Estimation of surface diffusion coefficient in reversed-phase liquid chromatography using methanol/water mobile phases

An estimation procedure for the surface diffusion coefficient (D-s) is proposed for a reversed-phase liquid chromatographic system using an octadecylsilyl-silica gel and methanol/water mixtures. The procedure is based on a restricted molecular diffusion model for surface diffusion, in which D-s is correlated with molecular diffusivity by introducing restriction energy (E-r) due to an adsorptive interaction between adsorbates and adsorbents. The ratio of E-r to isosteric heat of adsorption (Q(st)) is linearly correlated with 1/(-Q(st)). It is indicated that the linear correlation between gamma (=E-r/(-Q(st))) and 1/(-Q(st)) can be observed when a linear free-energy relation and an enthalpy-entropy compensation effect in adsorption equilibrium are established. Though both E-r and gamma are empirical parameters, D-s at given temperatures and mobile phase compositions can be consistently calculated with an error less than about 20% from only one datum of adsorption equilibrium constant.