions (i.e., anionic–nonionic interaction) in the mixed micelles predominate like interactions in pure micelles. In the case of the CTAB Brij 30 system, fairly constant β values (β = −4.2) were obtained (21). However, in the case of CTAB + Brij 35, β values were found to vary, indicating that cationic–cationic interaction of CTAB and cationic headgroup–oxonium ion interaction is a secondary effect. As shown in Figures 1 and 2, SDS + Brij 35 systems exhibit a large negative deviation from ideality in the case of electrolyte. In general, an increase in the electrolyte constant of the aqueous phase produces a decrease in the negative value of β. This is true for the SDS + Brij 35 systems, suggesting that interaction between them is, at least partly, electrostatic. For this reason, the negative values of β can


80

H. AKBAS¸ ET AL.

be attributed to complex formation between Na+ and the ether oxygens of the polyoxyethylene chain, resulting in its acquiring a positive charge, which increases the strength of its interaction with the anionic surfactant.

REFERENCES 1. Ogino, K., H. Uchiyama, M. Ohsato, and M. Abe, Fading Phenomenon of Azo Oil Dye in Anionic–Nonionic Surfactant Solutions, J. Colloid Interface Sci. 116:1,81 (1987). 2. Attwood, D., V. Mosquera, M. Garcia, J. Rodriguez, and M.J. Suarez, Apparent Molal Volumes and Compressibilities of NaF Solutions, Ibid. 157:168 (1993). 3. Marangoni, D.G., A.P. Rodenhiser, J.M. Thomas, and Kwak, Solubilization and Aggregation Number in Micellar Mixtures of Anionic and Cationic Surfactant with Tetraethylene Glycol and Tetraethylene Glycol Dimethyl Ether, Langmuir 9:438 (1993). 4. Akbas¸, H., and M. Is¸can, Effects of Added Salt on the Solubilization of Sudan Red B in Surfactant Solution, Turk. J. Chem. 18:80 (1994). 5. Sivakumar, A., P. Sumasundoran, and S. Tharch, Micellization and Mixed Micellization of Alkylxylenesulfonates—A Calorimetric Study, Colloids Surf. A 70:69 (1993). 6. Hu, Y., S.Q. Wang, and A.H. Jamieson, Kinetic Studies of a Shear Thickening Micellar Solution, J. Colloid Interface Sci. 56:31 (1993). 7. Ratman, J.F., and J.F. Scamehorn, Counterion Binding on Mixed Micelles, J. Phys. Chem. 88 (1984). 8. Ishikawa, M., K. Matsumura, K. Esumi, and K. Megura, Mixed Micelle Formation Between Anionic Surfactant and α,ω-Type Cationic Surfactant in Aqueous Solutions: Sodium Dodecyl Sulfate and N,N′-1,12-Dodecanediylbis (triethylammonium bromide) System, J. Colloid Interface Sci. 141:1,10 (1990). 9. Meguro, K., H. Akasu, M. Veno, and T. Satake, Effects of Polyoxyethlene Chain Length upon Mixed Surfactant Solution, Colloid Interface Sci. Proc. Int. Conf. 50th 2:421 (1976). 10. Varma, R.D., and R. Dayal, Conduction Behavior of Aqueous Solution of Barium, Strontium, and Nickel Soaps, J. Am. Oil Chem. Soc. 53:39 (1975). 11. Yu, Z.J., and G.X. Zhao, Physicochemical Properties of Aqueous Mixtures of Cationic–Anionic Surfactants, J. Colloid Interface Sci. 130:2,414 (1988).

12. Nguyen, C.M., J.F. Rathman, and J.F. Scamehorn, Thermodynamics of Mixed Micelle Formation, Ibid. 112:2438 (1986). 13. Rosen, M.J., and K. Shinoda, in Colloid Surfactants, edited by K. Shinoda, B. Tamamushi, and T. Nakagawa, Academic Press, New York, 1963, pp. 144–162. 14. Funasaki, N., The Thermodynamics of Micellization of Surfactants in the Presence and Absence of Salt, J. Colloid Interface Sci. 67:384 (1979). 15. Rubingh, D.N., in Solution Chemistry of Surfactants, edited by K.L. Mittal, Plenum Press, New York, 1979, pp. 337. 16. Fanasaki, N., and S. Hada, Surface Tension of Aqueous Solutions of Surfactant Mixtures. The Composition of Mixed Micelles, J. Phys. Chem. 83:19,1471 (1979). 17. Mukerjee, P., and Y.S. Yang, Nonideality of Mixing of Micelles of Fluorocarbon and Hydrocarbon Surfactants and Evidence of Partial Miscibility from Different Conductance Data, J. Phys. Chem. 80:1388 (1976). 18. Abuin, E., and E. Lissi, Tl+/Na Competitive Binding at the Surface of Dodecylsulfate/Brij 35 Mixed Micelles, J. Colloid Interface Sci. 151:597 (1992). 19. Matson, T.P., and M.F. Cox, An Approach to Formulating Cold-Water Laundry Products, J. Am. Oil Chem. Soc. 61:1272 (1984). 20. Cox, M.F., N.F. Borys, and T.P. Matson, Interaction Between LAS and Nonionic Surfactants, Ibid. 62:1139 (1985). 21. Desai, T.R., and S.G. Dixit, Interaction and Viscous Properties of Aqueous Solution of Mixed Cationic and Nonionic Surfactant, J. Colloid Interface Sci. 177:471 (1996). [Received January 5, 1999; accepted November 8, 1999]

Professor Dr. Mehmet Is¸can completed his B.A. in 1969 at the University of Istanbul and received Dr. ret. nat. in chemistry at the same university in 1972. His specializations include surfactants, micelle theory, microemulsions, and solubilizations. He now works for Trakya University Chemistry Department. Assistant Professor Halide Akbas¸ graduated from Technical University of Istanbul and received her Ph.D. at Trakya University in 1992. She has studied mixed surfactants, solubilization, and mixed micellization and works for Trakya University Chemistry Department. Taliha Sidim is a Ph.D. student at Trakya University and has worked on mixed micelles.
