A study of the miscibility of bile components in mixed monolayers at the air-liquid interface I. Cholesterol,lecithin, and lithocholic acid

Experimental isotherms, obtained by compressing monomolecular layers formed by a number of compounds involved in gallstone formation (cholesterol, L-α-phosphatidylcholine, and lithocholic acid), are studied. Mixed monolayers consisting of pairs of these substances are also analyzed. The additivity rule for molecular areas is used for characterizing the possible interactions between such components. Applying the phase rule to the surface systems studied in conditions of extended/condensed liquid equilibrium allows the miscibility of the components in both phases to be determined. Some hypotheses are given regarding the kind of molecular rearrangements that take place when each system suffers a phase transformation. The general conclusion reached is that, at the experimental conditions used (pH=6.00 and T =298 K), significant interactions occur between the molecules of the compounds studied.