Swelling Pressure of Montmorillonite

Abstract A montmorillonite suspension was placed in a miniature compression chamber bounded on one side by a cellulose membrane supported by a water saturated porous Alundum disk, and on the other by a rubber film. Changing the ambient pressure on the film caused water to move across the membrane until the swelling pressure of the suspension equalled the applied pressure. Existing theory, based upon the Gouy‐Chapman description of the diffuse double layer and upon its anticipated osmotic activity, was used to predict swelling pressures for different suspensions. During initial compression, measured swelling pressures of both Na‐ and Ca‐montmorillonite exceeded predicted values. Subsequent decompression and recompression cycles were reversible except for a slight but measurable hysteresis, but the position of the reversible portion of the curve depended upon the terminal pressure of the initial compression. When the latter was large (25 to 50 atm.), measured swelling pressures of decompression‐recompression cycles for Na‐montmorillonite agreed quantitatively with theory over the range 0.2 to 50 atm. Under similar conditions, Ca‐montmorillonite exhibited pressures appreciably less than predicted by theory.