Analysis of surface diffusion phenomena in reversed-phase liquid chromatography

Surface diffusion data obtained for a reversed-phase liquid chromatographic system (octadecylsilyl silica gel and a 70/30 (v/v) methanol/water solution) were analyzed in relation to their molecular diffusivity (D-m). The adsortate-adsorbent interactions between the studied compounds and the stationary phase were taken into account. The surface diffusion coefficient (D-s) depends on the mobile-phase composition, especially on the nature and concentration of the organic modifiers. Differences between the values of D-s measured under various conditions stem probably from differences in D-m. It also seems that D-s tends toward D-m with decreasing retention factor. The surface diffusion mechanism was assumed to be a restricted molecular diffusion in a potential held of adsorption. A restriction energy for this diffusion (E-r) was introduced to correlate D-s with D-m. The ratio of E-r to the isosteric heat of adsorption (-Q(st))was found to be nearly constant, irrespective of the retention factor, with an average value of 0.32 for our phase system. An estimation procedure for D-s using the enthalpy-entropy compensation effect for the adsorption equilibrium is proposed. From the adsorption equilibrium constant at 298 K only, D-s could be estimated at different temperatures with an error less than similar to 50%.