Effects of Clay Fraction and Temperature on the H₂O Self-Diffusivity in Hectorite Gel: A Pulsed-Field-Gradient Spin-Echo Nuclear Magnetic Resonance Study

Abstract

Abstract--Self-diffusion coefficients of H20 molecules in Na-rich hectorite gel were measured by IH nuclear magnetic resonance (NMR). Spin-echo pulse sequences with magnetic field gradient pulses for the translational diffusion measurement were applied to the hectorite gel at the Larmor frequency of 20 MHz. Effects of clay fraction (0-51.2 wt. %) and temperature (20.0-60.3~ were studied. The results show: (1) Phenomenologically, the self-diffusion coefficient, D, of IH20 in the clay gel is expressed by the normalized diffusivity, D/D o = exp(-0.0257w), where D O is the water self-diffusivity in bulk water at temperature and w is the weight fraction of the hectorite (wt. %). (2) The activation energy of H20 diffusivity in the hectorite gel is nearly equal to that in bulk water. Hence, the normalized diffusivity, D~ Do, obeys a temperature-independent curve. (3) The exponential dependence of D/D o on w for w <30 wt. % is explained by a random-walk model, in which free or unbound H20 molecules migrate in the geometrically complex and tortuous pore structure of randomly scattered clay-mineral grains.

Diffusion of H2O in Smectite Gels: Obstruction Effects of Bound H2O Layers

“…The other is the synthetic clay, smecton HE 1 , obtained from Kunimine Industries Co. Ltd. Its chemical composition was given by Nakashima (2000). The MBC value is 126 meq/100 g and r clay is 27 nm.…”

“…Thus, several water-rich gel samples of the Tsukinuno montmorillonite and stevensite were measured as part of the present study. No PGSE NMR experiments were performed for the smecton HE 1 gels because extensive NMR data were published by Nakashima (2000).…”

“…This PGSE NMR technique was used because it permits non-destructive, fast, precise measurements (e.g. Fripiat et al, 1984;Grandjean and Laszlo, 1989;Nakashima, 2000). The NMR spin-echo signal intensity, I, is given by:…”

“…where a, b and a' are dimensionless constants, w is the clay weight fraction of the gel (wt.%), and D 0 is the H 2 O self-diffusion coefficient in bulk water (Nakashima, 2000(Nakashima, , 2001a(Nakashima, , 2002a. Equations 1 and 2 are useful for predicting D in water-rich clay gels at arbitrary temperatures and with arbitrary clay fractions.…”

Continuous wave MRI diffusion study of water in bentonite clay

“…Diffusion in clays was successfully studied by field gradient NMR techniques for high water-content clay gels and for two-layered, non-swellable clay materials such as kaolinite [1,2]. At low water contents, water in non-swelling clays such as kaolinite still exhibits sufficiently long transverse relaxation times for field gradient NMR experiments.…”