2) H and (139) La NMR Spectroscopy in Aqueous Solutions at Geochemical Pressures.
TL;DR: A new NMR probe designed for solution spectroscopy at geochemical pressures allows experiments on aqueous solutions to pressures corresponding roughly to those at the base of the Earth's continental crust.
Abstract: Nuclear spin relaxation rates of (2) H and (139) La in LaCl3 +(2) H2 O and La(ClO4 )3 +(2) H2 O solutions were determined as a function of pressure in order to demonstrate a new NMR probe designed for solution spectroscopy at geochemical pressures. The (2) H longitudinal relaxation rates (T1 ) vary linearly to 1.6 GPa, consistent with previous work at lower pressures. The (139) La T1 values vary both with solution chemistry and pressure, but converge with pressure, suggesting that the combined effects of increased viscosity and enhanced rates of ligand exchange control relaxation. This simple NMR probe design allows experiments on aqueous solutions to pressures corresponding roughly to those at the base of the Earth's continental crust.
Summary (3 min read)
- A common geochemical model of electrolyte solutions, the Helgeson-Kirkham-Flowers (HKF) model, describes the partial molar properties of solutes using an expression containing the dielectric constant of water and solutespecific variables derived from fits to experimental data.
- These new pressure and temperature limits inspired the design of a nuclear magnetic resonance (NMR) probe for examining solute speciation at higher pressures (Pautler et al., 2014; Ochoa et al., 2015).
- In contrast, the design described in the present paper has a 10–15 microliter sample volume (Fig. 1) and can reach pressures of 2.0 GPa.
- The CsCl and LaCl3 solutions were chosen because Jonas’ group not only measured the T1 values for these solu- tions, but also measured viscosities via the rolling-ball method at high pressures.
2. EXPERIMENTAL METHODS
- All cesium chloride (CsCl) solutions were prepared by dissolving the anhydrous salt into deuterium oxide solvent (D2O).
- This highpressure NMR probe is distinct from previous designs (e.g., Ballard et al., 1996, 1998) because it employs a small solenoid coil made of Berylco-25 wire wrapped around cylindrical capsule of PEEK (Polyether ether ketone) tubing containing 10–15 lL experimental solution.
- The cylindrical capsule is sealed at each end (Fig. 1.A) with waterproof epoxy.
- Pressure was monitored in situ using ruby fluorescence where movement of the R1 peak with pressure (Piermarini et al., 1975; Mao et al., 1986) could be monitored using an Ocean Optics HR4000 UV–vis spectrometer.
- Temperature in the probe was checked with a Type T thermocouple.
3. SOLUTION NMR
- Rapid tumbling of small molecules in a strong magnetic field causes the longitudinal and transverse relaxation times to become equivalent (T1 = T2).
- Here estimates in the pressure-variation of viscosity can be determined via measurements of the diffusion coefficient of H2O using the alternative expression: D ¼ kT 6pgr ð3Þ 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 (Edward, 1970).
- In the PGSE experiment, the apparent signal-decay time constant is measured with a spin-echo experiment as function of the strength of the applied magnetic-field gradient pulses.
- These intensity data are then fit to Eq. (4) in order to calculate D, the apparent diffusion coefficient, with uncertainties established as the precision of four trials.
- An assumption is made that the scaling does not vary with pressure, which is reasonable since controls of the magnetic-field gradients are exterior to the probe.
4.1. 2H and 133Cs NMR relaxation rates in CsCl solutions compared to LaCl3 solutions
- The results using the microcoil probe are bracketed by the previous work at near temperatures (Fig. 2.B).
- The maximum in T1 values with pressure has generally been interpreted to indicate pressure-enhanced structuring of the solvent.
- The microcoil design has much less precise control over pressure at P < 0.4 GPa than the large-volume design of Lee et al. (1974), but it can recapture the characteristic maxima in 2H T1 values observed for all CsCl solutions.
4.2. Diffusion coefficients for H2O at pressure
- As described above, the viscosity can be estimated from measurement of an apparent diffusion coefficient using the PGSE pulse sequence.
- The apparent diffusion coefficients for H2O in both pure water and in a 1.0 m CsCl solution measured here are shown in Fig. 4.A.
- Note that the D values for both solutions decrease uniformly with pressure and that these trends match well the pressure variation of T1 values shown in Figs. 2 and 3. Solutions viscosities were calculated via Eq. (3) and are reported in Table 1.
- If changes in viscosity alone caused the measured variation in T1 values with pressure, then the ratio of D/T1 would be independent of pressure.
- As one can see, the ratios for all conditions lie within 10 % of each other and are independent of pressure, within experimental error.
4.3. Evidence for suppressed freezing
- Pressure was applied to all CsCl solutions until they froze, which was inferred from the disappearance of the 133Cs NMR peak, or when the 2H linewidth increased beyond the bandwidth of the detector.
