Gradient NMR Research Articles

Li‐ion hopping conduction enabled by associative Li‐salt in acetonitrile solutions

AbstractTo date, ionic conduction in nonaqueous electrolytes has been explained through the vehicle‐type migration mechanism. Yet, new research hints at another conduction mode: ion‐hopping, seen in highly concentrated solutions with multi‐coordinating solvents. Our research uncovers that Li‐ion hopping conduction also occurs in monodentate acetonitrile (AN) electrolytes, enabled by a highly associative Li‐salt. Using techniques like pulse‐field gradient NMR, Raman spectroscopy, and dielectric relaxation spectroscopy, we examined AN solutions with lithium trifluoroacetate (LiTFA) and lithium bis(fluorosulfonyl)imide (LiFSI). Results showed that Li‐ion diffusion in LiTFA‐AN was faster due to an anion‐bridge structure formed by the associative nature of LiTFA. In contrast, the LiFSI‐AN solution demonstrated slower Li‐ion movement. In practical applications, like LiFePO4 symmetric cells, 4 M LiTFA‐AN outperformed 1 M LiTFA‐AN in rate performance, despite its lower ionic conductivity. This challenges the belief that associative Li‐salts are unsuitable for battery electrolytes and prompts reconsideration of other associative Li‐salts.

(Invited) Phase Separation in Mixed Polysaccharide Gels

Studies on phase separation behavior for the mixed polysaccharides of kappa carrageenan (k-carr) and iota carrageenan (i-carr) has been reviewed. It was reported that the temperature dependence of G' and G'' for the mixture solution measured by dynamic rheological measurement showed two step increases in cooling process, suggesting the individual network formation of k- and i-carr chains at gelling temperatures (T g) corresponding to T g for pure each solution of. DSC measurements for the mixture showed two exothermic peaks at the T g’s of pure k- and i-carr, supporting the individual network formation. Diffusions of a probe polymer added in the mixed gel were measured by gradient NMR with a view to revealing the morphological structure in the gel, but adequate information was not obtained. Particle tracking method for fluorescent particles with diameter of 100nm in the mixture was demonstrated suggesting the formation of phase separated structure with k-carr rich and i-carr rich phases from the micro rheological diversity.

Signal attenuation of PFG restricted anomalous diffusions in plate, sphere, and cylinder

Pulsed field gradient (PFG) NMR is a noninvasive tool to study anomalous diffusion, which exists widely in many systems such as in polymer or biological systems, in porous material, in single file structures and in fractal geometries. In a real system, the diffusion could be a restricted or a tortuous anomalous diffusion, rather than a free diffusion as the domains for fast and slow transport could coexist. Though there are signal attenuation expressions for free anomalous diffusion in literature, the signal attenuation formalisms for restricted anomalous diffusion is very limited, except for a restricted time-fractional diffusion within a plate reported recently. To better understand the PFG restricted fractional diffusion, in this paper, the PFG signal attenuation expressions were derived for three typical structures (plate, sphere, and cylinder) based on two models: fractal derivative model and fractional derivative model. These signal attenuation expressions include two parts, the time part Tn(t) and the space part Xn(r). Unlike normal diffusion, the time part Tn(t) in time-fractional diffusion can be either a Mittag-Leffler function from the fractional derivative model or a stretched exponential function from the fractal derivative model. However, provided the restricted normal diffusion and the restricted time-fractional diffusion are in an identical structure, they will have the same space part Xn(r) as both diffusions have the same space derivative parameter β equaling 2, therefore, they should have similar diffractive patterns. The restricted general fractional diffusion within a plate is also investigated, which indicates that at a long time limit, the diffusion type is insignificant to the diffractive pattern that depends only on the structure and the gradient pulses. The expressions describing the time-dependent behaviors of apparent diffusion coefficient Df,app for restricted anomalous diffusion are also proposed in this paper. Both the short and long time-dependent behaviors of Df,app are distinct from that of normal diffusion. The general expressions for PFG restricted curvilinear diffusion of tube model were derived in a conventional way and its result agree with that obtained from the fractional derivative model with α equaling 1/2. Additionally, continuous-time random walk simulation was performed to give good support to the theoretical results. These theoretical results reported here will be valuable for researchers in analyzing PFG anomalous diffusion.

