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 (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.
The solution in the Laplace domain of the system of equations of the general kinetic model of chromatography provides equations relating the first absolute moment and the second central moment of elution bands to the characteristics of the retention equilibrium and the mass transfer kinetics, respectively. For continuous porous rod (i.e., monolithic) columns, these moment equations have the same form as those for conventional columns packed with spherical particles of a suitable packing material. However, some of the coefficients in the equation of the second central moment are different for conventional and monolithic columns. The results of the calculations made with the moment equations derived here for monolithic columns agree well with some typical characteristics of the experimental behavior observed for monolithic columns, showing the validity of these new moment equations. They should be useful for a detailed analysis of the retention equilibrium and of the mass transfer kinetics in monolithic-type columns. These moment equations are also applicable to the study of the adsorption characteristics of cylindrical adsorbents, such as activated carbon fibers.
Surface diffusion data obtained for a reversed-phase liquid chromatographic system (octadecylsilyl silica gel and a 70/30 (v/v) methanol/water solution) were analyzed in relation to their molecular diffusivity (D m ). The adsorbateadsorbent interactions between the studied compounds and the stationary phase were taken into account. The surface diffusion coefficient (D s ) depends on the mobilephase composition, especially on the nature and concentration of the organic modifiers. Differences between the values of D s measured under various conditions stem probably from differences in D m . It also seems that D s tends toward D m with decreasing retention factor. The surface diffusion mechanism was assumed to be a restricted molecular diffusion in a potential field of adsorption. A restriction energy for this diffusion (E r ) was introduced to correlate D s with D m . The ratio of E r to the isosteric heat of adsorption (-Q st ) was found to be nearly constant, irrespective of the retention factor, with an average value of 0.32 for our phase system. An estimation procedure for D s using the enthalpy-entropy compensation effect for the adsorption equilibrium is proposed. From the adsorption equilibrium constant at 298 K only, D s could be estimated at different temperatures with an error less than ∼50%.
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