- The estimated pressures when each solution froze are reported in Fig. 5, with respect to various phase boundaries.
- Ice-V and Ice-VI phase boundaries were reproduced from the results of Wagner et al. (1994).
- Lines in Fig. 5 also identify the solid–liquid phase boundary in the presence of different NaCl concentrations in H2O (Journaux et al., 2013).
- The points plotted on Fig. 5, however, correspond to a crude estimate of the actual freezing pressure – the authors cannot identify frozen samples in their NMR probe.
5.1. Relaxation in fully dissociated electrolytes
- The differences in T1 relaxation values at ambient conditions for solutions of various CsCl and LaCl3 concentrations are expected since these electrolytes affect the structure of water differently.
- Electrolytes are classified as ‘structure-breaking’ or ‘structure-making’ according to their effects on NMR relaxation and the structure of water (Cox and Wolfenden, 1934; Hribar et al., 2002; Marcus, 2009).
- Structure-breaking ions, like Cs+, allow solvent molecules to rotate more freely when added to pure water and structure-making ions, like La3+, restrict the rotation of solvent molecules by binding tightly to the bulk solvent lattice.
- Pressure eliminates the differences in T1 values for different solutions.
5.2. Viscosity of aqueous solutions at pressure and NMR spectroscopy
- The broadening with pressure of these signals can, under ideal circumstances, be interpreted to indicate pressure-induced changes in reaction rates and thus activation volumes.
- The contribution of viscosity will be increasingly important as high pressures are reached.
- As one can see in Tables 1 and 2, the increases in solution viscosities with pressure are monotonic and can probably be predicted from a simple regression.
- This agreement is important because the data in Table 2 were measured by Jonas’ group using a completely different method.
- They employed a rolling sphere to estimate the viscosity from optical measurements.
5.3. Suppressed freezing and metastability
- There are two causes of the apparent overpressurization of these solutions.
- The figure demonstrates solution freezing at 1.75 GPa after 15 min, as gauged by the slow disappearance of the NMR signal.
- As mentioned above, temperature estimates seem to be accurate and indicate that the sample is not heated by the radio frequency pulse in the saturation-recovery pulse sequence.
- Similar over-pressurization of the samples is observed when either fluorocarbon liquid or Daphne 7373 oil is used as the pressure medium.
- The simplest explanation for the relatively high freezing pressures is that higher pressures ure for pure H2O and 4.5 m CsCl in D2O at 10 C and 30 C (Jonas ed in Table 1 that were measured independently via NMR methods, can be reached via the combined effects of freezing-point depression by the electrolyte and solution metastability.
6. CONCLUSIONS AND IMPLICATIONS FOR GEOCHEMISTRY
- There are several important geochemical results from this study.
- As mentioned in the Introduction, the range of geochemical models for solution thermodynamics was recently extended via moleculardynamic estimates of the dielectric properties of water to 6.0 GPa and 1200 C (Pan et al., 2013; Sverjensky et al., 2014).
- It is reasonable to expect a similar effect to that which is found for NaCl solutions (Journaux et al., 2013).
- Finally, transport properties of molecules can be measured easily at pressure, such as the diffusion coefficients shown in Fig.
- At the high pressures of this study, contributions to the NMR linewidth from increased solvent viscosity are appreciable, but can be estimated directly and easily (Fig. 5).
Did you find this useful? Give us your feedback
Cites background from "2) H and (139) La NMR Spectroscopy ..."
...Nuclear magnetic resonance (NMR) studies are now being extended to high-density fluids with the potential to help identify the nature of metal complexes (Pautler et al. 2014; Ochoa et al. 2015; Augustine et al. 2017)....
Related Papers (5)
Frequently Asked Questions (2)
Q1. What have the authors contributed in "Nmr spectroscopy of some electrolyte solutions to 1.9 gpa" ?
The longitudinal-relaxation times ( T1 ) for H compare well with those reported in the previous studies of Lee et al. ( 1974 ), who examined lower pressures, and indicate that the probe functions properly.
Q2. What are the future works in "Nmr spectroscopy of some electrolyte solutions to 1.9 gpa" ?
These pressures, however, are well beyond the current capabilities of conventional hydrothermal solution spectroscopies, yet through judicious choice of nonmagnetic, high-strength alloys ( e. g., Uwatoko et al., 2002 ), it should not be difficult to extend the 2. Furthermore, and most interestingly, the crystallization of high-pressure ices from these solutions at pressure is sufficiently slow that NMR spectra can be acquired. The authors, of course, know nothing of the phases that precipitate when they lose liquid-like NMR signals from these solutions, but these questions await further experimentation. Finally, transport properties of molecules can be measured easily at pressure, such as the diffusion coefficients shown in Fig.