Hydration of Na⁺, Ni²⁺, and Sm³⁺in the Interlayer of Hectorite: A Quasielastic Neutron Scattering Study

Abstract

Quasielastic neutron scattering experiments were performed with Na-hectorite, Ni-hectorite, and Sm-hectorite samples in order to find out whether Sm 3+ is present in the clay interlayer as a fully hydrated cation (outer-sphere complex), or, as it follows from neutron diffraction data analysis, it is dehydrated and bound to the clay surface (inner-sphere complex). The results obtained for the Sm-hectorite were compared with other interlayer cations: strongly hydrated Ni 2+ and relatively weakly hydrated Na+. It was found that water mobility in the Sm-hectorite sample is very close to the water mobility in Ni-hectorite. This is only possible if the Sm 3+ ion is fully hydrated. It was shown that water molecules hydrating Ni 2+ and Sm 3+ exhibit diffusion 2 mobility measurable with backscattering spectrometers. The diffusion coefficients of the exchangeable cations were found using the slow exchange approximations D Ni = (0.05 -0.14)x 10 -9 m 2 /s and D Sm = (0.04 -0.18) x 10 -9 m 2 /s.

Molecular dynamics simulations of the electrical double layer on smectite surfaces contacting concentrated mixed electrolyte (NaCl–CaCl2) solutions

“…Thirdly, calculation of D || from the mean-square displacement of molecules that are continuously located in a layer during a time interval τ implicitly assumes that the probability of a molecule leaving the layer is uncorrelated with its meansquare displacement. This assumption, which appears not to have been noted in previous studies, in fact may not be true; for example, if a water layer contains two populations of water molecules that have unequal diffusion coefficients and that exchange slowly on the timescale of measurement of the mean-square displacement (e.g., water molecules solvating divalent metal cations vs. other water molecules [99,123]), the more slowlydiffusing water population will be increasingly over-represented at large τ-values. Our hypothesis on the existence of this artifact is consistent with the observation that, at large τ, the D || values for first-and second-monolayer water in Fig.…”

Temperature dependent structure and dynamics in smectite interlayers: 23Na MAS NMR spectroscopy of Na-hectorite

“…44 Hectorite containing a wide variety of exchangeable species has been studied by NMR methods, 1,8,9,22,23,44 X-ray diffraction (XRD), 1,[45][46][47][48] thermogravimetric analysis (TGA), 1,44,46 and quasi-elastic neutron scattering (QENS). [24][25][26][49][50][51] In parallel, there have also been several computational molecular modeling studies of it using principally MD methods. 10,14,16,21,25,43,49,52 The results here show complex, temperature dependent dynamical behavior that depends greatly on the water content of the sample.…”

Structure, Energetics, and Dynamics of Cs+ and H2O in Hectorite: Molecular Dynamics Simulations with an Unconstrained Substrate Surface

“…[36][37][38]44,47,50,[52][53][54][55][56][57][58] The swelling energetics is strongly controlled by the clay composition, layer charge and cation hydration energy. For instance, the 4 swelling of the smectite mineral hectorite and its interactions with metal ions and H2O molecules have been probed in detail using X-ray diffraction (XRD), [59][60][61] thermo-gravimetric analysis (TGA), 59,62,63 neutron scattering, [64][65][66][67] and NMR spectroscopy. 10,38,59,62,[68][69][70] Interlayer structure and dynamics have been investigated by NMR 10,59,62,[68][69][70] and neutron scattering studies.…”

“…10,38,59,62,[68][69][70] Interlayer structure and dynamics have been investigated by NMR 10,59,62,[68][69][70] and neutron scattering studies. [64][65][66][67] Hectorite is widely used in NMR studies of smectites, 10,59,62,68,71 because its low Fe content minimizes paramagnetic effects on the probe nuclei. The results show that metal cations with higher hydration energies and smaller ionic radii are more prone to be hydrated and less often coordinated to the basal surface in inner sphere coordination than those with lower hydration energies and large ionic radii.…”