Phase behavior and induced interdigitation in bilayers studied with dissipative particle dynamics

Bilayers formed by coarse-grained models of amphiphilic surfactants are studied using dissipative particle dynamics combined with a Monte Carlo scheme to achieve the natural state of a tensionless bilayer. We address the issue of the influence of the molecular structure and the level of coarse graining on the bilayer properties by studying two models of different complexity: a single tail and a double tail surfactant. We compute the area per surfactant, the bilayer thickness, and the orientational order parameters and show how their dependence on the surfactant structure and temperature. We reproduce the gel to liquid crystalline phase transition, and we study the conditions that induce a gel interdigitated phase in the bilayer. We show how the interdigitated state in a bilayer containing double-tail surfactants can be reproduced by adding an extra bead in the surfactant headgroup, which mimics an ester-linkage to the phosphate group. In a bilayer formed by single tail surfactants, we induce interdigitation by changing the strength of the repulsive interaction between the headgroups, which corresponds with adding salt to the system. We are then able to derive a phase diagram as function of temperature and repulsion parameter for surfactants of different chain lengths.