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Faux DA, Istók Ö, Rahaman AA, McDonald PJ, McKiernan E, Brougham DF. Nuclear spin relaxation in aqueous paramagnetic ion solutions. Phys Rev E 2023; 107:054605. [PMID: 37328976 DOI: 10.1103/physreve.107.054605] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2022] [Accepted: 04/20/2023] [Indexed: 06/18/2023]
Abstract
A Brownian shell model describing the random rotational motion of a spherical shell of uniform particle density is presented and validated by molecular dynamics simulations. The model is applied to proton spin rotation in aqueous paramagnetic ion complexes to yield an expression for the Larmor-frequency-dependent nuclear magnetic resonance spin-lattice relaxation rate T_{1}^{-1}(ω) describing the dipolar coupling of the nuclear spin of the proton with the electronic spin of the ion. The Brownian shell model provides a significant enhancement to existing particle-particle dipolar models without added complexity, allowing fits to experimental T_{1}^{-1}(ω) dispersion curves without arbitrary scaling parameters. The model is successfully applied to measurements of T_{1}^{-1}(ω) from aqueous manganese(II), iron(III), and copper(II) systems where the scalar coupling contribution is known to be small. Appropriate combinations of Brownian shell and translational diffusion models, representing the inner and outer sphere relaxation contributions, respectively, are shown to provide excellent fits. Quantitative fits are obtained to the full dispersion curve of each aquoion with just five fit parameters, with the distance and time parameters each taking a physically justifiable numerical value.
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Affiliation(s)
- David A Faux
- Department of Physics, University of Surrey, Guildford, GU2 7XH, United Kingdom
| | | | | | | | - Eoin McKiernan
- School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
| | - Dermot F Brougham
- School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
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Zhang Y, Klein JM, Akolkar R, Gurkan BE, Maginn EJ. Solvation Structure, Dynamics, and Charge Transfer Kinetics of Cu 2+ and Cu + in Choline Chloride Ethylene Glycol Electrolytes. J Phys Chem B 2022; 126:6493-6499. [PMID: 35976689 DOI: 10.1021/acs.jpcb.2c04798] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Experimental measurements and classical molecular dynamics (MD) simulations were carried out to study electrolytes containing CuCl2 and CuCl salts in mixtures of choline chloride (ChCl) and ethylene glycol (EG). The study focused on the concentration of 100 mM of both CuCl2 and CuCl with the ratio of ChCl/EG varied from 1:2, 1:3, 1:4, to 1:5. It was found that the Cu2+ and Cu+ have different solvation environments in their first solvation shell. Cu2+ is coordinated by both Cl- anions and EG molecules, whereas Cu+ is only solvated by EG. However, both Cu2+ and Cu+ show strong interactions with their second solvation shells, which include both Cl- anions and EG molecules. Considering both the first and second solvation shells, the concentrations of Cu2+ and Cu+ that have various coordination numbers in each solution were calculated and were found to correlate qualitatively with the exchange current density trends reported in previous experiments of Cu2+ reduction to Cu+. This finding makes a connection between atomic solvation structure observed in MD simulations and redox reaction kinetics measured in electrochemical experiments, thus revealing the significance of the solvation environment of reduced and oxidized species for electrokinetics in deep eutectic solvents.
