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

Abstract

The interaction of water with a series of Ca 2+/Na + , Ca 2+/NH 4 +, and NH 4 +/Na +-exchanged hectorite and saponite clays in water- and clay-dominated systems was studied using 2H NMR. Variabletemperature 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 2H 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 ≪ 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 D 2O (0.436 s), with the rotational correlation time ( τ c) approaching 2.93 × 10 −12 s. A more recent article places τ c at 1.95 × 10 −12 s. The clay systems in this study have clay/ water ratios (=0.0238 g-clay/mL-D 2O), and the quadrupolar splittings are of the order of 100 Hz or less. Splittings of <100 Hz indicate that approximately 1 water molecule is bound to the clay for every 1.7 × 10 3 “free” water molecules. In clay-dominated systems with extremely low water contents (clay/water ratio > 1 g-clay/mL-D 2O), 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-D 2O), the quadrupolar splittings are of the order of 12 kHz, and for a Na-exchanged saponite (4.05 g-clay/mL-D 2O), 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.