Adsorption of Polar Gases on Model Silica Gel

Two models are developed to study the influence of polar surface groups present in silica gel on adsorption of polar gases. Both are based on the composite sphere model of Kaminsky and Monson ( J. Chem. Phys. 1991, 95, 2936−2948). The surface groups are incorporated either as discrete dipoles or as a continuum (smeared) dipolar bilayer. The behavior of each of these descriptions is explored through the calculation of Henry's constants in various microstructures, water adsorption isotherms, isosteric heats of adsorption, and adsorbate structural correlation functions. Comparison to experimental data shows that while both models produce good qualitative agreement, only the discrete approximation results in a model adsorbent that is sufficiently heterogeneous to predict the correct trends in the isosteric heats of adsorption.