Water sorption modeling and monolayer of biological and food materials

Abstract

Despite water molecules are strongly hydrogen bonding to hydrophilic solids, the Brunauer-Emmet-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) surface adsorption models have been widely accepted to describe water sorption properties of biological and food materials. These mathematically quite similar models use parameters which include a theoretical ‘monolayer’ value and one or two other parameters which describe ‘surface affinity’ in thermodynamic terms as heat of sorption. A high surface affinity indicates a strong surface binding of water molecules and preferential monolayer adsorption prior to multilayer sorption. Conversely, biological materials are water plasticized and suffer physicochemical changes at various stages of water sorption. Dynamic water sorption measurements often show regions where time-dependent changes in material characteristics, such as viscous flow and crystallization, may occur as a result of the glass transition. The two or three parameters of the BET and GAB models, respectively, are useful in the analysis of water sorption characteristics of individual substances, although the existence of a monolayer particularly in multicomponent materials may not be justified. Fitting the models to experimental data may occur rather for empirical than theoretical reasons.