Strongly Correlated Rafts in Both Leaves of an Asymmetric Bilayer

I consider a model of a bilayer characterized by two order parameters, one in each leaf. That of the inner leaf represents the difference in mole fractions of lipids with large spontaneous curvature, phosphatidylethanolamine (PE), and those with small spontaneous curvatures, phosphatidylcholine (PC) and phosphatidylserine (PS). Similarly, the order parameter in the outer leaf represents the difference in mole fractions of lipids with small spontaneous curvature, PC, and large curvature, sphingomyelin (SM). Each order parameter is coupled to the variations in the height of the membrane that is assumed to be of constant thickness. The couplings are of different strength. I show that, with reasonable parameters, a microemulsion is formed in each leaf, and that the two microemulsions are strongly coupled. Their characteristic size of domains is found to be on the order of 75 nm. In this picture, rafts consist of regions of SM in the outer leaf and PC and PS in the inner leaf, floating in a sea of PC in the outer leaf and PE in the inner leaf. I argue that microemulsions have been observed, but not identified as such, in model systems.