Molecular Interactions of Cationic and Anionic Surfactants in Mixed Monolayers and Aggregates

Abstract

The properties of anionic-rich and cationic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were investigated with conductometry and surface tension measurements and by determining the surfactant NMR self-diffusion coefficients. The critical aggregate concentration (CAC), surface tension reduction effectiveness(gamma(CAC)), surface excess(Gamma(max)), and mean molecular surface area (A(min)) were determined from plots of the surface tension (gamma) as a function of the total surfactant concentration. The compositions of the adsorbed films (Z) and aggregates (chi) were estimated by using regular solution theory, and then the interaction parameters in the aggregates (beta) and the adsorbed film phases (beta(sigma)) were calculated. The results showed that the synergism between the surfactants enhances the formation of mixed aggregates and reduces the surface tension. Further, the nature and strength of the interaction between the surfactants in the mixtures were obtained by calculating the values of the following parameters: the interaction parameter, beta, the size parameter, rho, and the nonrandom mixing parameter, P*. These results indicate that in ionic surfactant mixtures the optimized packing parameter has the highest value and that the size parameter can be used to account for deviations from the predictions of regular solution theory. It was concluded that, for planar air/aqueous interfaces and aggregation systems, this nonideality increases as the temperature increases. This trend is attributed to the increased dehydration of the surfactant head groups that results from increases in temperature. Further, our conductometry measurements show that the counterion binding number of mixed micelles formed in mixtures with a high CTAB content is different to those with a high SDS content. This difference is due to either their different aggregation sizes or the different interactions between the head groups and the counterions.

Specific ion effects on the micellization of aqueous mixed cationic/anionic surfactant systems with various counterions

“…Mixed micellization of cationic/anionic surfactants has attracted increasing interest over the past 20 years due to the strong synergism arising from the strong electrostatic attraction between the oppositely charged headgroups [36][37][38][39][40][41][42][43][44][45]. Usually the micellar compositions of the mixed cationic/anionic surfactants at cmc are nearly equimolar over a wide range of mixing ratios [46][47][48][49], the addition of inorganic salt has little or no effect on the cmc of these mixed cationic/anionic surfactant systems.…”

Electrolyte effect on adsorption and the phase transition from microstructures to nanostructures in ionic/ionic surfactants mixture

“…We found that, by using this model, we could explain the experimentally observed behavior in terms of the free energy contributions to mixed micelle formation. We investigated the adsorption properties of binary surfactant systems containing the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solutions [20]. Also, we determined the mixed micelle and adsorption properties of binary surfactant systems containing the cationic and nonionic surfactants in the presence of various concentrations of sodium bromide [21].…”

Recent Strategies in the Development of Catanionic Vesicles

“…The size, charge, and permeability of the catanionic vesicles can be further adjusted by varying the temperature, concentration, molar ratio, and chain length of the cationic and anionic surfactants 15 . Moreover, catanionic vesicles can efficiently encapsulate solutes and retain the molecules over long periods 16,17 .…”