Langmuir 1999, 15, 2301-2306
Mixed Micelles Containing Alkylglycosides: Effect of the Chain Length and the Polar Head Group M. L. Sierra and M. Svensson* Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden Received April 10, 1998. In Final Form: January 27, 1999 The mixing behavior of binary mixtures of the pure alkylglycosides: β-decylglucoside (β-C10G), β-dodecylglucoside (β-C12G), β-decylmaltoside (β-C10M), and dodecylmaltoside (C12M) in combination with different common surfactants has been studied. First, the effect of the nonionic polar headgroup and chain length of the glycosidic surfactants in the mixed micellization with sodium dodecyl sulfate (SDS) was investigated. Further on, to analyze the effect of the ionic headgroup on the micellization of the glycosidic surfactant, β-C10G was mixed with different surfactants: dodecyltrimethylammonium bromide (DTAB), dodecylheptaethylene glycol ether (C12E7), and β-C10M. All the mixed systems under study adapt reasonably well to the model developed by Rubingh, with negative values for the interaction parameter, βm, indicating a favorable interaction between the mixed surfactants. In the mixtures with SDS and the glycosides, the interactions become stronger when the hydrocarbon chain length of the surfactant is shorter and the hydrophilic headgroup is larger, i.e., when the surfactants become more hydrophilic. The β-C10G mixes favorably with the other surfactants, the interaction becoming stronger in the order C12E7, β-C10M, SDS, DTAB. The strong interaction in micellization with DTAB is explained by assuming an anionic character in the β-C10G molecule, as shown by electroosmosis measurements. Finally, the favorable interaction with β-C10M is explained by considering the packing between the headgroups of both nonionic surfactants.
Introduction During recent years, the use of alkylglycosides has increased due to their good dermatological compatibility and biodegradability.1 They are now produced industrially in large scale from renewable raw material.2 The increased popularity of the surfactants has also led to an increasing number of studies of their physicochemical properties, which have been found to differ considerably from those of the nonionic ethoxylated surfactants.3-6 In most practical applications, surfactants are used in formulations comprising a mixture of several different compounds. The behavior of a mixture is often very different than that of a pure surfactant; commonly synergistic effects in the properties are observed.7,8 For this reason, the study of mixed systems has attracted more attention.9-11. The properties of mixed surfactant systems have been extensively studied by several techniques, such as surface tension,12,13 fluorescence,14 light-scattering,15 small-angle neutron scattering (SANS),16 adsorption at * Corresponding surfchem.kth.se.
E-mail: [email protected]
(1) Balzer, D. Tenside, Surfactants, Deterg. 1991, 28, 419. Balzer, D. Tenside, Surfactants, Deterg. 1996, 33, 102. (2) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996. (3) von Rybinski, W. Curr. Opin. Colloid Interface Sci. 1996, 1, 587. Nilsson, F. INFORM 1996, 7, 490. (4) Platz, G.; Thunig, C.; Po¨licke, J.; Kirchoff, W.; Nickel, D. Colloids Surf., A: Physicochemical and Engineering Aspects 1994, 88, 113. Platz, G.; Po¨licke, J.; Thunig, C.; Hofmann, R.; Nickel, D.; von Rybinski, W. Langmuir 1995, 11, 4250. (5) Balzer, D. Langmuir 1993, 9, 3375. (6) Nickel, D.; Nitsch, C.; Kurzendo¨rfer, P.; von Rybinski, W. Prog. Colloid Polym. Sci. 1992, 89, 249. (7) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989. (8) Rosen, M. J. Prog. Colloid Polym. Sci. 1994, 95, 39. (9) Hill, R. M. In Mixed Surfactant Systems; Ogiono, K. Abe, M., Eds.; Surfactant Science Series 46; M. Dekker: New York, 1993. (10) Holland, P. M. In Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; p 31. (11) Kronberg, B. Curr. Opin. Colloid Interface Sci. 1997, 12, 456.
