A unified description of ion exchange and complete adsorption isotherms, including cluster growth and cavity filling

Abstract

In the context of micro- and mesoporous materials, the interpretation of ion exchange and gas adsorption isotherms rarely covers the full information content of the experimental data. Based on our earlier work regarding the role of entropy in multiple equilibria, we have revisited cation exchange data of zeolite A in more detail and addressed the analysis of Ar and N2 adsorption isotherms of nonporous, microporous and mesoporous silicates. The previously reported analysis is thereby extended and improved. The theory is presented in a unified and compact manner through rigorous application of the concept of fractional amount, including particle distribution, cluster growth, and cavity filling. A result is that the structure of mesoporous silica MCM-41 can be described as consisting of the main channel and two types of cavities, which can be interpreted as surface roughness. In a further step, the information delivered by the inflection points in type I, II and IV gas adsorption isotherms is considered. The adsorption of simple gases begins with the formation of a monolayer on the pristine surface. The inflection point thereby represents the start of a new process, indicating that the adsorption isotherms must be understood as the signature of several sequential chemical equilibrium steps. Information can be extracted by inspecting this signature more closely. The results demonstrate the ability of the reported procedure to elicit additional insight including structural features from experimental equilibrium isotherm data.