Surface compression in adsorption systems

A new phenomenon, surface compression of adsorbates, is discussed for gases adsorbed on solids. The strong attraction to a surface causes adsorbate molecules to attain much higher densities than that of a normal liquid. Under these conditions, adsorbate molecules are so compressed that they repel each other. This phenomenon is discussed in terms of experimental data, results of Monte Carlo simulations, and theoretical models. An approximate model for monolayer adsorption of Lennard–Jones molecules on a surface is derived. This model predicts adsorbate–adsorbate repulsions for strong adsorbate–adsorbent interactions. This model is applicable to both monolayer adsorption and to the first layer in multilayer adsorption. A linear form of this new model allows one to determine the adsorbate–adsorbate interaction energy in the adsorbed layer from experimental data. Analysis of different systems (such as nitrogen, carbon dioxide, hydrocarbons on various adsorbents) shows that the energies of molecule–molecule interactions in the adsorbed phase near monolayer coverage are positive, indicating adsorbate–adsorbate repulsions, and, hence, that the adsorbates are compressed fluids. This should be taken into account in the analysis of equations of state for the adsorbed phase and in using adsorption to characterize porous materials. It also should have an effect on solidification of the adsorbed layer and on the rates of reaction on catalyst surfaces. The results presented here also show that the concept of ‘monolayer capacity’ needs revision because it is a function of the adsorbate–adsorbent interaction energy.