Landau theory for the gas-liquid transition in a monomolecular layer of amphiphilic molecules

We propose a general framework for the study of the first-order transition in monolayers of amphiphilic molecules. We introduce the variation of internal molecular parameters versus density and temperature. We obtain an asymmetry of the coexistence curve with a (Tc-T) power law. With a simple model we find results in good qualitative agreement with the experimental results of Won Kim and Cannell. In conclusion, we examine also the coexistence curves of monolayer with two polar-head molecules.