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Self-diffusion of H₂O in stevensite gel: effects of temperature and clay fraction

Exploration Geophysics Research Group, National Institute of Advanced Industrial Science and Technology, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan

^* E-mail: yoshito{at}aist.go.jp

(Received 3 January 2001; revised 11 April 2001)

Self-diffusion coefficients of water molecules (¹H₂O) in Na-stevensite gel were measured by pulsed-gradient spin-echo (PGSE) proton nuclear magnetic resonance (NMR). The effects of clay fraction (0.00–37.7 wt.%) and temperature (20.0–60.3°C) were studied. The results show: (1) phenomenologically, the H₂O self-diffusivity in the clay gel, D, is expressed by D/D₀ = exp(–0.0198w) where D₀ is the H₂O self-diffusivity in bulk water of the temperature and w is the clay weight fraction (wt.%). (2) The activation energy of the diffusivity in the stevensite gel is nearly equal to that in bulk water. Thus, the normalized diffusivity, D/D₀, obeys a temperature-independent master curve. (3) The exponential dependence of D/D₀ on w for w < 25 wt.% ( 12 vol.%) can be explained by a random walk model, in which unbound H₂O molecules diffuse in the geometrically tortuous pore structure of randomly scattered clay mineral grains. (4) The measured diffusivity can also be explained by a model of unbound H₂O diffusing in a polymer network with a specific mesh-size or characteristic interval of the crosslinkage.

KEYWORDS: clay gel structure, NMR, porous media, random walk, water molecule

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