Monte Carlo calculations for vapor-liquid phase equilibria in Langmuir monolayers

Configurational-bias Monte Carlo simulations in the Gibbs ensemble have been carried out to determine the vapor-liquid coexistence curve for a pentadecanoic acid Langmuir monolayer. Two different force fields were studied: (i) the original monolayer model of Karaborni and Toxvaerd including anisotropic interactions between alkyl tails, and (ii) a modified version of this model which uses an isotropic united-atom description for the methylene and methyl groups and includes dispersive interactions between the tail segments and the water surface. The calculated phase diagram for the Karaborni and Toxvaerd force field deviates significantly from the experimental observations, but the modified model gives a more quantitative description. In both cases, the shape of the coexistence curve can be fitted using the critical exponent of the three-dimensional Ising model. The two force fields yield qualitatively different structural properties.