Study of adsorbed water: Electric potential calculation and molecular orientation in the two layer hydrate of a Mg vermiculiteThe well characterized two-layer hydrate of Na-Llano vermiculite has been studied by continous wave (CW) wide-line NMR and by pulse NMR of IH 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 o and SO·C and orientation independent below -60·C. Below 6O·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 X 10-8 cm 2 sec-I. The diffusion coefficient of free water or protons is about 0.2X 10-6 cm 2 sec-I. From +50 to -IOO·C, the observed T \1 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-I at room temperature. This exchange occurs within the hydration shell or it is relayed by the "free" water molecules between the hydration shells.

Water vapour sorption isotherms on sodium and magnesium vermiculite of high charge density were measured at 25 and 50°C. Heats of immersion at various stages of hydration were measured at 25°C. According to X-ray observations during the sorption process, two discrete interlayer hydrates are obtained with one and two monolayers of water between the unit layers of the crystallites. The derived integral entropy of adsorption indicates a reduced freedom of motion of the interlayer water molecules compared with that for water molecules in the liquid state. Comparison of the apparent density of the hydrated clay with the calculated crystallographic density indicates that interlayer water is slightly more densily packed than liquid water. However, the bulk density of water in a sodium vermiculite suspension is normal for the water in excess of the two hydration layers.Nuclear pulse resonance results obtained on sodium vermiculite shows that water in the onelayer hydrate is organized and some hypotheses are presented regarding this organization. Water molecules in the two-layer complex show the same degree of orientation only below -65°C. Infrared absorption spectra for hydrated flakes of sodium vermiculite were obtained as a function of angle between the (001) plane of the crystallites and the i.-r. beam. The results indicate orientation of water molecules in the one-layer hydrate at room temperature.

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The electric potential in two-layer hydrate Llano and Kenya Mg vermiculite is calculated using the Ewald method, and is applied to the interpretation of the NMR spectra of protons of the water molecules of the interlamellar space.