DETECTION OF PHASE-TRANSITIONS IN MONOLAYERS USING RETARDATION OF EVAPORATION OF WATER

The role of cholesterol in inducing the phase transitions in mixed monolayers with arachidyl alcohol and oleic acid has been studied by surface pressure and evaporation resistance measurements of the monolayers. The resistance of the monolayers to water evaporation indicates the effect of molecular interactions in mixed monolayers. The evaporation resistance of monolayers has been found to be a more sensitive parameter for studying the molecular interactions in mixed monolayers than the average area/molecule. A minimum of 15-20 mol% of cholesterol is required in a cholesterol-arachidyl alcohol mixture to completely fluidize the two-dimensional solid alcohol film. The evaporation resistance sharply decreases to a lower value at a molar ratio of 20:80 for cholesterol:arachidyl alcohol due to the phase transition in the monolayer. Cholesterol fluidizes the arachidyl alcohol monolayer by disrupting the association of saturated hydrocarbon chains of the alcohol. In the case of oleic acid-cholesterol mixed monolayers, 75 mol% of cholesterol is required to show a maximum solidifying effect. The evaporation resistance has been found to be maximum at this particular molar mixture. It is proposed that cholesterol molecules occupy the molecular cavities present in two-dimensional liquid oleic acid monolayers and thus reduce the evaporation of water. © 1990.