Interaction of water with clay minerals as studied by H-2 nuclear magnetic resonance spectroscopy

The interaction of water with a series of Ca2+/Na+,Ca2+/NH+4, and NH+4/Na+-exchanged hectorite and saponite clays in water- and clay-dominated systems was studied using H-2 NMR. Variable temperature Nuclear Magnetic Resonance (NMR) (in the range of 189-299 K) was also used to determine the frequency of exchange and the rotation activation energy between water molecules hydrating the cations and the free water of the tactoid. The H-2 quadrupolar splitting and spin-lattice relaxation time (T-1) are related to the ratio of clay to adsorbed water in these systems. In water-dominated systems (where clay/water ratio is less than 1), motional averaging occurs between water hydrating the cations and the ''free'' or bulk water of the tactoid. In these systems, the spin-lattice relaxation time (T-1) is dominated by the relaxation of the ''free'' water on the external surface of the tactoids and has a value close to that for D2O (0.436 s), with the rotational correlation time (T-c) approaching 2.93 x 10(-12) s. A more recent article places T-c at 1.95 x 10(-12) s. The clay systems in this study have clay/water ratios (=0.0238 g-clay/mL-D2O), and the quadrupolar splittings are of the order of 100 Hz or less. Splitting of <100 Hz indicate that approximately 1 water molecule is bound to the clay for every 1.7 x 10(3) ''free'' water molecules. In clay dominated systems with extremely low water contents (clay/water ratio is-greater-than 1 g-clay/mL-D2O), less motional averaging occurs. Rotational correlation times are slower and greater residual quadrupolar splittings are also observed. Increased residual quadrupolar splittings are observed with increasing clay/water ratios. For a Na-exchanged hectorite sample (5.31 g-clay/mL-D2O, the quadrupolar splittings are of the order of 12 kHz, and for a Na-exchanaged saponite (4.05 g-clay/mL-D2O, the quadrupolar splittings are of the order of 11 kHz. Rotational activation energies (E(a)) of 37.8 kJ mole(-1) (9.0 kcal.mole(-1)) for adsorbed water molecules on Na-exchanged hectorite and 27.2 kJ.mole(-1) (6.5 kcal.mole(-1)) for Na-exchanged saponite were calculated from spin-lattice relaxation (T-1) measurements. These values compare with calculated activation energies of rotation (8.5 kcal.mole(-1)) for water of hydration adsorbed to a Na-exchanged vermiculite.