Water diffusion in a synthetic hectorite clay studied by quasi-elastic neutron scattering

We present a quasi-elastic neutron scattering study of water dynamics confined in a model clay system, a synthetic hectorite with Na+ compensating counter-ions. As shown by water adsorption gravimetry and neutron/X-ray diffraction, the clay system has, unlike its natural counterparts, very well-defined swelling characteristics, with a clear appearance of a mono-hydrated and a bi-hydrated state. This simplifies to a great extent neutron scattering analysis and interpretation. Initially, microscopic relaxation times as well as long-range self-diffusion coefficients for water in Na-hectorite at ambient temperature are determined using the time-of-flight (TOF) and neutron spin echo (NSE) neutron scattering techniques, applying a simple model of isotropic (three-dimensional) translational diffusion. Results from the two techniques are in excellent agreement, giving diffusion coefficient of approximately 1.5 * 10-10 m2 s-1 and 4.5 * 10-10 m2 s-1 for the mono-hydrated and bi-hydrated state, respectively. Concentrating on the mono-hydrated hectorite system, after an account is taken of short-time relaxation stemming from fast (vibration-like) motion, the data is analyzed using a geometrically more appropriate translational model: powder averaged two-dimensional diffusion. This analysis yields a two-dimensional diffusion coefficient in the plane of the clay layers of 2.8 * 10-10 m2 s-1. We demonstrate on model data that isotropic analysis applied to a system with powder averaged two-dimensional diffusion overall underestimates the diffusion coefficient by approximately 25%. (authors)