Thermodynamic equation of state for cholesteryl esters in surface phases

Phase behavior at the air-water interface for binary mixtures of cholesteryl esters with several phospholipids was determined and combined with a large body of data from other similar studies. Analysis of areas along the phase boundary between the monolayer phase and either a double layer or a bulk phase revealed a simple, additive relationship between lipid components. This indicates that at its limits the monolayer phase behaves as an ideal mixture of components with fixed partial molecular areas. This concept is supported by the independence of the values of the partial molecular areas from the nature of the other lipid component. General agreement with the model was also found for data from the double-layer-bulk phase boundary. However, in contrast to the monolayer phase boundary, the double-layer phase behaves as if comprised of components with not only fixed but also identical partial molecular areas. The lack of a surface pressure dependency for partial molar areas apparently contradicts earlier thermodynamic descriptions of surfaces. However, satisfactory resolution with theory can be obtained if along the phase boundary each lipid species has a stoichiometric number of water molecules associated with it. This concept is supported by the consistency and values of the parameters calculated from the data by using this model. In both phases, the activity coefficient of interfacial water was constant, and the cholesteryl esters were relatively more hydrated than the other lipids. Overall, this thermodynamic equation of state for phase boundaries provides the basis for modeling the surfaces of natural, cholesteryl ester rich lipid inclusions like lipoproteins and arterial lipid deposits.