Swelling Pressures of Sodium Montmorillonite at Depressed Temperatures

Abstract

Swelling pressures of oriented samples of sodium montmorillonite at 10 −4 and 10 −2 M NaCl in the pore-water were measured at controlled temperatures ranging from 1°C to 23°C. The specially designed apparatus allowed swelling against a movable piston connected to a mercury manometer through which the ambient pressure was applied. A decrease in swelling pressure with decrease in temperature is predicted theoretically from the Gouy-Chapman theory, taking the variation of temperature and dielectric constant into account. At a salt concentration of 10 −4 M NaCl, the measured swelling pressure at 120 Å average interparticle distance decreased from 48 cm of mercury at 23°C to 35 cm at 1°C, while the corresponding calculated pressure decrease was from 48 to 40 cm. While the measured pressures fell below calculated values at higher distances, the influence of temperature remained consistent. At 10 −2 M NaCl, a decrease in measured swelling pressure with decreasing temperature also occurred, but the measured swelling pressures were much higher than those predicted theoretically. Corrections were calculated, based upon the salt concentrations required by the theory to give the experimentally measured swelling pressures at room temperature. These were substituted into the approximate Langmuir solution of the combined Poisson-Boltzmann equation to describe the swelling pressure at 10 −2 M NaCl. With these corrections, the calculated swelling pressure at 120 Å average interparticle distance decreased from 30 cm at 23°C to 26 cm at 1°C while the measured pressure decreased from 30 to 23 cm. This agreement between the corrected, calculated pressure and measured pressure held from 100 to 250 Å interparticle spacing.