Nuclear Magnetic Resonance Study of Water-Polymer Interactions and Self-Diffusion of Water in Polymer Films

The diffusivity and distribution of water in the butyl methacrylate and methacrylic acid copolymer films swollen in water have been investigated using the NMR relaxation and pulse field gradient (PFG) NMR techniques. The contributions of polymer matrix protons, surface water and bound water have been determined from 1H NMR spectra and relaxation functions. PFG NMR experiments showed that the echo attenuation function depends on the diffusion time indicating that water inside the swollen film is trapped in restricted confinement. The data obtained have been discussed using published physical models for diffusion of water in polymeric materials. The sizes of pores inside the film were estimated using the published model approaches giving the range of 0.8-1.0 μm. Magnetization decays as well as the spin-spin relaxation times of water saturated polymer films were also determined in this study. NMR relaxation provided additional information on the water distribution in the porous microstructure. The volume-averaged water mobility decreased with increasing hydrophobic content of the polymers.

Molecular mobility and microscopic structure changes in κ-carrageenan solutions studied by gradient NMR

Changes in the molecular mobility of κ-carrageenan were observed by the pulsed field gradient stimulated echo (PGSTE) and Carr–Purcell–Meiboom–Gill (CPMG) methods for elucidating the molecular aspect of the sol-to-gel transition. The echo signal intensity of κ-carrageenan without a gradient, Ikap(0), decreased steeply near the sol-to-gel temperature (Tsg), suggesting that κ-carrageenan chains formed aggregates and a network structure. Below Tsg, the spin–spin relaxation time T2 and the diffusion coefficient of κ-carrageenan (Dkap) increased with decreasing temperature, indicating that the solute κ-carrageenan chains have a lower molecular weight Mw than chains involved in the aggregation. The diffusion coefficient of pullulan (Dpul) added as a probe molecule in κ-carrageenan solutions was measured, and the characteristic hydrodynamic screening length, ξ, was then estimated from the degree of diffusion restriction. Below a certain temperature, Dkap reached a higher value than that of Dpul, suggesting that the Mw of solute κ-carrageenan became lower than that of pullulan. GPC measurements confirmed the presence of κ-carrageenan chains with a lower Mw than that of pullulan. A simple physical model of the structural change in κ-carrageenan solution was proposed with a bimodal distribution of κ-carrageenan with higher and lower Mw than the pullulan probe. The higher Mw chains form the gel network restricting the probe's diffusion, and the lower Mw chains increase the effective viscosity. The concentration of the high Mw solute κ-carrageenan chains in 1%, 2% and 4% κ-carrageenan solutions was estimated from Ikap(0) and the total κ-carrageenan concentration, and the relation with pullulan diffusion was studied.

The Beauty of the Different Views on Diffusion

Guest diffusion in nanoporous materials reveals a wide spectrum of phenomena associated with random mass transfer in condensed matter quite in general. Taking advantage of their potentials for monitoring mass transfer over microscopic dimensions, micro-imaging (by interference microscopy and IR spectroscopy) and pulsed field gradient (PFG) NMR are exploited for providing typical examples highlighting these options. Starting with the surprise provided by the application of these microscopic measuring techniques to nanoporous materials, namely discrepancies of up to five orders of magnitude in comparison with the so far well-established data, the review covers some of the most impressive further results, including the determination of sticking coefficients on the surfaces of these materials and the specification and detection of conditions where molecular mass transfer is accelerated rather than hampered by counter fluxes.

Diffusion of probe polymer in gellan gum solutions during gelation process studied by gradient NMR

The diffusion coefficients of pullulan added in gellan gum solutions as a probe polymer were measured using pulsed-field-gradient stimulated-spin-echo (PFG-Ste) 1H NMR in order to investigate the gelation mechanism and gel structure. The echo intensity of gellan was steeply decreased at around the gelling temperature T gel indicating stiffening of the gellan chains upon aggregation and formation of the network. The diffusion coefficient of pullulan D pull increased with decreasing temperature below T gel. This result suggests that the decrease in concentration of solute gellan in the interspaces of the network below T gel, as evidenced by the decrease in its echo intensity, leads to an overall decrease of hydrodynamic interactions between gellan and pullulan. The characteristic hydrodynamic shielding length ξ was calculated from the relation D pull/ D pull,0 = exp(− R h/ ξ), where D pull,0 is D pull in dilute solution and R h is the hydrodynamic radius of pullulan. The temperature dependence of ξ, which was investigated for varying concentrations of several cations, was found to follow closely the gelling temperature, and in particular, showing a very similar thermal hysteresis.

