On the driving force of cation exchange in clays: Insights from combined microcalorimetry experiments and molecular simulation

Benjamin Rotenberg (1) , Jean-Pierre Morel (2) , Virginie Marry (1) , Pierre Turq (1) , Nicole Morel-Desrosiers (2)

1 PECSA - Physicochimie des Electrolytes, Colloïdes et Sciences Analytiques
2 LMGE - Laboratoire Microorganismes : Génome et Environnement

We study the origin of the ionic exchange enthalpy in montmorillonite clays using microcalorimetry measurements and molecular simulation. We first determine the standard reaction enthalpy for well-defined interlayer water contents. We then show by a detailed analysis based on thermodynamic cycles that replacing Na+ ions by Cs+ in the interlayer of montmorillonite clays is an endothermic process, and that the overall exchange is exothermic only because it is dominated by the exothermic replacement of Cs+ by Na+ in the aqueous phase. This conclusion from ionic exchange enthalpies supports the one of a recent study of the ionic exchange free energy by Teppen and Miller [Teppen B. J. and Miller D. M. (2006) Hydration energy determines isovalent cation exchange selectivity by clay minerals. Soil Sci. Soc. Am. J.70(1), 31–40] and contradicts long-held views on the role of ion–clay interactions in determining the ionic exchange thermodynamics. This calls for a paradigm shift for the origin of this exchange: The driving force is the “hydrophobicity” of Cs+ compared to Na+ and not its affinity for clay surfaces.

Domaines: Chimie théorique et/ou physique

Benjamin Rotenberg, Jean-Pierre Morel, Virginie Marry, Pierre Turq, Nicole Morel-Desrosiers. On the driving force of cation exchange in clays: Insights from combined microcalorimetry experiments and molecular simulation. Geochimica et Cosmochimica Acta, 2009, 73, pp.4034. ⟨10.1016/j.gca.2009.04.012⟩. ⟨hal-00391354⟩