Characteristics and mechanism of surface diffusion in reversed-phase liquid chromatography using various alkyl ligand bonded silica gels.

Kinetic and thermodynamic parameters concerning surface diffusion were derived in reversed-phase liquid chromatography (RPLC) systems using a series of silica gel packing materials bonded with C1, C4, C8, and C18 alkyl ligands. In this paper, a more detailed study on some characteristics and the mechanism of surface diffusion in the RPLC systems using the different alkyl ligand bonded silica gels was made from a thermodynamic point of view by analyzing the surface diffusion data with consideration of the correlation between the parameters concerning surface diffusion and those relating to the retention equilibrium. The mechanism of surface diffusion seems to be the same irrespective of the length of the alkyl ligands because of the presence of the extrathermodynamic relationships, i.e., the enthalpy-entropy compensation and the linear free energy relationship. It was indicated that the surface diffusion coefficient (Ds) of weakly retained compounds was of the same order of magnitude with corresponding molecular diffusivity and that Ds decreased with increasing retention. The activation energy of surface diffusion (Es) was formulated as a linear function of the isosteric heat of adsorption (Qst). A modified Arrhenius equation, which is derived by taking into account the linear correlation between the two thermodynamic parameters Es and Qst, appropriately explains the intrinsic characteristics and mechanism of surface diffusion.