Thermodynamic characteristics of surface diffusion in reversed-phase liquid chromatography

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 bend-breaking process. The former is dose to the activation energy of the mobile phase viscosity; the latter is correlated with the isosteric heat of adsorption (Q(st)), 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 (D-s) and the adsorption equilibrium constant (linear free energy relation), between D-s and the boiling point of the sample components, and between the ratio of D-s to the molecular diffusivity and Q(st). These results support a restricted diffusion model, proposed earlier as an approximation of the mass transfer mechanism of surface diffusion. The restricted diffusion model provides consistent interpretations for the intrinsic thermodynamic properties of surface diffusion in RPLC.