Tactoid Size and Osmotic Swelling in Calcium Montmorillonite

Various workers in recent years have found that the Gouy-Chapman theory of the diffuse electrical double layer provides useful predictions of swelling behavior of certain 2:1 clays in dilute solutions. Calcium-saturated montmorillonite was apparently an exception. It swelled appreciably in dilute CaCl₂ solutions, but much less than predicted by theory, and to an extent depending on previous compression history. Earlier swelling studies with calcium montmorillonite were repeated with similar (but not identical) results. In addition, specimens with various histories of compression were examined by X-ray diffraction methods. Every specimen showed [001] spacings of 18.8Å., regardless of degree of compression or reswelling. Analysis of the 18.8Å. diffraction peaks by literal use of the Scherrer equation indicated that in uncompressed material (prepared by resin exchange from the sodium form) packets, or “tactoids” averaging about 4.5 montmorillonite particles each, produced the diffracted beam. The indicated number of particles increased irreversibly to about 8 as the maximum pressure experienced by a specimen increased to 100 atm. These results suggest that volume changes of compressed specimens were not associated with elastic bending of montmorillonite particles. Literal use of the Scherrer equation permits estimation of average spacing between tactoids from water content data. The slope of curves relating this spacing to pressure were surprisingly close to that predicted by application of double-layer equations, but large “dead volume” corrections must be applied if this is to be taken as evidence that swelling in calcium montmorillonite is in reasonable agreement with double-layer theory. It is concluded that the lowest energy state for calcium montmorillonite in dilute CaCl₂ is represented by a single “crystal” with 18.8Å. [001] spacing. Where attainment of this ordered state is prevented by imperfect orientation, swelling can occur, and is not incompatible with osmotic repulsion inherent in the diffuse double-layer model.