ADSORPTION-ISOTHERMS OF CHOLESTEROL AND RELATED-COMPOUNDS IN NONAQUEOUS REVERSED-PHASE CHROMATOGRAPHIC SYSTEMS

The adsorption isotherms of cholesterol, cholesteryl acetate. cholesteryl formate and cholestanone were measured on two different chemically bonded C18 silica columns, using acetonitrile-dichloromethane or acetonitrile-n-hexane as mobile phases. In all instances the experimental isotherms are nearly but not extactly linear, exhibit significant deviations from Langmuir isotherm behavior and are fairly well described by a three-parameter quadratic equation. However, the best representation of the experimental data is obtained with a four-parameter quadratic isotherm. A model was worked out to account for these isotherms and to attempt an explanation of the physical meaning of the isotherm parameters on the basis of the limited solubility of the compounds studied in the bulk liquid phase, of two-layer adsorption and of an association between the sorbed molecules. A detailed study of the dependence of the isotherm coefficients on various experimental parameters (e.g., type and specific surface area of the C18 phase, type and concentration of the solvents in the mobile phase) gave results in qualitative agreement with the model.