Translational diffusion of water and its dependence on temperature in charged and uncharged clays: A neutron scattering study

Abstract

The water diffusion in four different, highly compacted clays [montmorillonite in the Na- and Ca-forms, illite in the Na- and Ca-forms, kaolinite, and pyrophyllite (bulk dry density ρ b =1.85±0.05 g cm3)] was studied at the atomic level by means of quasielastic neutron scattering. The experiments were performed on two time-of-flight spectrometers and at three different energy resolutions [FOCUS at SINQ, PSI (3.65 and 5.75 Å), and TOFTOF at FRM II (10 Å)] for reliable data analysis and at temperatures between 27 and 95 °C. Two different jump diffusion models were used to describe the translational motion. Both models describe the data equally well and give the following ranking of diffusion coefficients: Na- montmorillonite≤Ca - montmorillonite Ca - illite>Na - illitekaolinite> pyrophyllitewater, in both jump diffusion models. For clays with a permanent layer charge (montmorillonite and illite) a reduction in the water content by a factor of 2 resulted in a decrease in the self-diffusion coefficients and an increase in the time between jumps as compared to the full saturation. The uncharged clay kaolinite exhibited no change in the water mobility between the two hydration states. The rotational relaxation time of water was affected by the charged clay surfaces, especially in the case of montmorillonite; the uncharged clays presented a waterlike behavior. The activation energies for translational diffusion were calculated from the Arrhenius law, which adequately describes the systems in the studied temperature range. Na- and Ca-montmorillonite (∼11-12 kJmol), Na-illite (∼13 kJmol), kaolinite and pyrophyllite (∼14 kJmol), and Ca-illite (∼15 kJmol) all had lower activation energies than bulk water (∼17 kJmol in this study). This may originate from the reduced number and strength of the H-bonds between water and the clay surfaces, or ions, as compared to those in bulk water. Our comparative study suggests that the compensating cations in swelling clays have only a minor effect on the water diffusion rates at these high densities, whereas these cations influence the water motion in non-swelling clays. © 2008 American Institute of Physics.