Pulsed Field Gradient NMR Research Articles

Concentration- and Temperature-Driven Evolution of V5+ Solution Structure in Redox-Flow Battery Electrolytes Studied By Multinuclear NMR

The ongoing implementation of renewable energy generation systems into worldwide electrical grids increasingly demands additional energy storage platforms to maintain stability through their associated intermittency, with vanadium redox-flow batteries being a forefront solution owing to the decoupling of energy- and power-density that they afford.1 However, a lingering performance issue associated with this chemistry in its conventional format, with the vanadium ions dissolved in concentrated aqueous sulfuric acid, is the poor thermal stability of the V5+ cation in solution. This leads to precipitation as the hydrated penta-coordinated vanadate complex, stable at room temperature, is deprotonated and converted to insoluble V2O5 by a condensation reaction induced at elevated temperature.2 This process ultimately limits the concentration of the electrolyte – adversely impacting the available energy density – and its operation at temperatures above ~310 K. While alternatives to this relatively simple electrolyte composition have shown great promise in rectifying this thermal- and concentration-induced instability of the solvated V5+, via the introduction of mixed acid solvent systems3 or stabilizing additives,4 a complete understanding in the original formulation at a range of length- and time-scales and across a multitude of conditions will aid the further development of alternative systems, and provide a means to benchmark their performance at a very fundamental level.This study therefore aims to greatly expand on previous multinuclear NMR work2,5 on the elucidation of this precipitation mechanism by combining a thorough exploration of concentration effects on V5+ solvation structure and dynamics in conjunction with an investigation of their thermal behavior. Ten samples spanning the concentration range of 0 M to 1.7 M V5+ (all with 5 M H2SO4) were examined over temperatures from 298 K to 383 K ascending, and subsequently likewise descending. Combining 1H, 17O, 33S, and 51V linewidth and shift measurements, the evolving solution structure with increasing temperature – and the associated hysteresis on again decreasing it – is mapped as a function of the V5+ concentration. In particular, the extent of the thermal hysteresis previously established with 17O and 51V linewidth and shift measurements at high V5+ concentration2 is quantitatively examined as the concentration is lowered. This spectroscopic analysis is complemented by a similar variable-concentration and variable-temperature study of the 1H self-diffusivity via pulsed-field gradient (PFG) NMR and by 17O longitudinal (T1) relaxation measurements, to determine both the effect of this evolving solution structure on the dynamics and transport, and how they are impacted by increasing V5+ concentration via the associated activation energy trends. The 1H PFG-NMR measurements, coupled with high-accuracy dynamic viscosity measurements via the falling-ball method, are also used to examine the evolution of the effective proton hydrodynamic radius as the V5+ aggregation reaction occurs. The totality of the data, collected across such a wide range of concentration and temperature conditions, yields an unprecedented map of the V5+ solvation phenomena and solution dynamics which spans molecular to bulk length- and time-scales, and which will serve as a useful platform of comparison for analyzing the effectiveness of alternative solvent systems and stabilizing agents presently undergoing development. References M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011) https://iopscience.iop.org/article/10.1149/1.3599565.M. Vijayakumar et al., J. Power Sources, 196, 3669–3672 (2011) http://dx.doi.org/10.1016/j.jpowsour.2010.11.126.L. Li et al., Adv. Energy Mater., 1, 394–400 (2011) http://doi.wiley.com/10.1002/aenm.201100008.N. Kausar, A. Mousa, and M. Skyllas-Kazacos, ChemElectroChem, 3, 276–282 (2016) http://doi.wiley.com/10.1002/celc.201500453.M. Vijayakumar et al., Chempluschem, 80, 428–437 (2015) http://doi.wiley.com/10.1002/cplu.201402139.

Molecular and Ionic Diffusion in Ion Exchange Membranes and Biological Systems (Cells and Proteins) Studied by NMR.