About the Choice of the Protogenic Group in PEM Separator Materials for Intermediate Temperature, Low Humidity Operation: A Critical Comparison of Sulfonic Acid, Phosphonic Acid and Imidazole Functionalized Model Compounds

AbstractTraditionally, sulfonated polymers are used as separator materials in PEM fuel cells. Based on recent experimental results on model compounds this paper critically discusses the potentials and limits of sulfonic acid and alternatively phosphonic acid and heterocycles (imidazole) as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures (T > 100 °C) and low humidification. Apart from transport properties, the stability and reactivity of mono‐functionalized model compounds (1‐heptylsulfonic acid (S‐C7), 1‐heptylphosphonic acid (P‐C7) and 2‐heptyl‐imidazole (I‐C7)) and a few diphosphonic acids are examined under wet and dry conditions. These are characterized with respect to their proton conductivity (ac impedance spectroscopy), proton diffusion coefficient (pulsed‐field gradient NMR), thermo‐oxidative stability (TGA under air), electrochemical stability (cyclic voltammetry) and their hydration behavior (TGA under water vapor). The sulfonic acid functionalized compound shows reasonable properties only when a minimum hydration level is guaranteed, while phosphonic acid functionalized compounds combine satisfactory proton conductivity even in the water‐free state at intermediate temperatures (T < 200 °C), comparatively high thermo‐oxidative and electrochemical stability and electrochemical reactivity (hydrogen oxidation and oxygen reduction at platinum surfaces). The presence of water leads to moderate water uptake allowing for reasonable conductivities even at room temperature and prevents condensation reactions at higher temperature. The imidazole based system shows the largest electrochemical stability window, but its moderate proton conductivity and thermo‐oxidative stability and the very high overpotential for oxygen reduction on platinum turn out to be severe disadvantages for the envisaged application.

Relation Between Structure and Diffusion in Nanostructured Porous Solids and in Lipid Membranes

AbstractPulsed field gradient (PFG) NMR technique has been applied to study molecular transport in two different types of nanostructured materials, viz. in fluid catalytic cracking (FCC) catalysts and in lipid membranes. Diffusion studies have been performed for a broad range of molecular displacements covering displacements that are as small as a fraction of a micron. The effective diffusivities recorded on various length scales are used to evaluate the relevance of various transport modes in the particles of FCC catalysts for the rate of molecular exchange between catalyst particles and the surrounding atmosphere. This rate is shown to be primarily related to the diffusion in the meso- and macropores of the particles under the condition of fast molecular exchange between these pores and the zeolite crystals located in the particles. Studies of lipid membranes are focused on developing fundamental understanding of the influence of various types of domains on lateral mobility of lipids. A meaningful study of this influence requires an ability of monitoring lipid diffusion for different displacements that are smaller and larger than the domain size. First PFG NMR data along this direction are presented.

Poly(n-butyl acrylate-co-carbon monoxide-co-ethylene) Characterization by High-Temperature Two-Dimensional NMR at 750 MHz

Poly(n-butyl acrylate-co-carbon monoxide-co-ethylene) samples with varying monomer compositions and degrees of branching (g‘) were characterized using 750 MHz two-dimensional (2D) pulsed-field gradient (PFG) NMR spectroscopy. The NMR experiments were performed in solution at 100 °C to ensure sample homogeneity and sufficient mobility of the ethylene-based terpolymer. Since the 1H and 13C NMR spectra of the terpolymer samples were complex, 2D-1H/13C-heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond correlation (HMBC) experiments were conducted. The distinctive resonances of the ketone and ester carbonyl groups facilitated spectral assignments, such that chain-end resonances were apparent and the effects of short-chain-branching reactions and monomer sequence distributions were readily observed.

The association of dimethylsulfoxide and model membranes studied by pulse‐field gradient NMR

The use of dimethylsulfoxide (DMSO) as a cryoprotectant to reduce cellular injury during freezing is well known, however the intermolecular interactions between this amphiphilic molecule and biological membranes that form the basis of this protection are unknown. DMSO–dipalmitoylphosphatidylcholine (DPPC) vesicle interactions were investigated in pulsed‐field gradient NMR (PFGNMR) experiments and spectra analysis allowed for the determination of self‐diffusion coefficients for each species present. The mole fraction of DMSO associated with the DPPC vesicles was then calculated from the diffusion coefficients: the mole fraction increased from 14% to 42% as the membrane was heated from below to above the main phase transition temperature.

A Surprising Drop of the Diffusivities of Benzene in a Mesoporous Material of Type MCM-41 at Medium Pore Filling Factors

Pulsed Field Gradient (PFG) NMR is applied to study molecular diffusion of benzene in a commercial sample of MCM-41. Over a very small concentration range at medium pore filling factors the diffusivity is found to sharply drop by up to one order of magnitude, while it remains essentially constant over the total remaining range from vanishing concentrations up to over-saturation. Similarly extreme deviations from monotonous concentration patterns have so far not been described in the literature. The observed effect may be rationalised as a consequence of the onset of capillary condensation in the transport pores in the hyper-structure of the MCM-41 particles under study.

