Molecular Interactions of Surfactants in Mixed Monolayers at the Air/Aqueous Solution Interface and in Mixed Micelles in Aqueous Media: The Regular Solution Approach

Abstract

Experimental area/molecule data (Aexpt) at the air/aqueous solution interface after mixing, ideal mixing area/molecule data (Aideal), based upon area/molecule data at that interface before mixing, and regular solution theory have been used to explain the values of surfactant molecular interaction (β) parameters observed in mixed monolayers and mixed micelles. The value of the β parameter reflects the difference in surfactant−surfactant interactions before and after mixing. In ionic−nonionic surfactant mixtures, when surfactant−surfactant interactions are weak, reduction in electrostatic self-repulsion interaction energy of the ionic surfactant, due to the dilution effect upon mixing, is suggested as a major contributor to the negative β values observed for mixed monolayers and mixed micelles. Steric effects appear when surfactant molecular structure varies in the size of the headgroups and in the branching of the hydrophobic groups. It is suggested that the effect of an increase in the size of the hydroph...