The results of NMR, and especially pulsed field gradient NMR (PFG NMR) investigations, are summarized. Pulsed field gradient NMR technique makes it possible to investigate directly the partial self-diffusion processes in spatial scales from tenth micron to millimeters. Modern NMR spectrometer diffusive units enable to measure self-diffusion coefficients from 10−13 m2/s to 10−8 m2/s in different materials on 1 H, 2 H, 7 Li, 13 C, 19 F, 23 Na, 31 P, 133 Cs nuclei. PFG NMR became the method of choice for reveals of transport mechanism in polymeric electrolytes for lithium batteries and fuel cells. Second wide field of application this technique is the exchange processes and lateral diffusion in biological cells as well as molecular association of proteins. In this case a permeability, cell size, and associate lifetime could be estimated. The authors have presented the review of their research carried out in Karpov Institute of Physical Chemistry, Moscow, Russia; Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia; Kazan Federal University, Kazan, Russia; Korea University, Seoul, South Korea; Yokohama National University, Yokohama, Japan. The results of water molecule and Li+, Na+, Cs+ cation self-diffusion in Nafion membranes and membranes based on sulfonated polystyrene, water (and water soluble) fullerene derivative permeability in RBC, casein molecule association have being discussed.

New Technique for Evaluating Fracture Geometry and Preferential Orientation Using Pulsed Field Gradient Nuclear Magnetic Resonance

Summary In-situ evaluation of fracture tortuosity (i.e., pore geometry complexity and roughness) and preferential orientation is crucial for fluid flow simulation and production forecast in subsurface water and hydrocarbon reservoirs. This is particularly significant for naturally fractured reservoirs or postacid fracturing because of the strong permeability anisotropy. However, such downhole in-situ characterization remains a challenge. This study presents a new method for evaluating fracture tortuosity and preferential orientation based on the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) technique. Such an approach provides diffusion tortuosity, τd, defined as the ratio of bulk fluid diffusion coefficient to the restricted diffusion coefficient in the porous media. In the PFG NMR technique, the magnetic field gradient can be applied in different directions, and therefore anisotropy in diffusion coefficient and τd can be evaluated. Three 3D printed samples, characterized by well controlled variable fracture tortuosity, one fractured sandstone, and three acidized carbonate samples with wormhole were used in this study. PFG NMR measurements were performed using both 2- and 12-MHz NMR instruments to obtain τd in the three different principal directions. The results obtained from the NMR measurements were compared with fracture tortuosity and preferential orientation determined from the microcomputed tomography (micro-CT) images of the samples. The results showed that τd increases as the fracture tortuosity and pore geometry complexity increases, showing good agreement with the image-based geometric tortuosity values. Moreover, the lowest τd values were found to coincide with the preferential direction of fracture surfaces and wormhole body for a given sample, whereas the maximum τd values correspond to the nonconnected pathway directions. These results suggest that the implantation of directional restricted diffusion measurements on the NMR well logging tools would offer a possibility of probing tortuosity and determining preferential fluid flow direction via direct downhole measurements.

Concentration-Dependent Solvation Structure and Dynamics of Aqueous Sulfuric Acid Using Multinuclear NMR and DFT.

Sulfuric acid is a ubiquitous compound for industrial processes, and aqueous sulfate solutions also play a critical role as electrolytes for many prominent battery chemistries. While the thermodynamic literature on it is quite well-developed, comprehensive studies of the solvation structure, particularly molecular-scale dynamical and transport properties, are less available. This study applies a multinuclear nuclear magnetic resonance (NMR) approach to the elucidation of the solvation structure and dynamics over wide temperature (-10 to 50 °C) and concentration (0-18 M) ranges, combining the 17O shift, line width, and T1 relaxation measurements, 33S shift and line width measurements, and 1H pulsed-field gradient NMR measurements of proton self-diffusivity. In conjunction, these results indicate a crossover between two regimes of solvation structure and dynamics, occurring above the concentration associated with the deep eutectic point (∼4.5 M), with the high-concentration regime dominated by a strong water-sulfate correlation. This description was borne out in detail by the activation energy trends with increasing concentration derived from the relaxation of both the H2O/H3O+ and H2SO4/HSO4-/SO42- 17O resonances and the 1H self-diffusivity. However, the 17O chemical shift difference between the H2O/H3O+ and H2SO4/HSO4-/SO42- resonances across the entire temperature range is nevertheless strikingly linear. A computational approach coupling molecular dynamics simulations and density functional theory NMR shift calculations to reproduce this trend is presented, which will be the subject of further development. This combination of multinuclear, dynamical NMR, and computational methods, and the results furnished by this study, will provide a platform for future studies on battery electrolytes where aqueous sulfate chemistry plays a central role in the solution structure.

