On the Prediction of Water Contents in Na-Saturated Dioctahedral Smectites

Properties of swellable clay minerals in technical and geotechnical applications as well as in geological systems are determined by their water content. Thereby, the water content depends on the structure and morphology of the smectites and available water while smectites are the most heterogeneous clay minerals. Here, first-principles calculations are combined with ab initio thermodynamics to systematically study the hydration states of Na-saturated dioctahedral smectites. Total energy calculations based on density functional theory are carried out for a series of montmorillonites and beidellites with varying layer charge (0.125 ≤ ξ ≤ 0.5) and increasing number of water molecules per unit cell. The thermodynamic ground state was then defined with the help of the grand canonical potential as a function of chemical potential of water, which is directly related to the experimental conditions of pressure and temperature. For the first time, p, T -phase diagrams of stable hydration states together with relative humidity curves are shown. In the discussion, deviation of measured water contents is explained by the contribution of layer charge distribution, hydration of cations at the layer edges, and osmosis in mesopores at the edges of layer stacks. In addition, we showed that dioctahedral 2:1 layer silicates with a layer charge of 0.125 are swellable, which reduces the definition gap between uncharged and charged 2:1 layer silicates.