Comparison of the Characteristics of Adsorption Equilibrium and Surface Diffusion in Liquid−Solid and Gas−Solid Adsorption on C18-Silica Gels

Abstract

Four parameters characterizing the adsorption equilibrium, surface diffusion, and related thermodynamic properties were derived from pulse-response experiments in various reversed-phase liquid chromatography (RPLC) systems using C18-silica gels and aqueous solutions of three different organic modifiers, methanol, acetonitrile, and tetrahydrofuran. The results were compared with corresponding data similarly measured by gas−solid chromatography on the same type of surface-modified silica gel, with helium. Information on the solvent effect on the adsorption characteristics was provided by the comparison of these experimental results. While the adsorption equilibrium constant and the heat of adsorption at infinite dilution were much larger in the gas−solid than in the RPLC system, the surface diffusion coefficient (Ds) and the activation energy of surface diffusion (Es) were of the same order of magnitude in both systems. Regarding surface diffusion, the logarithm of the frequency factor was linearly correlated with Es by the same straight line, suggesting the fundamental similarity of the surface diffusion mechanism in the gas−solid and liquid−solid systems. Calculations made on the basis of a surface-restricted diffusion model provide an explanation for the comparable values of Ds and Es in the two systems. In conclusion, the liquid phase in RPLC influences the thermodynamic parameters of surface diffusion as well as those of adsorption equilibrium, but similar values of Ds and Es are observed in both the two adsorption systems. A quantitative explanation of the similarities and differences of the characteristics and the mechanism of surface diffusion in gas−solid and liquid−solid adsorption systems is proposed.