Analysis of the surface diffusion of alkylbenzenes and p-alkylphenols in reversed-phase liquid chromatography using the surface-restricted molecular diffusion model.

Surface diffusion of alkylbenzene and p-alkylphenol derivatives was measured in reversed-phase liquid chromatography on a C18-silica column with a methanol/water mixture (70/30, v/v) as the mobile phase. They were analyzed on the basis of the restricted molecular diffusion model for surface diffusion. The temperature dependence of the surface diffusion coefficient (Ds) arises probably from that of molecular diffusivity, suggesting a correlation between surface and molecular diffusion. Other correlations were also observed: (1) an enthalpy-entropy compensation of the retention equilibrium constant, (2) a linear free energy relationship between the retention and surface diffusion, and (3) a linear correlation between the restriction energy of surface diffusion and the isosteric heat of adsorption. The physical meaning of parameters involved in the surface-restricted molecular diffusion model is discussed on the basis of these results. A practical and convenient procedure for the estimation of Ds is also suggested. This study demonstrates how surface diffusion phenomena can be accounted for on the basis of the correlation between surface and molecular diffusion.