Thermodynamic Characteristics of Surface Diffusion in Reversed-Phase Liquid Chromatography

Abstract

The thermodynamic properties of surface diffusion in reversed-phase liquid chromatography (RPLC) were analyzed in connection with the thermodynamics of surface diffusion in gas chromatography systems and of phase equilibrium in RPLC. The results suggest that the activation energy of surface diffusion in RPLC consists of only two contributions, corresponding to a hole-making and a bond-breaking process. The former is close to the activation energy of the mobile phase viscosity; the latter is correlated with the isosteric heat of adsorption (Qst), by a numerical coefficient between 0.5 and 0.6. The validity of these results was proved by the analysis of correlations between the surface diffusion coefficient (Ds) and the adsorption equilibrium constant (linear free energy relation), between Ds and the boiling point of the sample components, and between the ratio of Ds to the molecular diffusivity and Qst. These results support a restricted diffusion model, proposed earlier as an approximation of the mass tra...