The Henry constant and isosteric heat at zero loading for adsorption on energetically heterogeneous solids absolute versus excess

Abstract

We present a self-consistent analysis of adsorption of gases on energetically heterogeneous solids and derive expressions for the Henry constant and the isosteric heat at zero loading, qst(0), for both excess and absolute amounts of molecules; the latter being amenable to thermodynamic analysis. Results obtained by Monte Carlo integration are shown to depend on the system size for calculations using the total number of molecules in the adsorbed and gas phases of the system. In the limit when loading approaches zero, the thermodynamic variables obtained from the fluctuation theory in a grand canonical simulation are in agreement with the Henry constant and the isosteric heat at zero loading calculated from Monte Carlo integration, establishing the consistency of the analysis of isotherm and isosteric heat for the full range of loading. The equations for the isotherm (and the Henry constant), and the isosteric heat for a complex substrate, can be written in terms of the local isotherms and the local isosteric heats of the components comprising the substrate for all loadings. This results in a significant saving in computing time for the calculation of the thermodynamic variables of a complex substrate.