Molecular thermodynamic modeling of specific ion effects on micellization of ionic surfactants

Specific ion effects are ubiquitous in biological and colloidal systems. The addition of electrolytes to ionic surfactant solutions has pronounced effects on micellar properties, such as critical micelle concentration (cmc), micellar size, and shape. Ions play an important role in colloid stability and aggregation behavior of ionic surfactant solutions. Despite extensive experimental data, there is no well established molecular theory on specific ion effects. Published molecular thermodynamic theories for ionic surfactants do not properly account for ion-specific effects such as the inversion of the lyotropic series for the cmc of alkyl sulfates and carboxylates. In this work, we present a molecular thermodynamic theory for ionic surfactant solutions to take into account the headgroup-counterion specificity and address ion-specific effects on the cmc and aggregation number. We assume that the charged headgroup and the counterion at the Stern layer form solvent-shared ion pair with different degrees of cosphere overlap. The thickness of the Stern layer is estimated from molecular structures of hydrated surfactant heads and hydrated counterions, and from the knowledge of the qualitative strength of headgroup-counterion interaction in line with Collins' concept of matching water affinities. Our proposed thermodynamic model properly predicts the cmc of both anionic and cationic surfactants of various counterions, and the effect of different inorganic salts on micellization of ionic surfactants.