Water desorption and absorption isotherms of sodium montmorillonite: a QENS study

Understanding the adsorption, movement and release of water in bentonites under unsaturated conditions remains a critical factor in improving design of hydraulic barrier technologies to minimise transport of contaminants. In situ neutron scattering measurements during controlled exposure to water vapour partial pressures provide an opportunity to probe dynamics of adsorbed water over a large range of unsaturated conditions. The hydration state of a sodium montmorillonite (Na + -Mt) sample was monitored in situ by both the position of the (001) reflection and the elastic scattering signal amplitude to ensure that equilibrium was achieved at the chosen vapour pressures (p/p o = 0.85, 0.75, 0.55, 0.35 and 0.01 during dehydration and rehydration). Quantification of total adsorbed water, confirmed by separate gravimetric analyses on the same sample, was achieved by integrating the total scattering contribution between the limits ± 2.0 meV. A further integration between ± 0.8 meV allowed separating total adsorbed water into mobile and immobile fractions. Hysteresis was observed for the p/p o = 0.55 and 0.35 vapour pressures, but little or no hysteresis was observed for the p/p o = 0.85 and 0.75 states and these were representative of hydration intermediate between pseudo-monolayer and pseudo-bilayer hydrate states. Fitting of the quasielastic neutron scattering spectra to the water model resulted in rational changes in translational and rotational diffusion coefficients (D t and D r , respectively), mean residence time, τ 0 and the mean relaxation time, τ r , of the water with respect to experimental p/p o . The behaviour of D t and τ 0 suggested that on rehydration at low p/p o , water initially occupied the external surfaces before entering the interlayer spaces.