Hydration, self-diffusion and ionic conductivity of Li+, Na+ and Cs+ cations in Nafion membrane studied by NMR

Hydration of Nafion 117 perfluorinated sulfonic cation-exchange membrane in alkaline ion forms was investigated by high resolution 1H NMR. Hydration numbers of Li+, Na+ and Cs+ cations were 5 ± 1, 6 ± 1 and 1 ± 0.2, correspondingly for membrane equilibrated with water vapor at 98% RH. As opposed to Li+ and Na+, which form separate ion pair, Cs+ cation directly contacts with membrane sulfonate group. Cation self-diffusion coefficients were measured by pulsed field gradient NMR technique on 7Li, 23Na and 133Cs nuclei for the first time. Self-diffusion coefficients are changed in the next rows Li+ ≤ Na+ > Cs+. Self-diffusion activation energies of Li+ and Na+ cations are about 20 kJ/mol which is close to water self-diffusion activation energy in these membranes, but Cs+ self-diffusion activation energy is distinctly more (25 kJ/mol). Ionic conductivities calculated on the basis of Nernst–Einstein equation from cation self-diffusion coefficients 1.6∙10−2, 2∙10−2, 6∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, are closely approximating to conductivities measured by impedance spectroscopy: 1.3∙10−2, 1.1∙10−2, 2.3∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, but calculated values are appreciably more compared with experimental meanings.

Use of Solid-State NMR Spectroscopy for the Characterization of Molecular Structure and Dynamics in Solid Polymer and Hybrid Electrolytes.

Solid-state NMR spectroscopy is an established experimental technique which is used for the characterization of structural and dynamic properties of materials in their native state. Many types of solid-state NMR experiments have been used to characterize both lithium-based and sodium-based solid polymer and polymer–ceramic hybrid electrolyte materials. This review describes several solid-state NMR experiments that are commonly employed in the analysis of these systems: pulse field gradient NMR, electrophoretic NMR, variable temperature T1 relaxation, T2 relaxation and linewidth analysis, exchange spectroscopy, cross polarization, Rotational Echo Double Resonance, and isotope enrichment. In this review, each technique is introduced with a short description of the pulse sequence, and examples of experiments that have been performed in real solid-state polymer and/or hybrid electrolyte systems are provided. The results and conclusions of these experiments are discussed to inform readers of the strengths and weaknesses of each technique when applied to polymer and hybrid electrolyte systems. It is anticipated that this review may be used to aid in the selection of solid-state NMR experiments for the analysis of these systems.

Factors Influencing Preferential Anion Interactions during Solvation of Multivalent Cations in Ethereal Solvents

Most multivalent secondary batteries have employed electrolytes composed of cyclic ether solvents such as tetrahydrofuran or linear glycol ether solvents (glymes) such as 1,2-dimethoxyethane (G1). A robust understanding of multivalent cation solvation tendencies in these classes of solvents provides insight into corresponding structure–property relationships which, in turn, promotes the design and discovery of improved electrolytes. In this work, our goal is to systematically address how electrolyte constituent properties, namely, ether solvent structure and dication size, direct the solvation interactions of divalent electrolytes and their resultant properties. This study utilizes pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy in conjunction with Raman spectroscopy and ionic conductivity measurements to elucidate the preferential interactions between multivalent cations, anions, and solvent molecules along with their correlated ion dynamics. These investigations incorporate two representative divalent cations (Ca2+ and Zn2+) as well as two ethereal solvent representatives from both the cyclic ether and glyme structural classes. The results reveal that anions coordinate more readily with divalent cations in cyclic ethers than in glymes. Furthermore, the coordination of the anions with Ca2+, i.e., contact-ion pair (CIP) formation is more pronounced than with Zn2+ in a glyme solvent of limited chain length (G1), providing insight into cation size effects that are important for translating solvation behavior across various multivalent electrolytes. Importantly, we find that specific anion coordination is more strongly controlled by solvent structure than by salt concentration in the practical range of 0.1–0.5 M. However, simply reducing these inner-sphere inter-ionic interactions by changing solvent structure does not necessarily de-correlate ionic motion. Instead, concentration-dependent changes in molar ionic conductivity suggest that second-shell interactions, i.e., solvent separated ion pairs (SSIPs), are prevalent in these electrolytes and that the solution dielectric constant, which is increased by the presence of dipolar ion pairs, is critical for controlling these interactions. These findings thus provide a basis for understanding the physical chemistry of multivalent battery electrolytes.

