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

• Published in: Geochimica et cosmochimica acta 2009-07, Vol.73 (14), p.4034-4044
• Main Authors: Rotenberg, Benjamin, Morel, Jean-Pierre, Marry, Virginie, Turq, Pierre, Morel-Desrosiers, Nicole
• Format: Article
• Language: English
• Subjects: Chemical Sciences; or physical chemistry; Theoretical and
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2009.04.012

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.