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Adsorption of water is one of the most important properties of clay minerals. Water molecules are adsorbed on the surfaces of the layers of the clay mineral (interlayer spaces and external surfaces). For the first time, the thermodynamic properties of water in clay are obtained, from (i) comparison of the thermodynamic properties of anhydrous and hydrated minerals, between 0 and 350 K, and (ii) water vapor adsorption isotherms, between 300 and 380 K.
The studied samples are international reference clays : smectite MX-80 (Wyoming, USA) and mixed-layer illite-smectite ISCz-1 (Slovakia).
Solution isothermal calorimetry is used to determine the enthalpies of formation of the minerals, by dissolution in a HF-HNO3 solution at 1 bar and 298 K of (1) the clay sample and (2) the mechanical mixture of the constituents of the clay mineral. Comparison of the results, for the anhydrous and the hydrated minerals, leads to the enthalpies of hydration at 298 K.
Adiabatic calorimetry measurements give the heat capacities of the minerals, from 0 to 350 K (Debye law type extrapolation, between 0 and 5 K). Entropies, enthalpies and Gibbs free energies of formation, and then, of hydration (comparing anhydrous and hydrated minerals), between 0 and 350 K, are finally obtained. Comparison of two slightly different hydration states leads to the entropy, enthalpy and Gibbs free energy of the water adsorption reaction.
The Cp(T) curve, for the heat capacity of water in clay – i.e., the difference between the heat capacities of the hydrated and the anhydrous minerals –, shows that water in clay undergoes a first glass transition at around 190 K and – for highly hydrated minerals – a second glass transition at 260 K, and behaves as free liquid water above 273 K. The two glass transitions might correspond to two types of water molecules : (i) first adsorbed water molecules, bound to the interlayer cations of the clay mineral ; (ii) last adsorbed water molecules, not bound to the interlayer cations.
Water vapor adsorption isotherms are obtained from 298 to 378 K (magnetic suspension thermobalance) and represented by a BET type model (3 adsorbed layers). From the isotherms, Gibbs free energies of the adsorption reaction and of hydration can be determined. The values are similar to the above ones. |