Diffusion Measurements of Hydrocarbons in Zeolites with Pulse-Field Gradient Nuclear Magnetic Resonance Spectroscopy

Pulse field gradient NMR technique was used to determine self-diffusivity of heptane and pentadecane at room temperature for microporous catalysts, used both as powders and shaped with a binder extrudates. The results showed that diffusivities increased with increasing specific surface area, micro- and mesopore volume of the studied catalysts. The presence of Bindzil binder together with H-Beta-25 decreased hydrocarbon diffusivities. Self-diffusivities of heptane and pentadecane were smaller for extrudates than for the powder catalysts. The detailed information about mass transfer limitations is needed to further process optimization since effective diffusivity is directly correlated with self-diffusion coefficients. The estimates of the ratio of porosity and tortuosity were also determined. The diffusion measurements with relatively long observation times Δ (20 up to 1000 ms) and catalysts fully immersed in pentadecane revealed that a small portion of sites exhibits very small diffusivities in H-Beta-25-Bindzil extrudates, which is correlated with a low ratio of mesopore to micropore volumes of this material.

Determining the effect of dissolved CO2 on solution phase Li+ diffusion in common Li-ion battery electrolytes

Electrolyte decomposition in Li-ion batteries (LIB) and its entailing gas evolution significantly impacts cell performance. CO2 is one of the most abundantly evolved gases from common LIB electrolytes and could potentially affect Li+ solution transport during LIB operation. To this end, a comparative analysis of the Li+ diffusion coefficient (DLi+) which governs mass transport is required. Herein a methodology is established to saturate common battery electrolytes with a soluble gas without introducing other contaminants, so as to determine DLi+ in saturated and unsaturated samples. As a proof of concept, the values of DLi+ are determined in 1 M LiPF6 and 1 M LiClO4 in ethylene carbonate (EC):dimethyl carbonate (DMC) with and without dissolved CO2, confirmed by gas chromatography–mass spectrometry (GC–MS). The results obtained by pulse field gradient nuclear magnetic resonance (PFG-NMR) diffusion measurements agree for both electrolytes; saturating the electrolyte with CO2 has no measurable effect on DLi+ and therefore does not hinder Li+ mass transport.

Measurement of the diffusion of multiple nuclei in restricted spaces by pulsed field gradient NMR

In this study, the restricted diffusion of a solvent, anion, and cation in an electrolyte solution was measured by pulsed field gradient (PFG) NMR for 1H, 19F, and 7Li nuclei. Further, the time dependences of the diffusion coefficients were measured for a 1 M LiPF6 electrolyte solution in porous polyethylene, which has pores with sizes of tens of micrometers. The decreasing ratio of the diffusion coefficients of the solvent, cation, and anion based on the diffusion time can be scaled similarly for each diffusion distance. The experimentally obtained time dependences of the diffusion coefficients of the solvent, anion, and cation agreed with the results of the analytical equation with the same structural parameters. Furthermore, the abovementioned experimental results were produced via Monte Carlo simulation in the same model-restricted structure for the solvent, anion, and cation. Based on PFG-NMR, it can be concluded that the solvent, anion, and cation exhibit the same restricted diffusion behavior in polyethylene pores measuring tens of micrometers. It was confirmed that measuring the time dependences of the diffusion coefficients via PFG-NMR with multiple nuclei is effective for studying the diffusion mechanisms of electrolyte solutions in restricted spaces.

Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol).

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.

Solvation Structure and Dynamics of Mg(TFSI)2 Aqueous Electrolyte

Using ab initio molecular dynamics (AIMD) simulations, classical molecular dynamics (CMD) simulations, small‐angle X‐ray scattering (SAXS), and pulsed‐field gradient nuclear magnetic resonance (PFG‐NMR), the solvation structure and ion dynamics of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) aqueous electrolyte at 1, 2, and 3 m concentrations are investigated. From AIMD and CMD simulations, the first solvation shell of an Mg2+ ion is found to be composed of six water molecules in an octahedral configuration and the solvation shell is rather rigid. The TFSI− ions prefer to stay in the second solvation shell and beyond. Meanwhile, the comparable diffusion coefficients of positive and negative ions in Mg(TFSI)2 aqueous electrolytes have been observed, which is mainly due to the formation of the stable [Mg(H2O)6]2+ complex, and, as a result, the increased effective Mg ion size. Finally, the calculated correlated transference numbers are lower than the uncorrelated ones even at the low concentration of 2 and 3 m, suggesting the enhanced correlations between ions in the multivalent electrolytes. This work provides a molecular‐level understanding of how the solvation structure and multivalency of the ion affect the dynamics and transport properties of the multivalent electrolyte, providing insight for rational designs of electrolytes for improved ion transport properties.

