CRITICAL PRESSURES IN MULTICOMPONENT LIPID MONOLAYERS

Epifluorescence microscopy has been used previously to study coexistin g liquid phases in lipid monolayers of dihydrocholesterol and dimyrist oylphosphatidylcholine at the air/water interface. This binary mixture has a critical point at room temperature (22 degrees C), a monolayer pressure of approx. 10 mN/m, and a composition in the vicinity of 20-3 0 mol% dihydrocholesterol. It is reported here that this critical pres sure can be lowered, raised, or maintained constant by systematically replacing molecules of this phosphatidylcholine with molecules of a ph osphatidylethanol amine, or an unsaturated phosphatidylcholine, or mix tures of the two, while maintaining the dihydrocholesterol concentrati on at 20 mol%. Thus, even complex mixtures of lipids may be characteri zed by a single, well-defined second-order phase transition. In princi ple, such transitions might be found in biological membranes.