A predictive model of thermodynamic entities of hydration for smectites: Application to the formation properties of smectites

A predictive model is developed for estimating the amount of clay water and the hydration thermodynamic properties as a function of relative humidity (R.H.), for any 2:1 clay composition. From a methodology performed by Gailhanou et al. (2017) upon MX80-Na smectite, based on experimental water adsorption isotherms and structural data, a regular solid solution between anhydrous and highly hydrated clay end-members is parameterized, providing a set of 4 thermodynamic parameters ΔCem.WH , ΔCem.WS, WH and WS characterizing the theoretical mechanisms of adsorption of the clay water versus the relative humidity. To complete literature data, water adsorption isotherms are acquired at 25 °C and 45 °C for five natural 2:1 clays, with various compositions, interlayer charges and interlayer cations (Na, Ca, K, Mg). The amounts of clay water are extracted from the total adsorbed water amounts by removing the contribution of capillary water and the thermodynamic entities of the hydration reaction are calculated and are closely related to the nature of the clay (characterized by its interlayer and tetrahedral charges and the nature of the interlayer cation), allowing two predictive models, for di-octahedral smectites and for tri-octahedral 2:1 clays. These two models are assessed by using experimental data for other clay minerals provided by literature. For a given 2:1 clay composition, the model allows - to predict quite well the clay water adsorption isotherm, - to provide hydration thermodynamic properties (G, H, S) for the di-octahedral model and for the tri-octahedral model and-to predict the thermodynamic properties of formation for some hydrated clays of fictive compositions.