Field Gradient Nuclear Magnetic Resonance Research Articles

Diffusion in Nanoporous Materials: Novel Insights by Combining MAS and PFG NMR

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) allows recording of molecular diffusion paths (notably, the probability distribution of molecular displacements over typically micrometers, covered during an observation time of typically milliseconds) and has thus proven to serve as a most versatile means for the in-depth study of mass transfer in complex materials. This is particularly true with nanoporous host materials, where PFG NMR enabled the first direct measurement of intracrystalline diffusivities of guest molecules. Spatial resolution, i.e., the minimum diffusion path length experimentally observable, is limited by the time interval over which the pulsed field gradients may be applied. In “conventional” PFG NMR measurements, this time interval is determined by a characteristic quantity of the host-guest system under study, the so-called transverse nuclear magnetic relaxation time. This leads, notably when considering systems with low molecular mobilities, to severe restrictions in the applicability of PFG NMR. These restrictions may partially be released by performing PFG NMR measurements in combination with “magic-angle spinning” (MAS) of the NMR sample tube. The present review introduces the fundamentals of this technique and illustrates, via a number of recent cases, the gain in information thus attainable. Examples include diffusion measurements with nanoporous host-guest systems of low intrinsic mobility and selective diffusion measurement in multicomponent systems.

High Lithium Transference Number Electrolytes Containing Tetratriflylpropene's Lithium Salt.

Electrolytes with a high lithium transference number linked with high ionic conductivity are urgently needed for high power battery operation. In this work, we present newly synthesized lithium tetra(trifluoromethanesulfonyl)propene as a salt-in-glyme-based "salt-in-solvent" electrolyte. We employ impedance spectroscopy in symmetric Li/electrolyte/Li cells and pulsed field gradient nuclear magnetic resonance spectroscopy to investigate the lithium conduction mechanism. We find predominant lithium conductivity with very high lithium transference numbers (∼70% from the polarization experiments) and three times higher ionic conductivity compared to well-known lithium triflate in diglyme electrolyte. This is a consequence of the reduced mobilities of large anions linked with improved ionic dissociation.

Mechanisms of Physical Stabilization of Concentrated Water-In-Oil Emulsions Probed by Pulse Field Gradient Nuclear Magnetic Resonance and Rheology through a Multiscale Approach.

The long-term physical stability of surfactant-stabilized (Span 80 and Tween 20) concentrated water-in-mineral oil (W/O) emulsions in the presence of an electrolyte (NaCl) was studied. Pulse field gradient NMR and rheology (bulk and interfacial) were used to probe the response at the macroscopic, microscopic, and molecular levels, rendering a multiscale approach. The results show that: (1) Emulsions prepared with NaCl exhibit higher values of the elastic shear modulus ( GwithNaCl' > GwithoutNaCl') even after ∼20 days. (2) The stabilization effect of salt against the coarsening of droplets is not due to the differences in droplet size (and thus G') or the energy incorporated through emulsification. (3) NaCl relaxes the liquid-liquid interface via a salting-in effect, which results in a lower interfacial shear elasticity ( GwithNaCls' < GwithoutNaCls') and a higher resistance to coarsening events because of the changes in the adsorption density of the layer.

The ion and water transport properties of K+ and Na+ form perfluorosulfonic acid polymer

In this work, we explore the behavior of membranes under conditions that are relevant to ion-exchange membranes used in a variety of electrochemical engineering applications, extending beyond the heavily-studied fuel cell application and previous studies of fully hydrated systems. We report the water uptake, density and conductivity of membranes determined at various hydration states of K+, Na+ and K+/Na+ mixed form perfluorosulfonate membranes. Water uptake decreases with increasing K+ content in the membrane, consistent with increasing average cation size and decreasing hydration energy. However, for membranes at the same water content, the conductivity increased with increasing K+ fraction, in spite of the fact that the size of K+ is larger than that of Na+. Analysis of data on the basis of percentage conducting volume reveals that the membranes with higher K+ content show a much higher conductivity (with higher cation mobility) at percentage conducting volumes higher than 10%. One reason for this behavior may be due to a higher extent of dissociation for K+ from the fixed anion site. TEM results show a larger cluster size in K+ form membranes, another possible reason for faster K+ transport. Pulsed field gradient (PFG) Nuclear Magnetic Resonance (NMR) shows that water diffusion coefficient in the membranes with higher K+ fraction is higher than for samples with lower K+ fraction. FT-IR bands shift with the change of cation content, supporting the suggestion of a higher degree of cation-sulfonate dissociation and weaker hydrogen bonding interactions between cation and water molecules for membranes with higher K+ content.

