Enthalpy Changes Accompanying the Collapse of Montmorillonite Layers and the Penetration of Electrolyte into Interlayer Space

The enthalpy changes, ΔH, in suspensions of Li–, Na–, and K–montmorillonite were determined as functions of the molal concentrations of LiCl, NaCl, and KCl, respectively, by microcalorimetry. These changes were compared with the corresponding changes in interlayer distance, λ, as determined by X-ray diffraction. It was found that ΔHvalues accompanying the collapse of the montmorillonite layers from λ > 14 nm to λ ≅ 1.0 nm (or 0.6 nm for K–montmorillonite) were essentially negligible; however, ΔHvalues accompanying the collapse of layers of Li– and Na–montmorillonite from λ ≅ 1.0 nm (three-water-layer hydrate) to λ ≅ 0.6 nm (two-water-layer hydrate), and the collapse of layers of K–montmorillonite from λ ≅ 0.6 nm (two-water-layer hydrate) to λ ≅ 0.3 nm (one-water-layer hydrate), were not negligible. No additional collapse of Li– and Na–montmorillonite layers occurred after λ ≅ 0.6 nm, and for K–montmorillonite layers after λ ≅ 0.3 nm, but ΔHcontinued to change with increasing salt addition. This change was attributed to the penetration of electrolyte into the interlayer space. A notable observation was that the value of ΔHwas positive for Li–montmorillonite, but negative for Na– and K–montmorillonite. These observations were interpreted to mean that ΔHchanges in the montmorillonite suspension were largely attributable to reorganization of the interlayer space by the added electrolytes.