the solid/liquid interface,17,18 electrophoresis,19 and selfdiffusion NMR.20,21 However, only a few papers have dealt with the mixtures of alkylglycosides with other surfactants.21-25 Critical micelle concentrations (cmc) and the composition of mixed micelles (xi) can be calculated considering ideal mixing of the components.26 However, the ideal model does not account for mixtures where the surfactants are dissimilar.27 When ionic surfactants are mixed with nonionic surfactants, the electrostatic interactions between the headgroups become significant and the mixture deviates from ideality. The nonideal model, developed by Rubingh28 and based on the regular solution theory, introduces a fitting parameter, the so-called interaction (12) Funasaki, N.; Sakae, S. J. Phys. Chem. 1979, 83, 2471. Hoffmann, H.; Po¨ssnecker, G. Langmuir 1994, 10, 381. Janczuk, B.; Gonza´lez, M. L.; Bruque, J. M.; Dorado-Calasanz, C. Tenside, Surfactants, Deterg. 1996, 33, 379. Desai, T. R.; Dixit, S. G. J. Colloid Interface Sci. 1996, 177, 471. (13) Carrio´n-Fite´, F. J. Tenside, Surfactants, Deterg. 1985, 22, 225. (14) Turro, N. J.; Kuo, P.-L.; Somasundaran, P.; Wong., K. J. Phys. Chem. 1986, 90, 288. Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (15) Nishikido, N. J. Colloid Interface Sci. 1987, 120, 495. (16) Bucci, S.; Fagotti, C.; Degiorgio, V.; Piazza, R. Langmuir 1991, 7, 824. (17) Hey, H. J.; Mctaggart, J. W.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1986, 82, 805. (18) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177, 222. (19) Zhang, H.; Dubin, P. J. Colloid Interface Sci. 1997, 186, 264. (20) Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (21) Griffiths, P. C.; Stilbs, P.; Paulsen, K.; Howe, A. H.; Pitt, A. R. J. Phys. Chem. B 1997, 101, 915. (22) Hines, J. D.; Thomas, R. K.; Garrett, P. R.; Rennie, G. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 9216. (23) Kameyama, K.; Muroya, A.; Takagi, T. J. Colloid Interface Sci. 1997, 196, 45. (24) Arai, T.; Takasugi, K.; Esumi, K. J. Colloid Interface Sci. 1998, 197, 94. (25) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103. (26) Clint, J. H. J. Chem. Soc. 1975, 71, 1327. (27) Nagarajan, R. Langmuir 1985, 1, 331. (28) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1.
10.1021/la9804177 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/10/1999
2302 Langmuir, Vol. 15, No. 7, 1999
parameter, β, which quantifies the degree of interaction between the components. Most hydrocarbon-hydrocarbon surfactant mixtures exhibit favorable interactions, mainly due to electrostatic interactions. These are most significant in the case of cationic/anionic mixtures, followed by anionic/nonionic, cationic/nonionic, and nonionic/nonionic mixed systems.28 In this paper, we have first analyzed the micellization of the anionic surfactant sodium dodecyl sulfate (SDS) with four different alkylglycosides: β-decylglucoside (βC10G), β-decylmaltoside (β-C10M), β-dodecylglucoside (βC12G), and dodecylmaltoside (C12M). A comparison of the behavior of these mixed systems allowed us to analyze (i) the effect of the presence of a second glucose unit in the glycosidic nonionic headgroup and (ii) the effect of increasing the alkyl chain length of the glycosidic nonionic surfactant with two more methylene groups. Second, we studied the effect of the headgroups on mixed micellization with alkylglycosides by also looking at mixtures of β-C10G with the cationic surfactant dodecyltrimethylammonium bromide (DTAB) and the nonionic surfactant dodecylheptaethylene glycol ether (C12E7). Finally, β-C10G was mixed with β-decylmaltoside (β-C10M) to investigate the effect of mixing two glycosidic surfactants with different headgroups. The interactions in the mixture were studied by analyzing the variation of cmc with the mole fraction of the nonionic surfactant in the mixture, as determined from the surface tension values. The differences found are explained on the basis of the higher hydrophobicity of the surfactants, electrostatic interactions between the headgroups, and better packing of the surfactants in the micelles. Experimental Section SDS (purity >98%) was supplied by BDH; the alkylglycosides β-C10G, β-C12G, C12M (a mixture of R and β anomers), and β-C10M (purity >99%) were supplied by Fluka. These alkylglycosides are referred to as β-CnGm, where “n” is the number of C atoms in the alkyl chain and “m” is the number of glucose units in the headgroup (CnG1dCnG and CnG2dCnM). The nonionic surfactant C12E7 was supplied by Nikkol Chemicals and DTAB by Sigma (purity >99%). All the surfactants were used without further purification. All the solutions were prepared using MilliQ water and left for at least 2 h before carrying out the measurements. Surface tension was measured with a KSV Sigma 70 instrument using the du Nou¨y ring, with a Methrom Dosimat titration unit attached. The surface tension (γ) vs concentration (overall surfactant concentration) plots were obtained by titration with a concentrated stock solution of the mixture at a fixed nonionic to ionic surfactant molar ratio (R). The measurements were performed in glass beakers cleaned in bichromate sulfuric acid and rinsed in double distilled water. All measurements were performed at room temperature. At least 20 determinations were carried out for each measurement, observing a constancy of the surface tension value with time at all concentrations. The ζ-potential of β-C10G adsorbed on hydrophobized silica was obtained by measurement of electroosmosis induced at a flat plate of hydrophobized silica. Experimental details for the preparation of silica29 and measurements of electroosmosis are given elsewhere.30
Sierra and Svensson
tant system under the pseudophase separation model, an equation can be derived that relates the cmc of the mixture to the cmc and activity coefficients of each of the pure surfactants.
