Understanding the vacancy solution theory

Abstract The vacancy solution theory is one of the approaches used to describe adsorbed phase non-ideality that leads to a flexible pure component isotherm and allows to predict or correlate multicomponent adsorption equilibrium. This contribution identifies the correct reference state and presents a new derivation of the theory applicable with any activity coefficient model. The definition of the reference state and the derivation identify clearly the requirement for thermodynamic consistency, which as for any model that reduces to the Langmuir multicomponent isotherm is that the saturation capacities of all components must be the same. A numerical example based on the thermodynamically consistent dual site Langmuir model for a hypothetical mixture that has an azeotrope is used to discuss the coupling of the non-random two liquid model with the vacancy solution theory and test its predictive ability.