Saponite clay belongs to the phyllosilicate family and is comprised of layers of Si(IV) tetrahedra and Al(III) or Mg(II) octahedra with definite interlayer spacing. In these systems, the trivalent ion substitutions in the tetrahedral layers lead to negative charge on the layers. Here we report the dynamics of water contained in [Si(6.97)Al(1.03)][Ni(6.00)]O(20)(OH)(4)[Na(1.03)]·28H(2)O (SAP-1) and [Si(7.13)Fe(0.86)][Ni(6.00)]O(20)(OH)(4)[Na(0.86)]·14H(2)O (SAP-2) saponite clays in the temperature range 200-310 K as studied by quasielastic neutron scattering technique. Particularly the effect of the ion substitution towards the dynamics of water is addressed here. Data analysis is carried out using the relaxing cage model. The existence of distribution in relaxation times indicated that the water molecules in saponite clay have a different local environment which leads to complex diffusion behavior. It is found that water exists in a supercooled state in the temperature range up to 235 K. However, some of the water molecules are found to be immobile in the temperature range 240-285 K. The fraction of immobile water decreases with increase in temperature. At higher temperatures, some of the water molecules in the hydration shells or those near the surface start participating in the diffusion process and at 293 K, almost all water molecules contribute to the dynamics. Diffusivity of water in both SAP-1 and SAP-2 are found to be lower in comparison to the bulk, and within the two samples of saponite clay diffusivity in SAP-1 is found to be lower compared to SAP-2; this has been explained on the basis of the charge on the tetrahedral layers and the charge balancing cations in the interlayer spacing.

Diffusion of water confined in sodium bentonite clay is studied using the quasi-elastic neutron scattering (QENS) technique. Hydration of bentonite clay is characterized by several methods including X-ray diffraction and thermogravimetric measurements. X-ray diffraction shows that the clay is having a well-defined crystalline structure with an interlayer spacing of 13 Å. The QENS experiment has been carried out on hydrated as well as dehydrated clay at room temperature. Significant quasi-elastic broadening was observed in case of hydrated bentonite clay whereas dehydrated clay did not show any broadening over the instrument resolution. Analysis of QENS data reveals that diffusion of water occurs through jump diffusion. Both Jump diffusion models having random and Gaussian distribution in jump length are used to describe the observed dynamics. Obtained diffusion coefficient is found to be lower than that of bulk water.

Thermally induced gelation forming based on methylcellulose is recently being explored as a simple and environmentally benign process. Alumina slurry containing 0.1 wt% methylcellulose is subjected to Quasi Elastic Neutron Scattering (QENS) and rheological measurements in gelation temperature regimes to evolve a possible mechanism of the forming process. A reduction in diffusivity of water in the slurry from 2.16 to 1.92 × 10^-5 cm ² ·s^-1after exposure to 55°C is observed with QENS. This is found to be well correlated with a steep increase in viscosity from 1.2 Pa.s till 50°C to 50,000 Pa.s at 55°C. QENS studies revealed the diffusion of water occurs by jump diffusion with the jump lengths distributed randomly. Further, for the entire sample much longer residence time is found as compared to bulk water, which is due to hydrophilic interaction of water molecules with the methylcellulose in the slurry. Reduction in diffusivity of water along with the steep increase in viscosity could be understood as the strong, cross-linked polymer-solvent irreversible gel formation in presence of alumina which is responsible for the retention of a consolidated shape of the ceramic green body. Samples maintained the integrity while heat treatments achieving close to theoretical density values of3.98 g·cm^-3 at 1550°C.

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