Using NMR displacement imaging to characterize electroosmotic flow in porous media

Pulsed field gradient nuclear magnetic resonance (PFG-NMR) and NMR imaging were used to study temporal and spatial domains of an electrokinetically-driven mobile phase through open and packed segments of capillaries. Characteristics like velocity distribution and an asymptotic dispersion are contrasted to viscous flow behavior. We show that electroosmotic flow in microchannel geometries can offer a significant performance advantage over the pressure-driven flows at comparable Peclét numbers, indicating that velocity extremes in the pore space of open tubes and packed beds are drastically reduced. An inherent problem of capillary electrochromatography that we finally address is the existence of wall effects when in the general case the surface zeta-potentials of the capillary inner wall and the adsorbent particles are different. Using dynamic NMR microscopy we were able resolve this systematic velocity inequality of the flow pattern which strongly influences axial dispersion and may be responsible for long time-tails of velocity distribution in the mobile phase.

Structure-related diffusion under molecular confinement

Molecular diffusion under confinement by highly ordered nanoporous structures is subjected to a hierarchy of particle-particle correlations. These correlations result in a variety of peculiarities of molecular diffusion. Three examples are discussed in detail. As a consequence of the confinement by the host matrix, molecular displacements in different directions often intimately depend on each other, resulting in an interdependence of the principal elements of the diffusion tensor (correlation rule of diffusion anisotropy). If intracrystalline channel systems confine molecular propagation to one direction, molecules with diameters sufficiently exceeding the channel radii are subjected to single-file diffusion, whose spatial-temporal dependence significantly deviates from normal diffusion. In networks of intersecting single-file systems with different affinities to the reactant and product molecules, the interplay of diffusion and reaction leads to deviations from a homogeneous pore filling over the sample and to an enhanced output of reaction products in comparison with channel networks of equal adsorption affinities (reactivity enhancement by molecular traffic control). The presented theoretical concepts are compared with experimental results, in particular with the findings of pulsed field gradient (PFG) NMR diffusion studies.

Investigation of the solution condition of lithium electrolyte solutions with LiCF3SO3 salt

Solution condition of the lithium electrolyte of LiCF3SO3 salt dissolved in propylene carbonate (PC), dimethoxyethane (DME) and tetrahydrofuran (THF) were investigated based on the measurements of the self-diffusion coefficients of lithium (DLi) and anion (DF) species using the pulsed-field gradient (PFG) NMR, viscosity and conductivity. DLi and DF decreased with the increase in the solution concentration for these electrolytes. This result reflects the change in the dissociation degree of the salt and the viscosity depending on the solution concentration. In the case of PC solution (the dielectric constant (∈) of PC: ∼65), measured DLi and DF were separated each other with the decrease in the concentration because the dissociation of the salt was promoted to increase the content of dissociated ions. With an increase in the concentration, the dissociation would be restricted and DLi and DF approximated each other, which means that the associated ion pairs are dominant in the solution. The effective ionic radius (reff) of cation species of PC solution estimated from the Stokes-Einstein relation using the diffusion coefficient and the viscosity results was four times greater than that of the naked lithium ion and independent of the dissociation degree. On the other hand, the reff of anion species changed depending on the dissociation degree. This suggests that the dissociated lithium ions are solvated by the PC solvent in the solution. DME and THF solutions did not show the solvation effect from the result of the change in reff with the concentration. This suggests that the dissociation degree of the dissolved salt was low due to the low ∈ of their solvents (∈ ∼ 7.2) and the ion pairs would dominant in the solutions independent of the solution concentration.

GELATION KINETICS OF POLYMERS CONFINED IN WATER/ OIL-SATURATED POROUS MATERIALS PROBED BY NMR

The gelation of polyvinylalcohol with glutaraldehyde confined in a water/ oil (= 3/1 in volume ratio) saturated sandstone (Bentheimer) was characterized by H NMR spin-lattice relaxation rate (1/ T)measurements and Pulse Field Gradient (PFG) NMR diffusion measurements. The gelation rate was found to be (12.8 ± 1.3) 10−1 5−1 at 340 K, which is approximately an order of magnitude faster than in bulk solution. In comparison, the gelation rate within the same porous material containing no oil phase was even faster, by a factor of 2.

