Out-of-plane phase segregation and in-plane clustering in a binary mixture of amphiphiles at the air-water interface

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Abstract
We have carried out extensive Monte Carlo simulations of a microscopic lattice model of a binary mixture of amphiphilic molecules, of two different lengths, in a system where water is separated from the air above it by a sharp well-defined interface. We have demonstrated an entropy-driven phase segregation in a direction perpendicular to the air-water interface when the initial total surface density of the amphiphiles is sufficiently high. We have also investigated (a) the conformations of the amphiphiles, (b) the distribution of the sizes of the clusters of monomers belonging to the long amphiphiles as well as of those belonging to the short amphiphiles in planes parallel to the interface, (c) the effects of varying the lengths, total concentration, and the ratio of the numbers of the two types of the amphiphilic molecules as well as those of varying the strength of the intermonomer interactions and temperature. We have critically examined the interpretations of some recent experimental results in the light of our observations.