Dispersivity of Bidisperse Packings of Spheres and Evidence for Distinct Random Structures.

The intrinsic longitudinal and transverse dispersivity of bidisperse random packings of spheres with size ratio 5∶1 was determined by pulsed field gradient nuclear magnetic resonance, in the dilute regime where small spheres occupy between 0% and 5% of the packings' volume. Small spheres plugging pores systematically raise the mechanical transverse and longitudinal dispersivity above that of reference packings of monodisperse spheres. NMR-derived porosities, widths of velocity distributions, and dispersivities reveal distinct states of structural disorder above and below a relative sphere concentration n/N=1, where n and N are the number densities of small and large spheres.

Determining the number of chemical species in nuclear magnetic resonance data matrix by taking advantage of collinearity and noise

The number of chemical species is crucial in analyzing pulsed field gradient nuclear magnetic resonance spectral data. Any method to determine the number must handle the obstacles of collinearity and noise. Collinearity in pulsed field gradient NMR data poses a serious challenge to and fails many existing methods. A novel method is proposed by taking advantage of the two obstacles instead of eliminating them. In the proposed method, the determination is based on discriminating decay-profile-dominant eigenvectors from noise-dominant ones, and the discrimination is implemented with a novel low- and high-frequency energy ratio (LHFER). Its performance is validated with both simulated and experimental data. The method is mathematically rigorous, computationally efficient, and readily automated. It also has the potential to be applied to other types of data in which collinearity is fairly severe.

Dispersed phase volume fraction, weak acids and Tween 80 in a model emulsion: Effect on the germination and growth of Bacillus weihenstephanensis KBAB4 spores

In foodstuffs, physico-chemical interactions and/or physical constraints between spores, inhibitors and food components may exist. Thus, the objective of this study was to investigate such interactions using a model emulsion as a microbial medium in order to improve bacterial spore control with better knowledge of the interactions in the formulation.Emulsions were prepared with hexadecane mixed with nutrient broth using sonication and were stabilized by Tween 80 and Span 80. The hexadecane ratio was either 35% (v/v) or 50% (v/v) and each emulsion was studied in the presence of organic acid (acetic, lactic or hexanoic) at two pH levels (5.5 and 6). Self-diffusion coefficients of emulsion components and the organic acids were measured by Pulsed Field Gradient-Nuclear Magnetic Resonance (PFG-NMR). The inhibition effect on the spore germination and cell growth of Bacillus weihenstephanensis KBAB4 was characterized by the measure of the probability of growth using the most probable number methodology, and the measure of the time taken for the cells to germinate and grow using a single cell Bioscreen® method and using flow cytometry.The inhibition of spore germination and growth in the model emulsion depended on the dispersed phase volume fraction and the pH value. The effect of the dispersed phase volume fraction was due to a combination of (i) the lipophilicity of the biocide, hexanoic acid, that may have had an impact on the distribution of organic acid between hexadecane and the aqueous phases and (ii) the antimicrobial activity of the emulsifier Tween 80 detected at the acidic pH value. The interface phenomena seemed to have a major influence. Future work will focus on the exploration of these phenomena at the interface.

