Miscibility critical pressures in monolayers of ternary lipid mixtures

When phospholipids are mixed with cholesterol in a monolayer at an air-wate r interface, coexisting 2-dimensional liquid phases can be observed if the surface pressure, pi, is lower than the miscibility critical pressure, pi(C ). Ternary mixtures of two phospholipid species with dihydrocholesterol hav e been reported to have critical pressures that are linearly proportional t o the relative composition of the phospholipids, However, we report here th at, if the acyl chains of the two phospholipids differ significantly in len gth or unsaturation, the behavior is markedly different. In this case, the critical pressure of the ternary mixture can be remarkably high, exceeding the critical pressures of the corresponding binary mixtures. High critical pressures are also seen in binary mixtures of phospholipid and dihydrochole sterol when the two acyl chains of the phospholipid differ sufficiently in length. Using regular solution theory, we interpret the elevated critical p ressures of these mixtures as an attractive interaction between the phospho lipid components.