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

Abstract

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.

Diffusion of Water in Li-Montmorillonite Studied by Quasielastic Neutron Scattering

“…An extensive series of NMR experiments on clay water systems was carded out by Fripiat and co-workers (Fripiat, 1977;Hougardy et al, 1976). Although the clays studied were not montmorillonites, making a strict comparison with the present results impossible, those containing monovalent counterions show features in common with each other, which suggest that these features are also to be expected for the montmorillonites.…”

“…The result from the experiments described here is that the protons are all undergoing translational diffusion. In principle, this could take place by proton transfer without any translational motion of the water molecules themselves (Hougardy et al, 1976). However, the concentration of hydrogen ions in the Li-montmorillonite system is far too low for proton transfer to contribute significantly to the scattering law.…”

“…For a two water-layer Na-vermiculite and a one-layer Li-hectorite, two T1 minima occur, which correspond to correlation times for proton motion at room temperature of about 10 -7 and 10 -l~ sec for the Na-clay and 10 -9 and 5 x 10 -11 sec for the Li-clay. Hougardy et al (1976) interpreted the shorter of these correlation times as arising from translational diffusion of protons in the system. They correlated their value of 10 -19 sec for the Na-vermiculite with a value estimated from the translational coefficient of diffusion of water in Na-montmorillonite measured by Olejnik et al (1970) using neutron scattering.…”

Proton Nuclear Magnetic Resonance Studies of “One-Layer” Hydrates of Oriented Hectorite

“…The central line observed for a synthetic fluoromontmorillonite by Hecht (1971) was attributed either to a H,O + triplet, or to proton exchange between water molecules. The latter interpretation was also proposed by Hougardy et al (1976) for Na-vermiculite "two-layer" hydrate. Proton exchange between water molecules may in the same way contribute to the central line which was observed for hectorite hydrates.…”

“…The "two-layer" hydrate of Na-vermiculite with low iron content (1700 ppm) was investigated by continuous-wave nuclear magnetic resonance (NMR) of H 1 and H 2 nuclei (Hougardy et al, 1976). The NMR absorption spectra consist of a doublet whose splitting obeys Pake's law and of a central line which does not depend on the angle 8 between the C ~ crystallographic axis and the steady magnetic field.…”