Prediction of Equilibrium Surface Tension and Surface Adsorption of Aqueous Surfactant Mixtures Containing Ionic Surfactants

Abstract

We report results of a theoretical study of the adsorption of mixtures of ionic and nonionic surfactants at the aqueous solution−air interface. A surface equation of state is developed by treating the adsorbed surfactant molecules as a two-dimensional gaslike monolayer consisting of hard disks interacting through attractive van der Waals interactions and repulsive electrostatic interactions. The hard-disk areas are calculated using known bond lengths and angles in the case of surfactants having compact hydrophilic heads and a Monte Carlo simulation approach in the case of surfactants having flexible, polymer-like hydrophilic heads. Attractive van der Waals interactions between the surfactant hydrocarbon tails are treated as a purturbation to the hard-disk repulsions using an expansion in surfactant surface concentration truncated at second order. The corresponding second-order virial coefficients are calculated using detailed molecular information about the surfactant species present in the monolayer. Ele...