Estimation of lateral species separation from phase transitions in nonideal two-dimensional lipid mixtures.

The two-dimensional statistical mechanical equations necessary to analyze the phase diagram of a nonideal lipid mixture are derived. The Prigogine approximation of the combinatorial term is used in the maximum term of the partition function which contains two parameters that describe the degree of nonideality in the liquid crystal and gel phases. These parameters are used to calculate the mole fraction of like-like and like-unlike lipid interfaces which describe the lateral species separation of like lipids in each phase. Data for several temperatures and compositions of the DMPC/DPPC, DPPC/DSPC, and DMPC/DSPC mixtures are analyzed.