Complex formation equilibria between cholesterol and diosgenin analogues in monolayers determined by the Langmuir method.

The purpose of this study was to investigate the interaction between diosgenin analogues [DioA: diosgenin acetate (DAc) and (25)-5α,6β-dihydroxyspirostan-3β-ol acetate (DSol)] and cholesterol (Ch) monolayers at the air/water interface. The surface tension of pure and mixed lipid monolayers at 22 °C was measured by using the Langmuir method with a Teflon trough and a Nima 9002 tensiometer. The surface tension values were used to calculate the π-A isotherms and to determine the molecular surface areas. The interactions between Ch and each DioA resulted in significant deviations from the additivity rule. The theory described in this work was used to determine the stability constants, the areas occupied by one molecule of Ch-DAc or Ch-DSol, and the complex formation energy (Gibbs free energy) values.