Interactions in Dilute Monolayers of Long-Chain Ions at the Interface between n-Heptane and Aqueous Salt Solutions

We present a theory for the electrostatic repulsions among ionic head groups of surfactants localized at the interface between oil and aqueous salt solutions. We follow two approaches: (i) a virial expansion, first explored by Vrij, and (ii) electrical double layer theory, introduced by Davies. The virial expansion, even with several refinements, predicts lateral pressures which are generally much too high when compared to recent data of Mingins et al. The Gouy-Chapman theory also predicts pressures which are too high. It is shown, however, that when the double layer theory is corrected for the discrete ion effect (ion self-energies) the theory predicts the data well for all salt concentrations.