SURFACE DIPOLE DENSITIES IN LIPID MONOLAYERS

Surface dipole density differences, mu, between coexisting liquid phases in lipid monolayers have been determined from an analysis of the Brownian motion of trapped lipid domains, as well as from surface potential measurements on each liquid phase. The coexisting liquid phases arise in binary mixtures of dimyristoylphosphatidylcholine (DMPC) with cholesterol or with dihydrocholesterol, Values of mu determined from Brownian motion, surface potential measurements, and measurements of domain electrophoretic mobility provide support for the theoretical equivalent dipole model used previously to describe these monolayers. Air oxidation and ozonolysis of cholesterol in monolayers is detected through the effect of an oxidation product-mediated reduction of interdomain line tension. It is shown that a cholesterol oxidation product, cholestenone, is strongly line active, reducing the line tension between liquid domains relatively rich in cholesterol and liquid domains relatively rich in DMPC.