Diffusion of H₂O and I⁻ in expandable mica and montmorillonite gels: Contribution of bound H₂O

Abstract

Self-diffusion coefficients of H₂O molecules in water-rich gels of Na-rich expandable mica synthesized using natural talc were measured by pulsed-gradient spin-echo ¹H nuclear magnetic resonance (NMR), and the dependence on mica fraction (0.00–43.8 wt.%) and temperature (30.0–60.9°C) was examined. On the basis of the NMR results, the self-diffusion coefficient of H₂O, D^water, in the gel can be expressed by ln(D^water/D ^water₀ ) = 1.64[exp( −0.0588w) - 1], where D ^water₀ is the self-diffusivity of bulk water at temperature and w is the weight fraction of the mica (wt.%). The activation energy of H₂O diffusivity in mica gel is nearly equal to that in bulk water. These findings indicate that the normalized diffusivity, D^water/D ^water₀ , is independent of temperature. The diffusivity of I⁻, D^iodine, in the gels was examined by X-ray computed tomography (CT) at 22°C, and the influence of the mica fraction (0.00–24.8 wt.%) was studied to determine the contribution of bound H₂O. The X-ray CT results show that the normalized I⁻ diffusivity, D^iodine/D ^iodine₀ , obeys the above-mentioned phenomenological curve where D ^iodine₀ is the I⁻ diffusivity in bulk water. I⁻ is non-sorbing, and thus its diffusion is restricted only by the geometrical complexity of the pore structure of gels. Therefore, the effect of bound H₂O molecules on average H₂O diffusivity is negligible for w <24.8 wt.%. Diffusivity is governed by free or unbound H₂O molecules diffusing in the geometrically complex and tortuous pore structure of mica-mineral grains. This is a result of the large population of unbound H₂O far from the grain surface compared to the small population of bound H₂O near the grain surface. The diffusion of I⁻ ions in montmorillonite gels was examined by X-ray CT for w <16.7 wt.% montmorillonite. The normalized iodine diffusivity, (D^iodine/D ^iodine₀ ) obtained is in reasonably close agreement with the literature data for the normalized diffusivity of H₂O and is similar to the master curve of expandable mica. Therefore, bound H₂O molecules near negatively charged clay surfaces do not play a major role in the H₂O diffusivity for water-rich montmorillonite gels.