Abstract
23Na MAS NMR spectroscopy of the smectite mineral hectorite acquired at temperatures from −120 °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 23Na 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 −120 °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 −60 °C. These sites have different numbers of H2O 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.
Highlights
The interactions among ions, water, and a substrate are fundamental processes in elds as diverse as biology, medicine, chemistry, materials science, geology, and space science
This paper reports the results of an experimental, variable temperature 23Na nuclear magnetic resonance (NMR) and computational molecular dynamics (MD) modeling study of the structural and dynamical behavior of Na+ in the widely investigated smectite clay, hectorite
Variable temperature 23Na NMR spectra of variably hydrated Nahectorite in combination with computational MD modeling shows the coexistence of several local hydration states of Na+ in an individual sample and that the average hydration state increases with increasing R.H
Summary
The interactions among ions, water, and a substrate are fundamental processes in elds as diverse as biology, medicine, chemistry, materials science, geology, and space science. In many of these elds, and especially materials science and the geosciences, the substrate for the interactions is o en a solid oxide or hydroxide material which can have wide ranging structural and chemical properties. The 23Na NMR behavior of hectorite has been studied previously,[1] but the results here provide more comprehensive understanding, because the spectra were obtained at a higher H0 eld strength (850 MHz 1H frequency) and over a wider range of temperatures and water contents. Hectorite is commonly used in NMR studies of smectites, because its low Fe content greatly reduces
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