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Abstract

Water dynamics was studied in three smectites—hectorite, montmorillonite (bentonite), and vermiculite having water contents of 0.5, 1, and 2 water layers, respectively—by quasielastic neutron scattering. The study began with the lowest stable hydration state, i.e., the hydration shell of the exchangeable cation. The water content related to the planar hydrate Li+●3 H20 adsorbed on hectorite was found to be equivalent to the presence of one half of a water monolayer. The water molecules were found to be involved in two uniaxial motions: (1) a slow motion of the entire hydrate around an axis parallel to the c-axis of the clay layer, and (2) a fast rotation of water molecules around their c2 axes. The slow motion occurring in the interlayer space was anisotropic, the component along the c axis was found to be nil. Neither motion was thermally activated between 300 and 210 K, and both stopped simultaneously at 190 K. The fast correlation time, τ1f was 2.7 × 10−12 s; the slow one was 21.4 × 10−12 s.

Similar measurements on Li+ ● 3 H2O adsorbed on the bentonite and vermiculite suggest that the correlation times of both motions were sensitive to the electrical charge of the clay structure; i.e., τ1f = 4.4 × 10−12 and 6.9 × 10−12 s and τ1s = 23.4 × 10−12 and 29 × 10−12 s, respectively, for Li-bentonite and Li-vermiculite. Both motions persisted with increasing water content, but slowed and could be thermally activated. For Li-hectorite, τ1f and τ1S changed to 4.1 × 10−12 and 23.9 × 10−12 s for one adsorbed water layer and 4.4 × 10−12 and 29 × 10−12 s for two adsorbed water layers.

The long-range motion of water appeared to be a jump diffusion in a bidimensional space as inferred from the Q dependence of half width at half maximum of the corresponding energy distribution having a residence time of 2 × 10−10 s and a jump length of 3 Å. For water contents greater than that necessary to form two layers, porosity water became predominant; i.e., its dynamics was found to be similar to that of bulk water. The molecule was involved in an isotropic rotational motion with a corresponding time τ1f of 3.1 × 10−12 s. The clay-water interfacial region can thus be described as follows: The water molecules are distributed over a superlattice (3a.b) and remain an average of 10−10 s on a site. During this time, they spin around their c2 axis and have a correlation time τ1f of 4–5 × 10−12 s. If they are in the hydration shell of a charge-balancing cation, they are also involved in the slow rotation of the entire hydrate that has a correlation time of ~3 × 10−11 s.

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