NMR study of adsorbed water. I. Molecular orientation and protonic motions in the two-layer hydrate of a Na vermiculite

Abstract

The well characterized two‐layer hydrate of Na‐Llano vermiculite has been studied by continous wave (CW) wide‐line NMR and by pulse NMR of ¹H nucleus in a wide temperature range. The absorption spectrum consists of a doublet and of a central line. The doublet splitting is orientation dependent between 0 and 50 °C and orientation independent below −60 °C. Below 60 °C, the symmetry of the hydration shell may be destroyed. The linewidth is compatible with rapidly rotating water molecules. An octahedral distribution of water molecules around the Na⁺ cations fits the experimental data, the six rotation axes around which the water molecules are spinning rapidly being tilted by about 65° with respect to the C* axis. The cation hydration shell is characterized by a rotational diffusion motion with an activation energy of 8.5 kcal, the diffusion coefficient at room temperature being about 0.5×10⁻⁸ cm² sec⁻¹. The diffusion coefficient of free water or protons is about 0.2×10⁻⁶ cm² sec⁻¹. From +50 to −100 °C, the observed T⁻¹₁ is contributed successively by the diffusion of the cation hydration shell, the diffusion of free water or proton with respect to the paramagnetic centers, and finally by the paramagnetic contribution. Water molecules within the hydration shell and/or water molecules ’’between’’ the hydration shells exchange protons with a frequency between 10⁻⁴ and 10⁻⁵ sec⁻¹ at room temperature. This exchange occurs within the hydration shell or it is relayed by the ’’free’’ water molecules between the hydration shells.