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

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.