The Toth isotherm is an empirical model that was developed to yield an improved fit versus traditional Langmuir isotherm modelling. It is often useful for describing heterogeneous systems. The Toth isotherm model approaches the Henry region at infinite dilution.
J. Toth, State equations of the solid gas interface layer, Acta Chem. Acad. Hung., 69 (1971) 311-317