Quasi-elastic neutron-scattering studies of the dynamics of intercalated molecules in charge-deficient layer silicates. Part. 1.—Temperature dependence of the scattering from water in Ca2+-exchanged montmorillonite

Abstract

Quasi-elastic neutron scattering (QENS) measurements on a Ca²⁺-exchanged montmorillonite containing two layers of adsorbed interlayer water have been made at three different temperatures. The data were analysed using a multi-dimensional non-linear least-squares minimisation method which separated the intensity into an elastic and a quasi-elastic part and fitted a broadening to the quasi-elastic part. The resulting broadenings were then fitted to two alternative jump-diffusion models which differed in the shape of the jump-length distributions assumed. The resulting jump correlation times (the mean time between jumps) were found to be model dependent, but for both models the temperature dependence of the correlation times gave a straight line on an Arrhenius plot, with the same activation energy, namely 11.0 ± 1 kJ mol^–1. However, because of the variation in the mean jump length with temperature, the effective diffusion coefficients did not show Arrhenius behaviour.

A detailed analysis of the correctly internormalised elastic and quasi-elastic intensities is presented. This suggests that both the water in the first hydration shell of the exchangeable interlayer cations and the protons of the hydroxyl groups within the aluminosilicate lattice have a small Debye–Waller factor. For the non-hydration shell water the Debye–Waller factor was found to be comparable with that for bulk water and to increase markedly with temperature in the range of the experiment. Water directly coordinated to the exchangeable cation was shown to be immobile on the time scale of the neutron measurements, but the apparent intensity of this ‘bound’ water fraction was found to decrease with increasing temperature. The elastic incoherent structure factors (EISF) were derived from the quasi-elastic intensities to avoid the complications due to coherent effects in the elastic intensities. The form of these functions suggested that the volume of the restricted diffusion (ca. 300 Å at 300 K) increased with increasing temperature. To complete the explanation of the observed peaks the coherent components in the total intensity were attributed to small-angle scattering caused by inhomogeneities in the clay layer and to the (001) reflection. The above description shows that water in the two-layer hydrate of Ca²⁺-exchanged montmorillonite can be separated into relatively ‘free’ and ‘bound’ components, the former exhibiting relatively rapid, spatially restricted motions.