Statistical thermodynamics of flexible-chain surfactants in monolayer films: Mixtures of different chain lengths

Recently, a simple mean-field theory was developed to describe structural and thermodynamic properties of fluid phases in monolayer films of flexible-chain surfactants [J. Chem. Phys. 91, 4423, 4431 (1989)]. This approach is extended here to examine films of mixed chain length. The high-density (LC-LE) phase transition predicted for the single component monolayers is found to exhibit nonideal mixing behavior, ranging from positive azeotropy to phase separation (in the LC phase) as the difference in chain lengths is increased. In contrast, the phase diagram in the LE-G transition region is predicted to exhibit almost ideal behavior, with a nearly linear bubble-point curve.