Hydration thermodynamics of the SWy-1 montmorillonite saturated with alkali and alkaline-earth cations: A predictive model

Thermodynamics of Hydration in Minerals: How to Predict These Entities

“…It appears that integral enthalpy of hydration modelled by Vieillard et al [63] (black line) based on experimental measurements of heat of adsorption from [15][16][17] encompasses nearly all experimental data. As the standard entropy of hydration is assumed constant in [66] and [68] models, the integral hydration enthalpies provided by these two previous models are nearly linear.…”

“…To quantify the effective amount of water involved in the hydration reaction, the amount of poral H2O must be quantified and subtracted from the total amount of H2O taken up by the clay sample. The number of external surface water molecules (expressed in mmol/g dry clay) can be expressed as a function of relative humidity [63]. There are numerous papers about the adsorption-desorption isotherms performed on various clays minerals, but few are devoted to the acquisition of enthalpy of All recent predictive models of the hydration of smectites are based on an approach that uses the solid solution initiated by Ransom & Helgeson [66].…”

“…The curve 3 (black line) in Figure 4A comes from the model of Vieillard et al [63] and is represented by the following equation: …”

“…In Figure 4B, a relationship between the standard state enthalpy of interlayer water with the nature of the interlayer cation located in the SWy-1 montmorillonite with a constant layer charge of 0.38, characterised by its electronegativity (ΔHO = M z+ (aq), Table 4) is proposed [63] and given as follows:…”

“…(39) has been settled for the same support, i.e., a constant layer charge. For natural montmorillonite-Na, most enthalpies of hydration of the interlayer water are consistent within a narrow range of interlayer charge, IC (0.31<IC<0.38) [63]. Due to the lack of calorimetric measurements of enthalpy of hydration for high charge (IC>0.5) and low charge (IC<0.3) montmorillonites, the variation of the standard hydration enthalpy as a function of the layer charge remains highly questionable.…”