Equilibrium thermodynamics of lipid monolayer domains

Coexisting thermodynamic phases of lipid molecules at the air-water interface exhibit domains with a variety of sizes and shapes. Both the sizes and shapes of these domains are affected by a competition between a line tension and long-range dipole-dipole repulsion. In the case of coexisting liquid phases, line tension favors large, circular domains, whereas the dipolar repulsions favor small and/or highly elongated domains. The present work discusses the thermodynamics of the equilibrium size and shape of these domains. It is shown that whereas there is only one stable equilibrium domain size, there is an infinite number of sizes that represent metastable equilibrium at different monolayer pressures. Circular domains that are far enough apart as to not interact with one another electrostatically have the same radii under this condition of metastable equilibrium. It is shown that, in principle, the metastable equilibrium radius reached in a monolayer can depend on the initial distribution of nonequilibrium domain sizes. Domains formed in multicomponent lipid mixtures are shown to have lipid compositions in the vicinity of the domain boundaries that are different from the bulk lipid compositions. These composition variations tend to stabilize domains with different radii under conditions of metastable equilibrium.