Heterogeneity Effects in Adsorption from Solution

Following the example set by gas-adsorption studies, a useful approach is to examine the more ideal systems such as simple binary organic liquids in equilibrium with inert homogeneous solids. If these systems can be understood, then it is probable that the effects of surface heterogeneity in more complex systems will be more easily discerned. However as a precursor to this, it is worthwhile considering adsorption from binary gas mixtures as this serves as a link with pure gas adsorption theory. 

Comparisons with Adsorption from Binary Gas Mixtures.—Compared to pure gas adsorption, the amount of research concerning adsorption of binary gas mixtures on heterogeneous surfaces is sparse. This is not surprising as the latter requires an understanding of the simpler pure gas systems in fact a popular approach in verifying mixed gas adsorption theories is to predict mixed gas isotherms from pure gas isotherms measured at the same temperature. 

The analogy with adsorption from solution is readily demonstrated by considering as a first approximation Langmuir s theory applied to the adsorption of a gas component 1 from a mixture of gases (1 and 2) at a total pressure p chosen so that one of the components (say 2), when adsorbed from the pure 

Both equations (116) and (117) may be generalized for a patchwise heterogeneous surface. In the case of equation (116) this becomes  

Equations (116) and (118) are special cases for mixed gas adsorption. The more general adsorption isotherm for a patchwise heterogeneous surface may be written as  

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