Evaluation of Adsorption Enthalpies on Real Surfaces

We have seen above that the microscopic picture of real column surfaces is so complex that it has little in common with the earlier assumed ideal homogeneous surface. The present knowledge, especially as the silica surface is concerned, can be summarized as follows  

These findings necessarily pose the question of how to account for the real heterogeneity when deducing the values of the adsorption parameters from the experimental data. The formulae of widespread use were originally derived exclusively for the ideal models of adsorption. Too straightforward application of these relations might have resulted in errors. 

We will attempt an analysis of the possible misconceptions. To begin with, let us consider only the heterogeneity, independent of whether the surface is bare or chemically modified. The topography of the adsorption centers is usually described as patch-wise, random and intermediate. It seems that the surfaces employed in the radiochemical studies have been almost solely of the random type. In the strict sense of the term, such a surface consists of sites with uncorrelated values of the adsorption energy. However, it seems that the potential barrier between two adjacent adsorption sites cannot be of completely random height it must be somewhat correlated with the depths of the partner wells. As to the adsorbate, we suppose that each molecular entity takes only one site and that the particles occupying adjacent sites do not interact with each other. 

A principally different method is based on thermal desorption. In it the temperature of an adsorbent-adsorbate sample is steadily raised and the desorption rate of the adsorbate is measured. The resulting graph is interpreted in the parameters of the adsorption heterogeneity of the surface. The method seems promising for problems of our interest, though to the best of our knowledge, it has not yet been used to study systems like ours. 

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