Methodology for determining the thermodynamic properties of smectite hydration

This study proposes an improved methodology for extracting the thermodynamic hydration properties of clay minerals from adsorption isotherms performed at two temperatures, by removing the capillary water contribution. The work focuses on Na-smectite. Its thermodynamic hydration properties were extracted as follows: (i) experimental measurement of water vapor adsorption isotherms at two temperatures (25 °C and 45 °C); (ii) removal of the capillary water contribution; (iii) calculation of the thermodynamic properties. The values obtained from the adsorption branch compare favorably with calorimetric measurements. We parameterized a strict regular solid solution model similar to that developed by Ransom and Helgeson (1994). From this we see how the thermodynamic properties of hydration evolve, so we can explain and sort the values obtained by previous authors. The calculation process, based on structural constraints from the literature, considers only half of the total adsorbed water as structurally related to the mineral, when relative humidity conditions are close to 100%. The approach proposed in this work enables generalization to other types of clay minerals and implementation of thermodynamic databases with thermodynamic properties that are intrinsic to each clay mineral and independent from contextual constraints.