R 1 1-R ) + cmc f1cmc1 f2cmc2
Subscript 1 denotes the nonionic surfactant (β-CnGm), while subscript 2 refers to the other surfactant in the mixture, f1 and f2 are the activity coefficients for the pure surfactants in the mixture. The critical micelle concentration of the mixture is denoted cmc; the cmc values of the pure surfactants are denoted cmc1 and cmc2, respectively, and R is the bulk mole fraction of the nonionic glycosidic surfactant in the solution. When both surfactants mix ideally, the activity coefficients are equal to unity. This is normally the case when there is no electrostatic contribution in the interaction between both surfactants,8 namely, in mixtures of nonionic surfactants and mixtures of surfactants with the same headgroup and different alkyl chain lengths.32 In other cases, the activity coefficients differ from unity and it is necessary to know f1 and f2 in order to solve eq 1. Using the regular solution approximation for nonideal mixtures of liquids, it is possible to deduce an expression for the activity coefficients as a function of the mole fraction of the mixed micelle, x1, and a fitting parameter, β.28,31 The β parameter, so-called interaction parameter, represents the departure from ideality of the mixture and takes into account the interaction energies between the single monomers (wii) and the monomers of the two surfactants forming the mixed systems (wij), thus being β ) (w11 + w22 - 2w12)/kT. Therefore, the value of β will give an idea of the degree of interaction between the monomers in the micelle. A negative value indicates net attraction between the monomers in the mixed micelle, a positive value net repulsion, such as some hydrocarbon and fluorocarbon mixtures.33 In this paper, only the micelle formation of the mixture has been analyzed, and we therefore refer to βm as the interaction parameter. It should be kept in mind that the nonideal model assumes that the formation of mixed micelles follows the regular solution theory. This means that the excess entropy of mixing is equal to zero, hence, the deviation from ideality is only due to enthalpic effects. For this reason, the theory is not sufficient to explain heat and counterion effects.31 In general, synergism is defined as an improvement in a certain property compared to that attained by either of the pure surfactants.10 Some authors consider the existence of synergism in a mixed system to be when its behavior departs from ideality. The point of maximum synergism then corresponds to the mole fraction where it deviates the most from the ideal behavior.34 In this paper, we have assumed the first definition. The conditions for the existence of synergism in various fundamental interfacial phenomena, e.g., efficiency and effectiveness in the reduction of the surface tension and mixed micelle formation, have been derived mathematically, based also
Results and Discussion The experimental data were fitted to the nonideal model for the mixed micelle formation developed by Rubingh.28,31 Considering the micellization process of a binary surfac(29) Jo¨nsson, U.; Ivarsson, B.; Lunstro¨m, I.; Berghem, L. J. Colloid Interface Sci. 1982, 90, 148. (30) Burns, N. L. J. Colloid Interface Sci. 1996, 183, 249. (31) Holland, P. M. Adv. Colloid Interface Sci. 1986, 26, 111.
(32) Motomura, K.; Yamanaka, M. Aratono, M.; Colloid Polym. Sci. 1984, 262, 948. Takiue, T.; Tornioka, M.; Ikeda, N.; Matsubara, A.; Aratono, M.; Motomura, K. Colloid Polym. Sci. 1996, 274, 470. (33) Mukerjee, P. Colloids Surf., A: Physicochemical and Engineering Aspects 1994, 84, 1. Ben-Ghoulam, M.; Moatadid, N.; Graciaa, A.; Marion, G.; Lachaise, J. Langmuir 1996, 12, 5048. Ravey, J. C.; Gherbi, A.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1989, 79, 272. de Lisi, R.; Inglese, A.; Milioto, S.; Pellerito, A. Langmuir 1997, 13, 192. (34) Jost, F.; Leiter, H.; Schwuger, M. J. Colloid Polym. Sci. 1988, 266, 554.
Mixed Micelles Containing Alkylglycosides
Langmuir, Vol. 15, No. 7, 1999 2303
Figure 1. Surface tension (γ) vs log c (overall surfactant concentration, mmol dm-3) for the β-C10G/SDS mixtures at different mole fractions of the nonionic β-C10G in the mixture (R1): (b) R ) 0; (3) R ) 0.1; (O) R ) 0.3; (4) R ) 0.5; (]) R ) 0.7; (0) R ) 0.9; (9) R ) 1.
Figure 2. Variation of the cmc vs the mole fraction of the nonionic β-C10Gm in the solution (R1) for the β-C10Gm/SDS mixtures: (b) β-C10G/SDS; (9) β-C10M/SDS; (O) β-C10G/SDS with recrystallized SDS.
upon the nonideal model.35 The two conditions which a mixture must obey in order to exhibit synergism in mixed micelle formation are (1) an attractive interaction between the surfactants, βm