Development and application of an exchange model for anisotropic water diffusion in the microporous MOF aluminum fumarate

Diffusion of water in aluminum fumarate was studied by means of pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Due to water molecules exchanging between the intracrystalline anisotropic pore space and the isotropic intercrystalline void space the model of intracrystalline anisotropic diffusion fails to describe the experimental PFG NMR data at high observation times. Therefore, the two-site exchange model developed by Kärger is extended to the case of exchange between an anisotropic and an isotropic site. This extended exchange model is solved by numerical integration. It describes the experimental data very well and yields values for the intracrystalline diffusion coefficient and the mean residence times of the respective sites. Further PFG NMR studies were performed with coatings consisting of small aluminum fumarate crystals, which are used in adsorptive heat transformation applications. The diffusion coefficients of water in the small crystal coating are compared to the values expected from the extended two-site exchange model and from the model of long-range diffusion.

NMR Diffusion Study of Ionic Liquids and Ionogels for Use in Lithium Ion Batteries

Solid-state electrolytes have been explored increasingly for use in Li+ batteries on account of their ability to address the safety issues, packaging constraints, and volumetric design concerns associated with liquid electrolytes. Recently, ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest because of their high ionic conductivity, stability, and solution processability. Due to the complexity of the trapped liquid, the nature of the ion interaction and movement is not well established. Understanding how the ions behave inside the matrix is paramount for increasing the ionic conductivity and Li+ transference number. The literature for these characteristics is lacking in several key areas including, but not limited to, variations in the electrolyte’s Li+ concentration, the choice of silica precursors, and diffusion under different conditions. The ionogel matrix was synthesized using a non-aqueous organic acid catalyzed sol-gel process to form a nanoporous silica matrix around an ionic liquid, BMIM TFSI with variable amounts of LiTFSI added, thus confining the electrolyte. The choice of silica precursors and use of an organic acid allows for a crack free monolith to form under rapid gelation conditions. The silica component maintains a highly porous network with surface areas up to 1300 m2/g and an average pore size of 20nm as measured through N2 adsorption. The ionic liquid (IL) electrolyte component constitutes 75 vol% of the ionogel. Thermal gravimetric analysis shows that the ionogel is thermally stable to 400 oC. Due to the interconnected porosity and large percentage of liquid, conductivities of ̴10-3 S/cm were measured at room temperature for the prepared monoliths using electrochemical impedance spectroscopy (EIS). The degree to which each ion contributes to the total conductivity can be determined by combining the conductivity measurements with self-diffusion measurements obtained by pulsed field gradient nuclear magnetic resonance (PFG NMR) methods. Furthermore the presence of ion pairing or aggregation can be ascertained by direct comparison between the NMR-predicted and EIS-measured conductivity. In this work, the PFG NMR stimulated echo sequence was used to measure the self-diffusion coefficients of the IL cation, Li+, and anion via 1H, 7Li and 19F, respectively, in the IL and ionogel with a concentration of 1M LiTFSI, at variable temperature covering the 0o – 100oC range. Two main confinement effects of all mobile ions in the ionogel are observed (i) a reduction in self-diffusion values of all ions compared to the pure IL; (ii) the presence of a relatively immobile fraction of the ions. Interestingly the effective Li transference numbers defined as D(Li)/[D(Li) + D(BMIM) + D(TFSI)] in the liquid electrolyte and the and ionogel are comparable, with a value of approximately 0.20.

The Ionic and Water Transport Properties Studies of Univalent Ion Exchanged Perfluorosulfonate Membrane

