Surface Diffusion of Alkylbenzenes on Octadecylsilyl−Silica Gel

Abstract

Surface diffusion phenomena of alkylbenzenes in reversed-phase liquid chromatography (RP-LC) using an octadecylsilyl−silica gel and methanol/water (70/30, v/v) were analyzed on the basis of a restricted molecular diffusion model, in which surface diffusion coefficient, Ds, was correlated with molecular diffusivity, Dm, by introducing a restriction energy, Er. The average ratio of Er to isosteric heat of adsorption Qst, γ, in RP-LC was about 0.3. It was also indicated that γ at different temperatures could be represented as a linear function of the reciprocal of −Qst when a linear free-energy relation held and an enthalpy−entropy compensation for adsorption equilibrium was established. An estimation procedure of Ds was proposed on the basis of the results. It was demonstrated that Ds at a given temperature could be calculated with an error of less than about 10% from only one datum of the adsorption equilibrium constant.