Interplay of Structure and Dynamics in Lithium/Ionic Liquid Electrolytes: Experiment and Molecular Simulation

Despite their promising use in electrochemical and electrokinetic devices, ionic-liquid-based electrolytes often exhibit complex behavior arising from a subtle interplay of their structure and dynamics. Here, we report a joint experimental and molecular simulation study of such electrolytes obtained by mixing 1-butyl 3-methylimidazolium tetrafluoroborate with lithium tetrafluoroborate. More in detail, experiments consisting of X-ray scattering, pulsed field gradient NMR, and complex impedance spectroscopy are analyzed in the light of molecular dynamics simulations to probe the structural, dynamical, and electrochemical properties of this ionic-liquid-based electrolyte. Lithium addition promotes the nanostructuration of the liquid as evidenced from the appearance of a scattering prepeak that becomes more pronounced. Microscopically, using the partial structure factors determined from molecular dynamics, this prepeak is shown to correspond to the formation of well-ordered positive/negative charge series and also large aggregates (Lin(BF4)4-m)(4-m+n)-, which develop upon lithium addition. Such nanoscale ordering entails a drastic decrease in both the molecular mobility and ionic conductivity. In particular, the marked association of Li+ cations with four BF4- anions and long ion pairing times, which are promoted upon lithium addition, are found to severely hinder the Li+ transport properties.

Benchmarking MD Models of Disordered Proteins using NMR Data on Translational Diffusion

MD trajectories can be viewed as “ultimate models” of disordered proteins. However, these models are in need of careful experimental validation. The coefficient of translational diffusion Dtr, measurable by pulsed-field gradient NMR, offers a measure of compactness for disordered proteins (especially useful for smaller proteins, where SAXS data are unavailable). Here we investigate, both experimentally and via MD modeling, the translational diffusion of 25-residue N-terminal fragment from histone H4 (N-H4).

Self-Diffusion of Fullerene С60 Derivatives in Aqueous Solutions and Suspensions of Erythrocytes According to Pulsed Field Gradient NMR Data

Self-Diffusion of Fullerene С60 Derivatives in Aqueous Solutions and Suspensions of Erythrocytes According to Pulsed Field Gradient NMR Data

Molecular-level insights into structure and dynamics in ionic liquids and polymer gel electrolytes

Designing new electrolytes requires a better understanding of the correlation between their transport properties and their molecular structure. In this work, we present a detailed study of ionic liquids and polymer gel electrolytes probing their structure and dynamics by nuclear magnetic resonance (NMR) spectroscopy. In particular, ammonium- and phosphonium-based ionic liquids (ILs) are combined with different LiTFSI concentrations, and then with different ratios of poly(methylmethacrylate) polymer. The temperature dependence of self-diffusion coefficients of mobile species, DLi+, DTFSI- and DP4441+ or DN4441+, measured by pulsed field gradient (PFG) NMR spectroscopy obeys the Arrhenius equation. Diffusivity of [P4441][TFSI] ILs is found to be greater than those of the [N4441][TFSI] ILs in both liquid and gel electrolytes. Solid state NMR experiments including 13C and 19F MAS, 13C{19F},13C{1H} CPMAS probe the local structure and molecular-level interactions between ions and polymer in gel electrolytes, particularly for samples with higher PMMA content (≥25 wt%). Finally, the fast field cycling relaxometry has been used to unveil the rotational and translational dynamics of P4441+ or N4441+ and TFSI− by measuring 1H and 19F R1 relaxation rate profiles at different temperatures. A comprehensive NMR analysis including relaxation studies at low magnetic field provides decisive new insights regarding the formation of ionic clusters and the interaction of ions with the polymer chain in the case of the gel electrolytes.

How the Morphology of Nafion-Based Membranes Affects Proton Transport.