5565775 Producible fluid volumes in porous media determined by pulsed field gradient nuclear magnetic resonance

The present invention is a method to determine the bound and free fluid index of fluids in porous media by pulsed field gradient (PFG) NMR diffusion measurements. The PFG NMR signal is evaluated using a two-fluid model that describes the self diffusion coefficients for the fluid fractions with low and high translational mobilities.

NMR Studies of Fluid Diffusion in Confining Geometries

AbstractDiffusion dynamics for fluids in sandstone rocks were studied by CPMG and pulsed field gradient (PFG) NMR measurements. Stretched exponential decays of the transverse magnetization were observed while varying the interpulse spacing, τ, in the CPMG experiment which sets the time for diffusion. A crossover from free to restricted diffusion is evident in the dependence of both T2 and the stretch exponent, β2, on the diffusion time τ. The T2(τ) data is fit to a model which interpolates between free and restricted diffusion and has only one free adjustable length scale despite the fact that the sandstone rocks are characterized by a pore size distribution. This NMR derived length scale correlates with a characteristic length derived from Hg injection experiments at the percolation threshold. We compare these results from the CPMG experiment with stimulated echo PFG-NMR diffusion measurements. This provides a complementary method of probing the diffusion dynamics since the longitudinal magnetization is insensitive to the dephasing effects of diffusion in internal field gradients.

Translational diffusion of glycine in erythrocytes measured at high resolution with pulsed field gradients

In the past we used relaxation measurements of [ 2,13C]glycine to estimate its intracellular translational diffusion coefficient and the viscosity of the cytoplasm in erythrocytes (I, 2). Here we report on a more direct approach to these measurements using pulsed field gradient (PFG) NMR (3) in a high-resolution mode at 9.4 T with low concentrations of solute. The results indicate that the intracellular glycine diffusion coefficient is one-third of the free solution value of 1.18 X 10 -9 m 2 s-l. The use of relaxation time measurements to estimate macroviscosity via the Debye equation (4) and thence translational diffusion coefficients using the Stokes-Einstein equation (5, 6) requires that the solvent mo lecules be “seen” as a cont inuum by the probe mo lecule. For a probe mo lecule in the cytoplasm of a red cell, the “solvent” mo lecules are hemoglobin and H20. Herrmann and M iiller ( 7) showed that the intracellular macroviscosity is almost exclusively determined by the hemoglobin concentration. Our results indicate that the glycine mo lecule does not see the solvent as a continuum, which is consistent with it being three orders of magn itude smaller than hemoglobin. Human blood was obtained from the Red Cross Blood Transfusion Service, Sydney, NSW. [ 2,‘3C]Glycine was from Merck Sharp and Dohme, Montreal, Canada. The blood was washed twice in saline ( 154 mmo l/liter of NaCl, 10 mmo l/liter of glucose, 277 K) and then bubbled with carbon monoxide. The cells were centrifuged to give a pellet of hematocrit -0.9; 5 m l of the pellet was then added to 5 m l of solution containing [ 2,13C]glycine ( 100 mmo l/liter) in saline ( 125 mo l/liter ofNaC1 and 20 mmo l/liter ofglucose in 20/80 ‘H20/ ‘H*O, pH 7.4) which had been bubbled with carbon monoxide. The cell suspension was sealed in a centrifuge tube and incubated at 3 10 K for 5 h. Following the incubation the cell suspension was saturated with carbon monoxide and centrifuged. The cells were then resuspended in saline ( 154 mmo l/liter of NaCl-saturated with carbon monoxide) constituted in 100% 2H20 and the cells were centrifuged to give a pellet with a hematocrit greater than 0.9. Lysates of the glycine-incubated cells were made by sonication with a Branson B 12 sonifier (Branson Sonic Power Company, Danbury, Connecticut). Packed cells ( -0.5 m l) were transferred to flat-bottom (9 m m o.d.) NMR tubes and a Teflon vortex plug was inserted. These tubes were placed in 10 m m o.d. tubes

Pulsed field gradient NMR determination of the temperature dependence of the tracer diffusion coefficient of hemoglobin.

AbstractWe have studied the temperature dependence of the tracer diffusion coefficient of carbonmonoxy hemoglobin A (HbA‐CO) by means of pulsed‐field gradient nmr (PFG‐nmr). Measurements were made over the temperature range from 15 to 35°C for samples having concentrations 7.4 and 16.7 g/dL. No significant deviations were found from the predictions of the Stokes‐Einstein relation. Thus, this work does not corroborate the recently proposed conformational change in hemoglobin at 22°C. The advantages of PFG‐nmr for the study of hemoglobin are discussed.