Kinetic derivation of common isotherm equations for surface and micropore adsorption

The Langmuir equation is one of the most successful adsorption isotherm equations, being widely used to fit Type I adsorption isotherms. In this article we show that the kinetic approach originally used by Langmuir for 2D monolayer surface adsorption can also be used to derive a 1D analogue of the equation, applicable in ultramicropores with single-file diffusion. It is hoped that such a demonstration helps dispel the idea that the Langmuir isotherm equation cannot apply to some micropores as more than a mathematical correlation. We furthermore seek to extend the insight provided by the simple kinetic derivation of the Langmuir equation to other isotherm equations capable of modelling Type I isotherms. The same kinetic approach is thus also used to derive the Volmer, Fowler–Guggenheim and Hill–de Boer equations, both for surface (2D adsorbed phase) and micropore adsorption (1D and 3D adsorbed phases). It is hoped that this will help make more intuitively clear that these equations can be used as phenomenological models in some instances of adsorption in micropores.