Calorimetric Effects in Adsorption

The behavior of the isosteric enthalpy of adsorption suggests that in many cases, surface heterogeneity plays an important role, and because such systems often follow one or other of the empirical isotherms, this is strong evidence that the models underlying such isotherms must incorporate a description of the heterogeneity of the surface. From the theory of mobile adsorption, it is evident that the related isosteric enthalpy of adsorption will be concave to the θ-axis, where θ is defined as in the Hill-de Boer equation, at low temperatures and convex at high temperatures. The form of the hard core interaction has no effect on the shape of the isosteric enthalpy curve. Among other isosteric enthalpy of adsorption data on homogeneous surfaces below the critical temperature, krypton adsorption on graphitized carbon black at 90 K may be mentioned. Thermal graphitization is not the only process that can be used to obtain homogeneous surfaces. The effects of surface heterogeneity will decrease as the number of adsorption layers increases. For the highest surface coverage, the shape of the C r ( T ) curve approaches that predicted by classical theories of critical phenomena. As the adsorption decreases, the peaks become more rounded as the effects of surface heterogeneity become increasingly important.