Perfluorosulfonic acid (PFSA) polymer membranes have been widely studied in the context of their applications as ion exchange medium/solid electrolytes in proton exchange membrane (PEM) fuel cells and other redox applications since 1970s(1). Water management plays a significant role in the performance of polymer membrane electrolytes, especially in fuel cells, because the ionic conductance dramatically decreases at low relative humidity, while excess water may flood the flow channels and hinder the transport of reagents such as gases (2). The conductivity and water diffusion coefficient of membranes therefore are always of concern because of their function in electrochemical devices. In this work, Li+, Na+, NH4 + and TEA+ form PFSA membranes were prepared based on H-form 3M PFSA membrane. The water uptake, density and conductivity of these membranes were investigated at various hydration levels. It’s found that the hydrophilicity of cations determines the water uptake of membranes with different cations, showing a series of water uptake in the order of: H+ > Li+ > Na+ > NH4 + > TEA+. As expected, H+ form membranes show the highest conductivity among all membrane forms due to the smallest charge carrier size and proton hopping transport mechanism. This mechanism may also contribute for NH4 + transport, resulting in a comparably high conductivity of NH4 + form membranes even though the latter take up much less water. Pulsed field gradient (PFG) Nuclear Magnetic Resonance (NMR) was exploited to observe the diffusion coefficient of water in the membranes. It was observed that hydrophilicity of cations and the hydrogen bonding interactions affect the water transport in the membranes. Based on density measurements, a partial molar volume analysis is carried out in this study. This parameter provides insight into the water and polymer interactions in various univalent PFSA membranes. Acknowledgement. We gretefully acknowledge the support of this work by the Office of Naval Research. We also appreciate the assistance of 3M company in providing 3M PFSA membrane samples.

Influence of Hydrogen Sulfide Exposure on the Transport and Structural Properties of the Metal–Organic Framework ZIF-8

The interaction between hydrogen sulfide and ZIF-8 was studied via structural characterizations and guest molecule diffusion measurements. It was found that hydrogen sulfide reacts with the ZIF-8 external particle surface to form a surface barrier that excludes the uptake of larger molecules (ethanol) and slows down the uptake of smaller molecules (carbon dioxide). Nonetheless, bulk transport properties were unaltered, as supported by pulsed field gradient nuclear magnetic resonance studies. Dispersion-corrected density functional theory calculations revealed that H2S is consumed by reactions occurring at the ZIF external surface. These reactions result in water and defect formation, both of which were found to be exothermic and independent of both crystallographic facets ({001} and {110}) and surface termination. We concluded that these surface reactions lead to structural and chemical changes to the ZIF-8 external surface that generate surface barriers to molecular transport.

Study of Dispersion in Porous Media by Pulsed Field Gradient NMR: Influence of the Fluid Rheology

Pulsed field gradient nuclear magnetic resonance (PFG-NMR) is used to measure the molecular displacements for the flow of a fluid through a capillary tube and a packed bed made of monodisperse PMMA beads. The molecules average displacement is studied using both the formalism of propagators and the cumulant method. In the Poiseuille case, the dispersion coefficients determined by the cumulant method compare satisfactorily with the theoretical values obtained. The technique is then extended to study the flow through a porous medium. We thus analyze Newtonian (water) and non-Newtonian (Xanthan) flows and put a particular emphasis on comparing the dispersion mechanisms between Newtonian and non-Newtonian fluids.

Effect of Structural Disorder on Hydrodynamic Behavior of Alpha-Casein According to PFG NMR Spectroscopy

The concentration dependences of self-diffusion coefficient for intrinsically disordered milk protein αS-casein were studied by pulsed field gradient nuclear magnetic resonance. The experimental data were analyzed in a view of phenomenological approach based on the frictional formalism of non-equilibrium thermodynamics by Vink. The results of αS-CN hydrodynamic study showed that at low- and high-protein concentrations, αS-CN exists in the different structural forms. At low concentrations in the rather broad concentration range, protein remains monomeric but with greater hydrodynamic size than have rigid globular proteins of the equal mass. At high concentrations beyond the definite protein content, αS-CN tends to form associates. The application of the Vink’s approach to αS-CN testifies that the role of flexible domains in the process of self-diffusion is mainly in increasing the friction of between αS-CN molecules due to their inter-entanglement. The latter physically means that when αS-CN molecules cling each other by their flexible domains, this phenomenon provides much more efficient friction than their interaction with solvent molecules.

Correlation between Ionic Liquid Cytotoxicity and Liposome-Ionic Liquid Interactions.

