  Using binary hard disk mixture models, we studied the two-dimensional
ordering of bacteriorhodopsins in a lipid bilayer. The phase diagrams were
calculated using the thermodynamic perturbation theory. We examined two types
of effective interactions to discuss the lateral depletion effects caused by
repulsive core interaction between lipid molecules. The results indicate that
the core repulsions drastically broaden the coexistence region for the
fluid--ordered phase.
