THERMODYNAMIC STUDY ON PHASE-TRANSITION IN MIXED MONOLAYERS OF CHOLESTEROL, LECITHIN AND LITHOCHOLIC ACID

For mixed monolayers of cholesterol, lecithin and lithocholic acid the surface pressure has been measured as a function of molecular area at various compositions and temperatures. The liquid-expanded (LE)/liquid-condensed (LC) phase transition takes place over a wide range of monolayer compositions. The cholesterol monolayers are of the condensed type, but they become gradually expanded as the composition of lecithin or lithocholic acid increases. As expected the transition surface pressure increases as temperature increases. From the temperature dependence of the transition surface pressure, the apparent molar entropy, enthalpy and internal energy associated with the phase transition have been calculated. The contribution of lecithin and lithocholic acid molecules to the phase transition are not equivalent. The condensing effect of cholesterol on lithocholic acid is more pronounced than that of lecithin on the bile acid.