NMR spectroscopy of some electrolyte solutions to 1.9 GPa

Abstract

Nuclear-magnetic resonance (NMR) spectra of CsCl and LaCl3 in D2O/H2O solutions were collected up to pressures of 1.9 GPa using a new NMR probe design that considerably extends the pressure range available for geochemical experiments. The longitudinal-relaxation times (T1) for 2 H compare well with those reported in the previous studies of Jonas et al. (1974), who examined lower pressures, and indicate that the probe functions properly. In some experiments, 133 Cs and 1 H NMR spectra could be taken on solutions to pressures well beyond the nominal freezing pressure of D2O or H2O to form Ice VI (near 0.9 GPa). Freezing to form the high-pressure ice is kinetically slow on an experimental time scale (minutes to hours). The data indicate that the electrolyte concentrations increase the freezing pressure of the solution. This result means that solution NMR spectra can be collected at pressures that are nearly twice the nominal freezing pressure of pure D2O or H2O. Pulsed-magnetic-field-gradient NMR methods are used to independently measure the self-diffusion coefficient of H2O in these solutions, which yields estimates of solution viscosity via the Stokes-Einstein relation. The increased viscosity accounts for the pressure variation of T1 values as rates of molecular tumbling are affected. Accounting for such changes is essential if NMR spectral line widths are used to infer pressureenhanced rates of geochemical reactions, such as interconversion of aqueous complexes.

17 O NMR as a Tool in Discrete Metal Oxide Cluster Chemistry

“…In 2013, however, molecular-dynamic simulations were used to estimate the dielectric constant to 1200°C and 5 GPa [130,131]. These new conditions motivated scientists to design new NMR probes to reach GPa pressures and that are suitable for solution spectroscopy [135][136][137][138]. Additionally, there are separate very promising efforts to establish chemical-shift accuracy in defective diamonds using optical detection [139].…”

“…38), D ¼ kT 6πηr (38) where D is the self-diffusion coefficient of molecules in the solvent, r is the hydrodynamic radius of the molecule and all other variables retain their usual definitions. Ochoa, Colla, Klavins, Augustine and Casey [137] built upon the extensive work by Jonas [141][142][143][144][145][146] to show that consistent results of solute diffusion could be achieved by using the clamp-cell and microcoil design and they then extended the measurements to 1.9 GPa. They used NMRmeasured diffusion coefficients to estimate viscosities at pressure via the Stokes-Einstein equation.…”

“…The increased viscosity contributes directly to broader NMR line widths by slowing molecular tumbling, even in the absence of chemical causes such as enhanced rates of chemical exchange. Although pressureenhanced increases in viscosity can usually be ignored for aqueous solutions in the 0.2-0.4 GPa range (but not in organic solvents [147]) [40][41][42][44][45][46][47][48]131,[134][135][136][137][138][139][140][145][146][147][148][149], increased viscosities in the pressure range 0.5-2.0 GPa causes significant broadening of an NMR line width. This broadening must be accounted in interpreting 17 O NMR spectra to yield rates of reactions, or changes in equilibrium constants (Fig.…”

“…9 Viscosities of aqueous solutions estimated from the measured diffusion coefficient of water in aqueous CsCl solutions and the Stokes-Einstein relation. The viscosity of water increases by a factor of ca 4 in the range 0.5-2.0 GPa [137]. If the resulting line broadening was misinterpreted as indicating a reaction volume, the result would be a À3 cm 3 /mol error.…”

29Si NMR of aqueous silicate complexes at gigapascal pressures

“…In recent years, developments of high-pressure solution NMR probes yielded information about the complexation of borate species, up to 1.2 GPa in pressure 25,26 . Efforts have also been made to increase the pressure threshold, robustness, and sensitivity of these types of probes, to the point where they can now reach 2.8 GPa [27][28][29] . Furthermore, there are active efforts to increase this pressure threshold but most designs limit the sample volume to sub microliter volumes where induction NMR detection of solutes is difficult (see Supplementary note 1) [30][31][32][33] .…”