The two-step collapse of mixed monolayers of cholesterol and its esters.

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Mixed monolayers of two insoluble components out of cholesterol and its acetate and propionate were spread on a dilute aqueous solution of disodium hexadecyl phosphate. The curves of the surface pressure against the molecular area were found to have two kink points, collapse points, although the curves of each component, and also that of the "concollapse" mixture, had only one collapse point. The diagram showing the relation between these two collapse pressures and the composition was entirely analogous to the phase diagrams of bulk mixtures which have an eutectic point. It has, therefore, been concluded that the mixed monolayer of two mutually immisible substances undergoes a two-step collapse. At the lower collapse point, one of the two components, i, starts separating from the monolayer. The lower collapse pressure, FCl′ varies with the mole fraction, Xi, according to FCl′=Fol′−(kT⁄Aol)lnXi. At the higher collapse point, the other component starts separating as well. Upon further compression, the surface pressure remains unchanged at the higher collapse pressure, which is equal to the equilibrium-spreading pressure and is independent of the composition.
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