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

Abstract

Na MAS NMR spectroscopy of the smectite mineral hectorite acquired at temperatures from À120 C to 40 C in combination with the results from computational molecular dynamics (MD) simulations show the presence of complex dynamical processes in the interlayer galleries that depend significantly on their hydration state. The results indicate that site exchange occurs within individual interlayers that contain coexisting 1 and 2 water layer hydrates in different places. We suggest that the observed dynamical averaging may be due to motion of water volumes comparable to the dripplons recently proposed to occur in hydrated graphene interlayers (Yoshida et al. Nat. Commun., 2018, 9, 1496. Such motion would cause rippling of the T-O-T structure of the clay layers at frequencies greater than $25 kHz. For samples exposed to 0% relative humidity (R.H.), the 23 Na spectra show the presence of two Na + sites (probably 6 and 9 coordinated by basal oxygen atoms) that do not undergo dynamical averaging at any temperature from À120 C to 40 C. For samples exposed to R.H.s from 29% to 100% the spectra show the presence of three hydrated Na + sites that undergo dynamical averaging beginning at À60 C. These sites have different numbers of H 2 O molecules coordinating the Na + , and diffusion calculations indicate that they probably occur within the same individual interlayer. The average hydration state of Na + increases with increasing R.H. and water content of the clay.

Role of Cations in the Methane/Carbon Dioxide Partitioning in Nano- and Mesopores of Illite Using Constant Reservoir Composition Molecular Dynamics Simulation

“…Natural gas produced from shale and other tight reservoirs is one of the most important resources for addressing increasing global energy demand, and the injection of carbon dioxide into these reservoirs has significant potential to enhance oil and gas recovery and also to facilitate geological carbon sequestration. − Clay minerals are important components of shales and other tight formations, , and because the pores in these rocks are so small, interaction of the pore fluids (e.g., H 2 O, CO 2 , and CH 4 ) with clay surfaces plays an important role in controlling the structural environments of fluid species, their partitioning between pores of different sizes, and their transport properties. , The behavior of water and cations at mineral surfaces and in the nanoscale interlayer galleries of swelling clays has been extensively studied experimentally and by computational molecular simulations, − and in recent years, there has been increasing focus on the partitioning of CO 2 and H 2 O into expandable clay interlayers and how factors like mineral composition, cation size and charge, and the saturation state of the fluid phase impact the partitioning characteristics. − Mixed CO 2 /CH 4 fluids are of significant potential importance, especially near injection wells, but little is known about their mutual behavior in the nano- and mesopores in shales and other tight formations. Most studies that address CO 2 and CH 4 interactions are focused on their single-component adsorption in the interlayers of expandable clay minerals (smectites), which show a strong preferential adsorption of CO 2 over CH 4 . − …”

Caffeine Adsorption on Natural and Synthetic Smectite Clays: Adsorption Mechanism and Effect of Interlayer Cation Valence

“…In the spectrum for Na + HT, one simple peak was observed at a chemical shift of −2 ppm, which, because the chemical shift was close to 0 ppm, indicated a nearest-neighbor coordination environment similar to that of Na + ions in the solution, i.e., Na + (6H 2 O). 40 In the spectrum for Caf-Na + HT, the same peak was observed but at a slightly lower chemical shift compared with that for Na + HT. In addition, the lowfrequency shoulder of the peak, centered around −10 ppm, had a higher intensity than that in the Na + HT spectrum.…”

“…Figure A shows the 23 Na MAS-NMR spectra for fresh Na + HT and for Na + HT after exposure to a 5 mmol L –1 aqueous solution of caffeine (Caf-Na + HT). In the spectrum for Na + HT, one simple peak was observed at a chemical shift of −2 ppm, which, because the chemical shift was close to 0 ppm, indicated a nearest-neighbor coordination environment similar to that of Na + ions in the solution, i.e., Na + (6H 2 O) . In the spectrum for Caf-Na + HT, the same peak was observed but at a slightly lower chemical shift compared with that for Na + HT.…”

Water and Ion Dynamics in Confined Media: A Multi-Scale Study of the Clay/Water Interface

“…For that purpose, numerous dynamical studies have been performed to probe, over a broad dynamical range, the mobility of ions [9,10] and molecules [11][12][13] physisorbed at the clay/water interface. As illustrated in Figure 1, we selected to use a complementary set of experiments, including Inelastic Neutron Scattering (INS) [14][15][16][17][18][19], Quasi-Elastic Neutron Scattering (QENS) [12,[20][21][22][23][24][25][26][27][28][29][30], Nuclear Magnetic Resonance (NMR) relaxometry [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49], Pulsed-Gradient Spin-Echo (PGSE) attenuation [11,[50][51][52], Magnetic Resonance Imaging (MRI)…”