Complementarity and clustering in a simple model mixed bilayer.

A bilayer of uniform thickness containing a mixture of long and short lipids is simulated using a parallel hard-rod model to illustrate the effect of transbilayer repulsions between the tails of the long component. Monte Carlo simulations show considerable entropy-driven clustering within each layer. Demixing reaches a maximum at the highest packing fraction of the liquid state and decreases as the system orders. The formation of complementary clusters of long and short rods on opposite sides of the bilayer increases translational freedom within each cluster by reducing constraints imposed by the opposing leaflet, an effect that becomes less important as rods lock into facing hexagonally ordered arrays.