This study aims at extending the understanding of the toxicity mechanism of ionic liquids (ILs) using various analytical methods and cytotoxicity assays. The cytotoxicity of eight ILs and one zwitterionic compound was determined using mammalian and bacterial cells. The time dependency of the IL toxicity was assessed using human corneal epithelial cells. Hemolysis was performed using human red blood cells and the results were compared with destabilization data of synthetic liposomes upon addition of ILs. The effect of the ILs on the size and zeta potential of liposomes revealed information on changes in the lipid bilayer. Differential scanning calorimetry was used to study the penetration of the ILs into the lipid bilayer. Pulsed field gradient nuclear magnetic resonance spectroscopy was used to determine whether the ILs occurred as unimers, micelles, or if they were bound to liposomes. The results show that the investigated ILs can be divided into three groups based on the cytotoxicity mechanism: cell wall disrupting ILs, ILs exerting toxicity through both cell wall penetration and metabolic alteration, and ILs affecting solely on cell metabolism.

Unexpected Diffusion Anisotropy of Carbon Dioxide in the Metal-Organic Framework Zn2(dobpdc).

Metal-organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is crucial for the practical application of these materials as adsorbents or in mixed-matrix membranes, since the rate of gas diffusion within the adsorbent pores impacts the required size (and therefore cost) of the adsorbent column or membrane. Here, we investigate the diffusion of CO2 within the pores of Zn2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. The residual chemical shift anisotropy for pore-confined CO2 allows PFG NMR measurements of self-diffusion in different crystallographic directions, and our analysis of the entire NMR line shape as a function of the applied field gradient provides a precise determination of the self-diffusion coefficients. In addition to observing CO2 diffusion through the channels parallel to the crystallographic c axis (self-diffusion coefficient D∥ = (5.8 ± 0.1) × 10-9 m2 s-1 at a pressure of 625 mbar CO2), we unexpectedly find that CO2 is also able to diffuse between the hexagonal channels in the crystallographic ab plane (D⊥ = (1.9 ± 0.2) × 10-10 m2 s-1), despite the walls of these channels appearing impermeable by single-crystal X-ray crystallography and flexible lattice MD simulations. Observation of such unexpected diffusion in the ab plane suggests the presence of defects that enable effective multidimensional CO2 transport in a metal-organic framework with nominally one-dimensional porosity.

Understanding transport mechanisms in ionic liquid/carbonate solvent electrolyte blends.

To unravel mechanistic details of the ion transport in liquid electrolytes, blends of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI), ethylene carbonate (EC) and dimethyl carbonate (DMC) with the conducting salts lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) were investigated as a function of the IL concentration. Electrochemical impedance, Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) and Raman spectroscopy supported by Molecular Dynamics (MD) simulations allowed the structural and dynamic correlations of the ion motions to be probed. Remarkably, we identified that though the individual correlations among different ion types exhibit a clear concentration dependence, their net effect is nearly constant throughout the entire concentration range, resulting in approximately equal transport and transference numbers, despite a monitored cross-over from carbonate-based lithium coordination to a TFSI-based ion coordination. In addition, though dynamical ion correlation could be found, the absolute values of the ionic conductivity are essentially determined by the overall viscosity of the electrolyte. The IL/carbonate blends with a Pyr14TFSI fraction of ∼10 wt% are found to be promising electrolyte solvents, with ionic conductivities and lithium ion transference numbers comparable to those of standard carbonate-based electrolytes while the thermal and electrochemical stabilities are considerably improved. In contrast, the choice of the conducting salt only marginally affects the transport properties.

Physical properties of tofu gel probed by water translational/rotational dynamics

The dynamic behavior of water molecules in tofu gels is observed using dielectric spectroscopy (DS) and pulsed field gradient nuclear magnetic resonance (PFG-NMR) methods. The breaking stress of tofu gels are dependent on the heating time, which affects the water structures characterized by the dielectric relaxation time, τ. On the other hand, the inhomogeneity of the gel structures decreases with increasing coagulant concentration was reflected as decreasing of the dielectric broadening parameter, β, that related for a fluctuation of the dynamic water structure. In addition, a decrease in the translational diffusion coefficient of water molecules is obtained with increasing coagulant concentration suggesting a reduction of the free space for the diffusional motion of water molecules. Negative correlations between the broadening parameter, β, and the inverse of diffusion coefficient, 1/D, are found in tofu gels and soymilk with distinct correlation coefficients of −0.95 and −0.87, respectively. These results suggest that water dynamics analysis and complementary analysis using DS and PFG-NMR measuring techniques can be an effective tool to evaluate food gel physical properties. Further these technique could also explain the molecular mechanisms of gel and liquid structures and properties via dynamic water structures.

