Quasi-elastic neutron-scattering studies of intercalated molecules in charge-deficient layer silicates. Part 2.—High-resolution measurements of the diffusion of water in montmorillonite and vermiculite

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Abstract
The relative influence of water–surface and water–cation interactions in hydrates of the charge-deficient expanding-lattice layer silicates montmorillonite and vermiculite have been studied using high-resolution quasi-elastic neutron-scattering (QENS) measurements made with the back-scattering spectrometer at ILL, Grenoble. Detailed measurements have been made Ca2+–montmorillonite equilibrated at seven different water-vapour partial-pressure (p/p0) values and for Ca2+, Mg2+ and Na+ vermiculite samples at fixed p/p0. The QENS data are fitted to a model consisting of an elastic and a single Lorentzian quasi-elastic component plus a flat background. The quasi-elastic linewidth and the relative intensities of the three components are adjustable parameters. The resultant quasi-elastic broadenings were approximately five times smaller than the values derived previously from measurements at lower energy resolution. Also, the quasi-elastic scattering intensity increased with p/p0 and decreased with increasing scattering angle, the latter being the reverse of the behaviour observed at the broader energy resolution. The low- and high-resolution measurements taken together indicate the occurrence of two different phases of translational motion of water molecules not directly coordinated to the exchangeable (Ca2+) cations. These are attributed (i) to rapid localized motions in ‘cages’ bounded by the silicate sheets and the hydrated cations and (ii) to slower, longer-range inter-cage diffusion. In addition, the relative intensities of the quasi-elastic scattering at high and low wavevector transfer suggest that the high-resolution measurements also observe rotations of the complete six-fold coordinated hydration shells of the cations about one of the C3 axes of the octahedron. To explain the observed width, this rotation must have a correlation time of ca. 10–10 at ambient temperature. The present QENS data yielded effective diffusion coefficients for water in Ca2+–montmorillonite which increased rapidly from < 1.0 × 10–10 m2 s–1 at p/p0= 0.15 to ca. 4.5 × 10–10 m2 s–1 at p/p0= 0.33. At higher values of p/p0 the effective diffusion coefficients remained approximately constant within the limits of experimental error. For water adsorbed by vermiculite having Na+, Ca2+ or Mg2+ as the exchangeable cation the diffusion coefficient measured at p/p0= 0.76 was found to be significantly smaller than the corresponding value for Ca2+–montmorillonite. This may be attributed to the hindering effect of the more densely packed network of hydrated cations in the more highly charged vermiculite.

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