Evaluation of Isotherm Models for Water Vapor Sorption Behavior of Expansive Clays

AbstractMeasuring water vapor sorption behavior has emerged as a potentially important alternative for characterizing expansive soil behavior. Mathematical models for representing measured water vapor sorption isotherms and for calculating properties such as heat of adsorption, monolayer coverage, and specific surface area (SSA), however, have not been evaluated in detail for clay-water systems. This paper evaluates fitting performance and correlations among four theoretical models previously developed for general sorbent-sorbate interactions (e.g., colloid science) for specific application to water vapor sorption on expansive clays. Models evaluated include those by Freundlich, Brunauer-Emmet-Teller, Frenkel-Halsey-Hill, and Guggenheim-Anderson-de Boer. Model performance is evaluated for a suite of bentonite-kaolinite mixtures representing a wide range of swelling potential and along adsorption and desorption paths. Trends in model parameters are evaluated as functions of bentonite content, correlated among the models, and discussed in the context of expansive soil characterization. Each model more accurately fits adsorption curves than desorption curves. Correlation is evident between bentonite fraction and model parameters reflecting sorption capacity, but is not evident between bentonite fraction and model parameters reflecting sorption strength. The SSA is systematically greater if determined from desorption isotherms rather than adsorption isotherms.