ABSTRACT
D. Estimation of the Surface Diffusivity The Restriction Energy and the Isosteric Heat of Adsorption As was illustrated in the previous section and will be demonstrated later in this review, surface diffusion plays an extremely important role on the intraparticle diffusion. In order to properly simulate the chromatographic separation processes in a column and to allow a rational design of chromatographic systems, accurate information on the mass-transfer kinetics is as necessary as the information required on the adsorption equilibrium of each component involved in the sample or the feed. All mass-transfer steps in the column contribute to peak spreading and to the column efficiency. Axial dispersion and fluid-to-particle mass-transfer phenomena have been studied in detail in the fields of chemical engineering and chromatography. It has become possible to estimate pore diffusivity and Knudsen diffusivity in adsorbent particles. By contrast, only a few methods have been proposed so far for the estimation of the surface diffusivity (D5 ) [106-108]. Furthermore, these methods cannot be applied directly to a given experimental system because D5 varies significantly with any change in the combination of the sample components, the nature or even the brand of the adsorbent, the nature of the solvents used, and their relative concentration. A suitable method must be developed for the estimation of D5 in RPLC systems [62,109-111].
