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 (Dm). The adsorbate−adsorbent interactions between the studied compounds and the stationary phase were taken into account. The surface diffusion coefficient (Ds) depends on the mobile-phase composition, especially on the nature and concentration of the organic modifiers. Differences between the values of Ds measured under various conditions stem probably from differences in Dm. It also seems that Ds tends toward Dm 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 (Er) was introduced to correlate Ds with Dm. The ratio of Er to the isosteric heat of adsorption (−Qst) 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 Ds using the enthalpy−entropy compensation effect for the adsorption equilibrium is proposed. From the adsorption equilibrium constant at 298 K only, Ds could be estimated at different temperatures with an error less than 50%.