Surface diffusion in reversed-phase liquid chromatography using silica gel stationary phases of different C1 and C18 ligand densities

Surface diffusion phenomena were studied from kinetic and thermodynamic points of view. The existence of a linear free energy relationship between surface diffusion and the retention equilibrium suggests that the mechanism of surface diffusion is the same irrespective of the density of C(1) and C(18) ligands. Surface diffusion coefficient (D(s)) of weakly retained compounds seems to be of the same order of magnitude with corresponding molecular diffusivity. There would be an intimate correlation between surface diffusion and molecular diffusion. The value of D(s) decreases with increasing retention strength. The magnitude of the restriction for surface diffusion is comparable to about one third the enthalpy change due to the sample retention. This means that it is necessary for sample molecules adsorbed to be partially desorbed from the stationary phase surface when they migrate by surface diffusion. The results of this study provide the fundamental information for developing an appropriate model of surface diffusion, which explains some intrinsic characteristics and mechanism of surface diffusion.