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Affiliation(s)
- Yong Zhang
- Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States
| | - Jeffrey M Klein
- Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
| | - Rohan Akolkar
- Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
| | - Burcu E Gurkan
- Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
| | - Edward J Maginn
- Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States
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Tang J, Luo Q, Zeng D, Zhang N. Thermodynamic and Kinetic Studies on the Conversion of Solvent-Shared to Contact Ion Pairs in Sparingly Soluble MF 2 (M = Mg 2+ and Ca 2+) Aqueous Solutions: Implications for Understanding Supersaturated Behavior and Association Constant Determination. J Phys Chem B 2022; 126:1566-1578. [PMID: 35167281 DOI: 10.1021/acs.jpcb.1c09135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The thermodynamic and kinetic behaviors of Mg2+-F- ion pairing in aqueous solution are investigated theoretically and experimentally and are contrasted to those of Ca2+-F-. Thermodynamically, similar to CaFx(H2O)142-x (x = 1 and 2), MgF(H2O)y+ (y = 14-20) contact ion pairs (CIPs) are more stable than their solvent-shared ion pairs (SSIPs), whereas the CIPs and SSIPs of MF2(H2O)y are almost isoenergetic. However, in kinetics, the conversion of SSIPs to CIPs for M2+-F- (M = Mg2+ and Ca2+) ion pairing must overcome a high energy barrier due to the strong hydration of Mg2+ and F-. The kinetics dominate after the thermodynamics and kinetics are balanced, which hinders the formation of M2+-F- CIPs in practical MF2 aqueous solutions (less than or equal to saturated concentrations). This result is also supported by the 19F nuclear magnetic resonance spectra of saturated MF2 solutions. Although the interaction between Mg2+ and F- is slightly stronger than that between Ca2+ and F- due to the smaller radius of Mg2+, the formation of Mg2+-F- CIPs needs to go through two rate-limiting steps, the dehydration and entrance of F- (i.e., via exchange mode) with a higher energy barrier, due to the ability of strongly bound water molecules and rigorous octahedral coordinated configuration of Mg2+, while the formation of Ca2+-F- CIPs only goes through a single rate-limiting step, the entrance of F- (i.e., via swinging mode) with a lower energy barrier, due to the flexible coordination configuration of Ca2+. This is responsible for precipitation in MgF2 aqueous solution requiring a larger supersaturation degree and a lower precipitation rate than in CaF2. These kinetic factors lead to the association constants previously reported for MF+ determined by a fluoride ion-selective electrode (ISE) combined with the titration method, where the MF2 solutions were always unsaturated at the titration end point, which actually corresponds to those of the ligand process going from completely free M2+ and F- to their SSIPs. A possible strategy to accurately determine the association constants of MF+ and MF2(aq) CIPs by fluoride ISEs is proposed. The present results suggest that judging the formation of M2+-F- CIPs in practical solutions from a theoretical calculation perspective requires significant consideration of the kinetic factors, except for the thermodynamic factors.
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Affiliation(s)
- Jianfeng Tang
- College of Science, Central South University of Forestry and Technology, Changsha, Hunan 410004, P. R. China
| | - Qiongqiong Luo
- College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China
| | - Dewen Zeng
- College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China
| | - Ning Zhang
- College of Science, Central South University of Forestry and Technology, Changsha, Hunan 410004, P. R. China
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Abstract
Basic studies on concentrated solutions are becoming more and more important due to the practical industrial and geological applications. The use in redox flow batteries is one of the most important applications of these solutions. Specifically, in this paper we investigated high-concentrated copper chloro-complexes solutions with different additives. The concentration of ligands and additives affects the physicochemical and electrochemical properties of 2 M solutions of Cu(I) and Cu(II). Solutions with calcium chloride and HCl as Cl− source were investigated with Cu:Cl ratios of 1:5 and 1:7, the 1:5 Cu:Cl ratio being the best performing. The substitution of calcium chloride with ammonium chloride increased the conductivity. However, while the effect on the positive electrode process was not very evident, the reversibility of the copper deposition–stripping process was greatly improved. Orthophosphoric acid could be a viable additive to decrease the complexation of calcium with chloride anions and to improve the stability of Cu(II) chloro-complexes. Absorption spectroscopy demonstrated that phosphate ions do not coordinate copper(II) but lead to a shift in the distribution of copper chloro-complexes toward more coordinated species. Electrochemically, the increased availability of chloride anions in solution stabilized the Cu(II)-rich solution and led to increased reversibility of the Cu(II)/Cu(I) redox process.
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Zhang N, Tang J, Luo Q, Wang S, Zeng D. Computational and solubility equilibrium experimental insight into Ca 2+-fluoride complexation and their dissociation behaviors in aqueous solutions: implication for the association constant measured using fluoride ion selective electrodes. Phys Chem Chem Phys 2021; 23:24711-24725. [PMID: 34709252 DOI: 10.1039/d1cp02087k] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Although the Ca2+-F- association is of great importance for aqueous environments and industrial systems containing F-, as well as for defluorination processes, many details of the association solvation structures and behavior remain unclear. Herein, a combination of classical/ab initio molecular dynamics simulations and density functional theory calculations was used to investigate the structure and hydration of CaFx2-x (x = 1, 2) and the association/dissociation behavior of Ca2+-F- in aqueous CaF2 solutions. The primary shell of Ca2+ is found to be very flexible in the association of Ca2+-F-, with coordination numbers dynamically oscillating in the range of 6-9, with 6 and 7 being the most favorable. The calculations show that for CaF(H2O)14+, the contact ion pair (CIP) is more favorable and occurs with no energy barrier, whereas the formation of CaF2(aq.) must overcome a ∼3.6 kJ mol-1 energy barrier; moreover, the CIP and solvent shared ion pair (SSIP) dynamically coexist for CaF2(H2O)14 in aqueous CaF2 solutions. Calculations for the dissociation process of CaF(H2O)6+ show a dramatic energy increase going from SSIP to free Ca2+ and F-, ascribed to the surprisingly long-range electrostatic attraction between Ca2+ and F- rather than to special F⋯H interactions. The energy increase results in the estimated association constant of CaF+ being larger than that previously measured using fluoride ion selective electrodes. This is attributed to the fact that the latter value might correspond to the ligand reaction of free Ca2+ and F- to form the Ca2+-F- SSIP. The combination of these results with CaF2(s) solubility measurements suggests that the higher-order Ca2+-F- complexes are absent in aqueous CaF2 solutions.
