Reactive vacancy solution theory for correlation and prediction of adsorption equilibria for physical and chemical adsorptions

An isotherm equation was developed in the framework of the vacancy solution theory. In the model, vacancies in the adsorbed phase are dealt with as a kind of reactive entity, which leads to more flexible adsorption isotherm equations. The model can be called as reactive vacancy solution theory (RVST). Practical RVST equations for single-component and binary-component adsorption systems were derived by using Wilson, NRTL and UNIQUAC equations for activity coefficients. The RVST equations were found to coincide with the Langmuir–Freundlich equation when the effect of activity coefficients is assumed to be negligible. Furthermore, it was also found that the RVST model conforms to several simple-theoretical or empirical isotherm equations when the model is simplified. Therefore, it can be stated that the reactive vacancy solution theory can be regarded as more universal and general mathematical description of adsorption equilibria. Validity of the RVST equations for single-component adsorption was tested. The results reveal that the RVST-W and RVST-N equations can appropriately correlate non-ideal adsorption equilibrium data.