This work represents a systematic and in-depth study of how Nafion 1100 membrane preparation procedures affect both the morphology of the polymeric film and the proton transport properties of the electrolyte. The membrane preparation procedure has non-negligible consequences on the performance of the proton-exchange membrane fuel cells (PEMFC) that operate within a wide temperature range (up to 120 °C). A comparison between commercial membranes (Nafion 117 and Nafion 212) and Nafion membranes prepared by three different procedures, namely (a) Nafion-recast, (b) Nafion uncrystallized, and (c) Nafion 117-oriented, was conducted. Electrochemical Impedance Spectroscopy (EIS) and Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) investigations indicated that an anisotropic morphology could be achieved when a Nafion 117 membrane was forced to expand between two fixed and nondeformable surfaces. This anisotropy increased from ~20% in the commercial membrane up to 106% in the pressed membrane, where the ionic clusters were averagely oriented (Nafion 117-oriented) parallel to the surface, leading to a strong directionality in proton transport. Among the membranes obtained by solution-cast, which generally exhibited isotropic proton transport behavior, the Nafion uncrystallized membrane showed the lowest water diffusion coefficients and conductivities, highlighting the correlation between low crystallinity and a more branched and tortuous structure of hydrophilic channels. Finally, the dynamic mechanical analysis (DMA) tests demonstrated the poor elastic modulus for both uncrystallized and oriented membranes, which should be avoided in high-temperature fuel cells.

Pulsed field gradient NMR diffusion measurement in nanoporous materials

Labeling in diffusion measurements by pulsed field gradient (PFG) NMR is based on the observation of the phase of nuclear spins acquired in a constant magnetic field with purposefully superimposed field gradients. This labeling does in no way affect microdynamics and provides information about the probability distribution of molecular displacements as a function of time. An introduction of the measuring principle is followed by a detailed description of the ranges of measurements and their limitation. Particular emphasis is given to an explanation of possible pitfalls in the measurements and the ways to circumvent them. Showcases presented for illustrating the wealth of information provided by PFG NMR include a survey on the various patterns of concentration dependence of intra-particle diffusion and examples of transport inhibition by additional transport resistances within the nanoporous particles and on their external surface. The latter information is attained by combination with the outcome of tracer exchange experiments, which are shown to become possible via a special formalism of PFG NMR data analysis. Further evidence provided by PFG NMR concerns diffusion enhancement in pore hierarchies, diffusion anisotropy and the impact of diffusion on chemical conversion in porous catalysts. A compilation of the specifics of PFG NMR and of the parallels with other measurement techniques concludes the paper.

Исследование ледовых композиционных материалов, армированных нанодисперсными волокнами оксида алюминия, методами ядерного магнитного резонанса

The aqueous suspensions of nanosized alumina fibers and ice composite materials based on them were studied using Pulsed-Field Gradient NMR and Magnetic Resonance Imaging methods. It is shown that the introduction of nanofibers does not lead to noticeable structural effects in suspensions in the concentration range of 1 - 10 wt. % Al2O3. It was found that a high concentration of filler significantly changes the morphology and texture of the ice matrix in composites: it becomes more homogeneous, with a high degree of continuity; when the melting front passes through, internal regions of thawing or failure of the integrity of the composite are not visualized. Yet the introduction of nanofibers into ice composites has a weak effect on the quantitative dynamics of heat transfer processes, making the rate of freezing/thawing front propagation to be similar in different samples at comparable temperatures of the process.

Dynamic ionic radius of alkali metal ions in aqueous solution: a pulsed-field gradient NMR study

The dynamic behavior of alkali metal ions, Li+, Na+, K+, Rb+ and Cs+ in aqueous solutions is one of the most important topics in solution chemistry. Since these alkali metals contain nuclear magnetic resonance (NMR) active nuclei, it is possible to directly measure the diffusion constants of the alkali metal ions using the pulsed field gradient (PFG) NMR method. In this paper, the 7Li, 23Na, 87Rb, 133Cs and 1H resonances are observed for diffusion constants in aqueous solution and the solvent H2O. Until now, the values of the diffusion constant have been lacking when discussing hydration effects around alkali metal ions. It is known that the static ionic radius (Rion) increases with increasing the atomic number, and the experimental diffusion constants also increase with increasing the atomic number, which is opposite to the Stokes–Einstein (SE) relation. It suggests that alkali metal ions diffuse through a space of 10−6 m accompanying the hydrated spheres with a time interval of 10−3 s. For each alkali metal ion, the dynamic ionic radius is evaluated.