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 adsorbate−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 field 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 ∼50%.