The phase transition in amphiphilic monolayers: isotherms in the cluster variation method

Abstract

The liquid-expanded (LE)-liquid-condensed (LC) phase transition in monolayers of amphiphilic molecules is investigated in the pair approximation of the cluster variation method. The model analysed is the model proposed by Firpo et al. (1984) which can be mapped into the Blume-Emery-Griffiths Hamiltonian. The signatures of the LE-LC transition are searched for as break points on the isotherms of the surface pressure-molecular area diagram. The results obtained in the pair approximation are improved with respect to those obtained in the Bragg-Williams approximation for a set of typical energy and entropy parameters in the case of pentadecanoic acid. In particular, the LE-LC phase transition is shown to be weakly first order. A comparison with experimental results gives satisfactory agreement.