Critical micelle concentrations (CMC) were obtained from tensiometric studies on several binary surfactant mixtures (anionic-anionic, cationic-cationic, anionic-nonionic, and cationic-nonionic) in water at different mole fractions (0-1). The composition of mixed micelles and the interaction parameter β, evaluated from the CMC data for different systems using Rubingh's theory, are discussed. Marked interaction is observed for ionic-nonionic systems, whereas it is weak in the case of similarly charged surfactants. The influence of counterion valence in the formation of mixed micelles was investigated, and results suggest that in similarly charged surfactant mixtures, the degree of counterion binding does have a major role in deciding the extent of interactions. Salt addition reveals a weakening of interactions in ionic-nonionic systems, and this is attributed to head group charge neutralization and dehydration of the ethylene oxide units of the nonionic surfactants. Cloud point and viscosity data on these systems support the observation.Paper no. S1110 in JSD 2, 213-221 (April 1999). KEY WORDS:Cloud point, mixed micelles, sphere-to-rod transitions, synergism.Adsorption characteristics of surfactants from solution onto different interfaces and the propensity of surfactants to form micelles and mesomorphic phases are useful in almost all practical applications such as foaming, dispersing, solubilizing, wetting, emulsifying and cleansing action (1,2). Owing to their improved action over single pure surfactants, mixed systems like surfactant/surfactant (3,4) or polymer/surfactant (5) are often used in formulations of finished products. It is therefore important to investigate the nature of interactions and factors affecting them in aqueous media so as to understand how these control the product performance. The tendency of different surfactants to form mixed micelles is governed by their attractive (synergistic) or repulsive (antagonistic) interactions and is often explained from the β parameter estimated using Rubingh ' s regular solution theory (6). Extensive studies have been carried out on various mixed surfactant systems like anionic-anionic (7-9), cationic-cationic (10-12), anionic-nonionic (13-15), cationic-nonionic (16,17), cationicanionic (18,19), and nonionic-nonionic (20). Considerable interaction has been reported for ionic-nonionic systems, whereas weak or negligible interaction has been observed for similarly charged surfactants. Interaction between anionic-cationic surfactants is generally very strong but such systems often lead to precipitation/coacervation as a result of the coulombic interactions between oppositely charged species. We report in this paper tensiometric studies on eight mixed systems where results are explained in terms of the β parameter. Critical micelle concentration (CMC) data for some anionic-anionic and cationic-cationic systems from the literature (11,12) are analyzed to compute a β parameter so as to investigate the role of counterion valence in the nature and strength of inte...

The interaction between an anionic surfactant (sodium dodecyl sulfate) and a nonionic surfactant [polyoxyethylene (9.5) octyl phenyl ether] in aqueous salt solution was investigated using the surface tension method. The critical micelle concentration values were determined for the individual surfactants and their corresponding mixtures. The interaction parameter between the surfactants in the mixed micelles, the activity and activity coefficients in the mixed micelles, and the thermodynamic parameters were calculated using various approaches, viz., Clint, Rubingh, and Maeda models. It was observed that the critical micelle concentration of the mixed surfactants system reveals little deviation from ideality.

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Critical micelle concentrations of cetyltrimethylammonium-p-toluene sulfonate (CTAT) and cetylpyridinium chloride (CPC) with sodium cholate (NaC) and sodium deoxycholate (NaDC) were determined in aqueous solutions by surface tension measurements. Interaction parameters and mole fraction of the components in mixed micelles were estimated using Rubingh's theory. Strong interaction was observed for each mixed system, a common feature shown by anionic-cationic mixtures. Dramatic effects on the viscosities of these cationic surfactant-bile salt mixtures were seen, and were markedly dependent upon the counterion of the cationic surfactant and the nature of bile salts. Micelles are small and spherical for cationic surfactants in the presence of NaC. Micelle growth was seen for CPC in the presence of NaDC by an increase in viscosity, but a CTAT solution showed an opposite effect on addition of NaDC. Conductance results supported this view. Different behavior of the two bile salts is explained on the basis of their orientation in cationic micelles. JSD 1, 507-514 (1998). KEY WORDS:Ideal mixing, interaction parameter, mixed micelles, sodium cholate, sodium deoxycholate, synergism.Recently, much interest has been expressed in studying mixed surfactant systems both from basic and applied viewpoints (1,2). Usually, a marked interaction is observed, resulting in an increased cloud point, decreased Krafft point, increased surface activity, and decreased critical micelle concentration (CMC). Each change contributes favorably to practical applications of surfactants. Superiority in the performance of surfactant mixtures compared to single pure surfactants is often attributed to synergistic interactions among surfactants (1). Much research has focused on mixed systems comprised of anionic-anionic (3-6), cationic-cationic (7,8), anionic-cationic (9,10), ionic-nonionic (11), and nonionic-nonionic (12,13) surfactants. The CMC of surfactant mixtures composed of similarly structured ionic surfactants (14) or nonionic surfactants (14) can be predicted by assuming that they obey ideal solution theory in the micellar phase. The surface and bulk properties of aqueous solutions of oppositely charged surfactants and those of ionic and nonionic surfactants are often greater than expected for the other types of mixed systems. Mixed systems composed of anionic and cationic surfactants (15-19) often result in strong coulombic interaction with remarkably lower CMC in mixtures than expected for ideal mixing. This deviation from ideal behavior was successfully explained by Rubingh (20) using the phase separation model of micellization and regular solution approximation. However, in most cases the complex formed is generally insoluble in water (increased Krafft point), limiting the studies and applications of such systems. Although for anionic-cationic combinations a high probability of precipitation through charge neutralization at comparable ratio is present, when one component is in excess, stable mixed micelles are generally formed (21,22).Bile ...