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Affiliation(s)
- Ning Zhang
- College of Science, Central South University of Forestry and Technology, Changsha 410004, Hunan, P. R. China.
| | - Jianfeng Tang
- College of Science, Central South University of Forestry and Technology, Changsha 410004, Hunan, P. R. China.
| | - Qiongqiong Luo
- College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China
| | - Shaoheng Wang
- College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China
| | - Dewen Zeng
- College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China
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Zhang N, He S, Li Y, Zhou Q, Zeng D, Zhao Z, Yan J, Zeng J, Hefter G. Spectroscopic Study of the Behavior of Mo(VI) and W(VI) Polyanions in Sulfuric-Phosphoric Acid Mixtures. Inorg Chem 2021; 60:17565-17578. [PMID: 34738803 DOI: 10.1021/acs.inorgchem.1c02133] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The solution chemistry of Mo(VI) and W(VI) in mixtures of sulfuric and phosphoric acids is relevant to the development of practicable hydrometallurgical processes for the recovery and separation of these two elements from low-grade scheelite ores. The behavior of Mo(VI) and W(VI) in such mixtures has been studied using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), nuclear magnetic resonance (NMR), and small-angle X-ray scattering (SAXS) spectroscopies, along with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). Where applicable, these techniques have produced a self-consistent picture of the similarities and differences between the chemical speciation of Mo(VI) and W(VI) as functions of solution composition, mostly at a constant phosphorous/metal (P/M; M = Mo(VI) or W(VI)) ratio of ∼1. In dilute acidic media (0.02 mol·kg-1 H+, without H2SO4), Mo(VI) exists mostly (∼60%) as P2Mo5O236- with the remaining ∼40% as β-Mo8O264-. Under the same conditions, W(VI) is largely present as NaPW11O396- (∼80%) and P2W5O236- (∼10%), with the remainder probably occurring as isopolytungstates such as W12O4212- and some tungstophosphate dimers such as P2W18O626-. At higher acid concentrations (≲5 mol·kg-1 H2SO4), polymeric Mo(VI) anions are broken down to form the oxocations MoO22+ and Mo2O52+ and their protonated forms, with the dimers becoming increasingly dominant at higher acidities (∼80% in 5 mol·kg-1 H2SO4). In stark contrast, W(VI) polyanions do not decompose at higher acidities but instead form (∼70% in 0.6 mol·kg-1 H2SO4) a Keggin ion, PW12O403-. Further acidification with H2SO4 results in the agglomeration of this Keggin ion, forming clusters of about 50 and 100 Å in diameter that ultimately produce crystalline precipitates, which could be identified in part by their X-ray diffraction patterns. Possible application of these findings to the hydrometallurgical separation of Mo and W using acidic solutions is briefly discussed, based on a limited number of batch solvent extractions.
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Affiliation(s)
- Ning Zhang
- College of Science, Central South University of Forestry and Technology, Changsha 410004, Hunan, P. R. China
| | - Shan He
- College of Science, Central South University of Forestry and Technology, Changsha 410004, Hunan, P. R. China
| | - Yongli Li
- School of Material Science and Engineering, Zhengzhou University, Zhengzhou 450001, Henan, P. R. China
| | - Qiuju Zhou
- College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, Henan, P. R. China
| | - Dewen Zeng
- College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China
| | - Zhongwei Zhao
- School of Metallurgy and Environment, Central South University, Changsha 410083, Hunan, P. R. China
| | - Jun Yan
- College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China
| | - Jianrong Zeng
- Shanghai Synchrotron Radiation Facility, Zhangjiang Laboratory, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201204, P. R. China
| | - Glenn Hefter
- Chemistry Department, Murdoch University, Murdoch, WA 6150, Australia
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