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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution 27
Al Pulsed Field Gradient, Diffusion-NMR Spectroscopy of Solvation Dynamics and Ion Pairing in Alkaline Aluminate Solutions
Trent R. Graham, Kee Sung Han, Mateusz Dembowski, Anthony James Krzysko, Xin Zhang, Jian Zhi Hu, Sue B. Clark, Aurora Evelyn Clark, Gregory K. Schenter, Kevin M. Rosso, and Carolyn I. Pearce J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b10145 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on November 13, 2018
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The Journal of Physical Chemistry
27Al
Pulsed Field Gradient, Diffusion-NMR Spectroscopy of Solvation Dynamics and Ion Pairing in Alkaline Aluminate Solutions Trent R. Graham†*, Kee Sung Han†, Mateusz Dembowski†, Anthony Krzysko‡, Xin Zhang†, Jianzhi Hu†, Sue Clark†‡, Aurora Clark‡, Gregory Schenter†, Kevin Rosso† and Carolyn Pearce† † Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡ Department of Chemistry, Washington State University, Pullman, Washington 99164, United States
Corresponding Authors *
[email protected] ACS Paragon Plus Environment
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ABSTRACT
Pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy was successfully applied to the
27Al
(I = 5/2) nucleus in concentrated electrolytes to investigate the diffusion of
aluminate ions [Al(OH)4-] in simulant high-level nuclear waste (3 M NaOH) between 25 and 85˚C. The temperature dependent diffusion coefficients obtained from 1H, 23Na and 27Al PFG-NMR were well fit by a Vogel-Fulcher-Tammann model and a power law equation. Comparison of
27Al
diffusion coefficients of 0.1 M Al(OH)4- in ~ 3 M MOH (where M = Na+, K+, (CH3)4N+) at room temperature varied in agreement with expected changes in solution viscosity via Stokes-Einstein relationship, confirming that the dominant Al species at these conditions are Al(OH)4- monomers. This 27Al PFG-NMR study extends an established methodology to a previously unexplored nucleus enabling this experimental technique to be leverage for exploring ion transport, speciation and solution structure in concentrated electrolytes.
KEYWORDS Quadrupolar Diffusion NMR, Electrolytes, Sodium Aluminate
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The Journal of Physical Chemistry
1.0 INTRODUCTION Concentrated electrolytes are common environments of diverse systems, including hypersaline biological systems in halophiles1 and industrial areas such as in nuclear waste processing.2–5 These low water activity liquids exhibit density inhomogeneities,6,7 which lead to complex material properties between those of an ideal liquid and a solid.8 Challenges persist in relating these properties to the underlying dynamics of solvation and ion-ion interactions at the molecular scale. For example, density inhomogeneity is often interpreted as molecular clustering or self-assembly, but significant uncertainty arises from the heterogeneous dynamics of assembly and disassembly9 and morphological variations of ion-ion aggregates.10 However, clustering has implicit effects on the transport properties of molecules and ions.11 Hence the ability to directly measure translational dynamics within these concentrated electrolytes can supply crucial information on the true extent of ion-ion aggregation.12,13 In turn, knowledge of these molecularscale properties leads to improved predictions of a host of other processes in concentrated electrolytes including, for example, adsorption and desorption at mineral-water interfaces.14 A common technique to investigate fluid transport properties is diffusion ordered, pulsed field gradient (PFG) NMR. Such studies have utilized several NMR nuclei including: 1H, 2H, 7Li, 13C, 19F, 21Ne, 23Na, 29Si, 31P, 129Xe,
and 131Cs.15,16 The simplest PFG-NMR pulse sequence, PFG-
echo, incorporates a pulse sequence composed of a pair of radio frequency (RF) pulses and a pair of spatially encoded pulsed magnetic field gradients applied before and after the second RF pulse with a mixing period for translocation of molecules, denoted the diffusion time (Δ), to encode and decode the initial and final positions, respectively, for all susceptible NMR active nuclei. If the molecular displacement is negligible relative to the PFG strength dependent encoded spatial resolution, the intensity of PFG-echo signal obtained after applying this pulse sequence is identical
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to that obtained after the first RF pulse. When the spatially encoded PFG is strong enough to detect the translocation of molecules, the signal will be reduced, enabling quantification of diffusion coefficients.17 In practice, an array of experiments are conducted in which the gradient strength function is incrementally increased. The incremental decay in signal intensity is related to the diffusivity through the Stejskal-Tanner relationship18,19 𝐼(𝐺) = 𝐼𝑜𝑒
[ ―𝐷(∆ ― 𝛿/3)(𝛾𝐺𝛿)2]
Eq. 1
where 𝐼(𝐺) is the intensity of the resonance at the gradient strength of 𝐺, 𝐼𝑜 is the intensity with G = 0, 𝐷 is the diffusivity, ∆ is the diffusion time, 𝛿 is the gradient pulse length, and 𝛾 is the gyromagnetic ratio of the observed nuclei. The challenges of conducting PFG-NMR experiments on low γ, quadrupolar nuclei can be seen through inspection of Equation 1. One challenge of PFG-NMR measurements is γ(27Al) = 11.094 MHz T-1 which is ~ 3.8 times smaller than γ(1H) = 42.577 MHz T-1. Consequentially, nuclei with low gyromagnetic ratios necessitate application of strong pulsed field gradients (approximately 15 times stronger for the case of 27Al) to attenuate the resonance compared to a proton (1H) resonance exhibiting identical diffusivity. Secondly and more importantly, quadrupolar nuclei exhibit fast spin-lattice (T1, longitudinal) and spin-spin (T2, transverse) relaxations20 which lead to poorly measurable signal intensity due to signal loss during the mixing time due to T2 (T1)
