An AFM study of lipid monolayers. 2. Effect of cholesterol on fatty acids

In this study the effect of cholesterol in Langmuir-Blodgett monolayers of fatty acids of varying chain lengths was investigated by atomic force micro scopy (AFM). Domain formation due to lateral ph ase separation was studied at different lipid compositions and surface pressures. A small amount of ch olesterol is miscible with palmitic acid (C16:0) and forms a flat monolayer while excess cholesterol forms a rougher cholesterol-rich phase. No miscib ility was observed in monolayers of lignoceric acid (C24:0) and cholesterol . For the ternary mixed monolayer (palmitic acid, lignoceric acid, and chol esterol) the two fatty acids formed separate domains and the miscibility of cholesterol in the two phases showed behavior corresponding to that of the binary fatty acid-cholesterol systems. From the shape, size, and height di fferences of the domains one can conclude that the driving force to minimiz e the interfacial length between different phases is reduced in the presenc e of cholesterol. This can be attributed to line active properties of chole sterol.