The Effect of Shape and Concentration on Translational Diffusion of Proteins Measured by PFG NMR

The concentration dependences of self-diffusion coefficient for irregular-shaped fibrinogen, for globular, spheroidal trypsin and α-chymotrypsin were studied by pulsed field gradient nuclear magnetic resonance. The experimental data were analyzed in a view of two known theoretical approaches—the hydrodynamic model of rigid spheres by Tokuyama and Oppenheim and the phenomenological approach based on the frictional formalism of non-equilibrium thermodynamics by Vink. The detailed discussion of their merits and drawbacks is presented. Our results testify that the Vink’s approach is quite universal, providing a satisfactory description of experimental data for proteins of complicated structure and different shape while the model of Tokuyama and Oppenheim is applicable only to proteins of more regular shape.

NMR investigation of water diffusion in different biofilm structures.

Mass transfer in biofilms is determined by diffusion. Different mostly invasive approaches have been used to measure diffusion coefficients in biofilms, however, data on heterogeneous biomass under realistic conditions is still missing. To non-invasively elucidate fluid-structure interactions in complex multispecies biofilms pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was applied to measure the water diffusion in five different types of biomass aggregates: one type of sludge flocs, two types of biofilm, and two types of granules. Data analysis is an important issue when measuring heterogeneous systems and is shown to significantly influence the interpretation and understanding of water diffusion. With respect to numerical reproducibility and physico-chemical interpretation, different data processing methods were explored: (bi)-exponential data analysis and the Γ distribution model. Furthermore, the diffusion coefficient distribution in relation to relaxation was studied by D-T2 maps obtained by 2D inverse Laplace transform (2D ILT). The results show that the effective diffusion coefficients for all biofilm samples ranged from 0.36 to 0.96 relative to that of water. NMR diffusion was linked to biofilm structure (e.g., biomass density, organic and inorganic matter) as observed by magnetic resonance imaging and to traditional biofilm parameters: diffusion was most restricted in granules with compact structures, and fast diffusion was found in heterotrophic biofilms with fluffy structures. The effective diffusion coefficients in the biomass were found to be broadly distributed because of internal biomass heterogeneities, such as gas bubbles, precipitates, and locally changing biofilm densities. Thus, estimations based on biofilm bulk properties in multispecies systems can be overestimated and mean diffusion coefficients might not be sufficiently informative to describe mass transport in biofilms and the near bulk.

Water sub-diffusion in membranes for fuel cells

We investigate the dynamics of water confined in soft ionic nano-assemblies, an issue critical for a general understanding of the multi-scale structure-function interplay in advanced materials. We focus in particular on hydrated perfluoro-sulfonic acid compounds employed as electrolytes in fuel cells. These materials form phase-separated morphologies that show outstanding proton-conducting properties, directly related to the state and dynamics of the absorbed water. We have quantified water motion and ion transport by combining Quasi Elastic Neutron Scattering, Pulsed Field Gradient Nuclear Magnetic Resonance, and Molecular Dynamics computer simulation. Effective water and ion diffusion coefficients have been determined together with their variation upon hydration at the relevant atomic, nanoscopic and macroscopic scales, providing a complete picture of transport. We demonstrate that confinement at the nanoscale and direct interaction with the charged interfaces produce anomalous sub-diffusion, due to a heterogeneous space-dependent dynamics within the ionic nanochannels. This is irrespective of the details of the chemistry of the hydrophobic confining matrix, confirming the statistical significance of our conclusions. Our findings turn out to indicate interesting connections and possibilities of cross-fertilization with other domains, including biophysics. They also establish fruitful correspondences with advanced topics in statistical mechanics, resulting in new possibilities for the analysis of Neutron scattering data.