Physicochemical Studies on the Micellization of Cationic, Anionic, and Nonionic Surfactants in Water-Polar Organic Solvent Mixtures

The effect of cosolvent (ethane-1,2-diol and dimethyl sulfoxide) on the self-assembly of three surfactants, N,N,N-trimethyl-1-dodecanaminium bromide (DTAB), sodium [dodecanoyl(methyl)amino]acetate (SDDS), and polyoxyethylene (20) sorbitan monolaurate (Tween-20) in aqueous solution have been investigated by conductometric, tensiometric, and viscometric techniques at 298 K. The main focus was on the effect of solvent on critical micelle concentration (cmc), free energy contribution to micellization (Delta G(0)(m)), tail transfer Gibbs free energy (Delta G(trans)(0)), Gibbs adsorption energy (Delta G(ads)(0)), and some micellar interfacial parameters, for example, Gibbs surface excess (Gamma(max)), minimum area per surfactant molecule (A(min)), surface pressure (Pi(cmc)), and pC(20)(= -log(C-20), where C-20 is the surfactant molar concentration required to reduce the surface tension of mixed solvent by 20 mN m(-1)). With increasing concentration of cosolvent in the binary mixture, the cohesive force decreases, and surfactant molecules are more soluble in mixed solvent. As a result, micellization process becomes less favorable, and an increase in cmc was obtained. Steady state fluorescence spectroscopy was used to determine the aggregation number (N-agg) of the surfactants in organic solventwater binary mixture and also the micropolarity of the mixed solvent. It was observed that Nagg decreased with the increase of organic solvent concentration. The micropolarity of the mixed solvent and packing parameter (P) were also determined.