Evaluation of the BET and GAB models for interpretation of soil water isotherms: A molecular simulation study

Mathematical characterization of soil water isotherm (SWI) plays a significant role in modeling various processes in soils and estimating the soil properties, and the Brunauer-Emmet-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models are most widely used. Each model uses the amount of adsorbed gas in the monolayer on the surface for area estimates. Although these two models are widely used with soils, studies on the applicabilities of the SWIs estimated with the two models and the reliabilities of the derived monolayer contents are noticeably lacking. This work presents detailed evaluations of the two models and the resulting SWIs and determines whether these provide meaningful monolayer contents. The SWIs for four representative soils were generated with Grand Canonical Monte Carlo (GCMC) simulations, and the data were evaluated along with a wealth of information on the physical states of the adsorbed water molecules. The results showed that the GAB model was much superior to the BET model in terms of fitting the SWIs. A series of analyses validated the soundness of the monolayer contents determined, including analyses of the layer thickness, the area occupied by the water monolayer, and the orientations of the water molecules. The molecular simulation also revealed the very different effects of the surface hydroxyls and cations in adsorption by considering the densities, space locations, and orientations of the adsorbed water. This work provides crucial information for rational application of the BET and GAB models to SWIs.