A new model for gas adsorption isotherm at high pressures

This contribution discusses the derivation of a new model for gas adsorption isotherm at high pressures, from the Redlich-Kwong's equation of state, instead of the virial equation used by the frequently employed Zhou model, where the involved virial coefficients have not direct physical information. The two models are based on the hypothesis that the adsorbate molecules behave like a real gas. Unlike Zhou model, in the proposed new model, the adsorption space (V_ad) appears as a fitting parameter. To validate the proposed model, the values obtained for V_ad from hydrogen adsorption isotherms recorded at 75 K and up to 10 bar, in a series of porous solids of the well-known crystalline structure were compared with the calculated accessible free volume in those solids. The difference between the calculated values of V_ad and the expected ones, from structural considerations, varies from 3.3 to 16.3% depending of the material nature. The effects of the model's parameters on the isotherm shape are discussed in detail. The proposed adsorption model herein discussed has potential applications for the evaluation of adsorption data for gases above their critical temperature, among them hydrogen, and also to use the hydrogen adsorption as a probe molecule to sense the pore volume in porous solid of small pore volume and narrow windows, which could be inaccessible for other probe molecules.