(A) Free energy per lipid as a function of the area per headgroup, a, for a mixed lipid bilayer with ns = 16 and xl = 0.50. (B) Free energy per lipid as a function of the concentration, xl, of the longer lipid. For each composition, the values of the free energy correspond to the tensionless membrane, i.e., the area at which the free energy is minimal (see A). The different curves correspond to different short lipids, as denoted in the figure. The stars show the compositions of coexistence between the two lipid layers.