Pulsed Field Gradient NMR Spectroscopy Research Articles

Direct Observation of Two Distinct Diffusive Modes for Polymer Rings in Linear Polymer Matrices by Pulsed Field Gradient (PFG) NMR

Using high resolution PFG NMR spectroscopy, we have studied the diffusion of well-characterized polymer rings in linear host matrices at various observation times Δ, varying both ring and host molecular weights. For the first time, to our knowledge, for higher Mw rings in entangled melts it was possible to directly distinguish two different diffusive modes: (i) fast diffusion that scales inversely with the host chain length and (ii) much slower diffusion depending much more strongly on the host molecular weight. Furthermore, we studied the diffusion of the linear chains in the host melts. The diffusion data were analyzed in terms of existing theories and compared to simulations. The two-mode structure directly verifies the hypothesis of qualitatively different mechanisms for ring diffusion in linear melts. The fast mode quantitatively agrees with the assertion of a special diffusion channel for once threaded rings, while the suggested diffusion of unthreaded rings was not found. The slow mode scales more ...

Influence of membrane-type and flow field design on methanol crossover on a single-cell DMFC: An experimental and multi-physics modeling study

The performance of a 5 cm2 single-cell direct methanol fuel cell (DMFC) was evaluated experimentally by using two different electrolyte membranes (Fumapem® F-1850 and Nafion® N-117) for assembling the electrodes and three different types of flow field design (a unique serpentine, four parallel serpentines, four inlet serpentines). A 3D multi-physics, multi-component, two-phase, and not-isothermal model was computed with Comsol® Multiphysics v4.4 platform, to analyze and understand the behavior of the various configuration tested. The model consists of Maxwell-Stefan, Stokes-Brinckman, extended two-phase Darcy Law, modified Butler-Volmer and Tafel equations to simulate the performance of the DMFC. Pulse Field Gradient (PFG) NMR spectroscopy was used to get a direct measurement of the diffusion coefficients of water and methanol through the membranes. These values were then implemented in the multi-physics model. The model well reproduces the cell performance of all the MEA tested regarding polarization curves obtained under various experimental conditions (varying the inlet mass flows, the methanol concentration, the type of oxidant, the temperature). Thus, the model was used as a tool to investigate anodic overpotentials, water and methanol crossover flow rates, current density distribution at the catalyst layer/membrane interface, understanding the relationship between flow fields and cell performance. At similar specific power density, and similar anodic overpotentials, the methanol crossover flow rate is one order of magnitude lower for Fumapem® F-1850 than for Nafion® N-117, notwithstanding the much lower thickness of the F-1850 membrane.

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

Ionic transport inside porous carbon electrodes underpins the storage of energy in supercapacitors and the rate at which they can charge and discharge, yet few studies have elucidated the materials properties that influence ion dynamics. Here we use in situ pulsed field gradient NMR spectroscopy to measure ionic diffusion in supercapacitors directly. We find that confinement in the nanoporous electrode structures decreases the effective self-diffusion coefficients of ions by over two orders of magnitude compared with neat electrolyte, and in-pore diffusion is modulated by changes in ion populations at the electrode/electrolyte interface during charging. Electrolyte concentration and carbon pore size distributions also affect in-pore diffusion and the movement of ions in and out of the nanopores. In light of our findings we propose that controlling the charging mechanism may allow the tuning of the energy and power performances of supercapacitors for a range of different applications. It is challenging to probe ion dynamics in supercapacitor electrodes, which has significant implications in optimizing their performance. Here, the authors develop in situ diffusion NMR spectroscopy to measure and illustrate the diffusion of the charge-storing ions in working supercapacitors.

Coupling and Decoupling of Rotational and Translational Diffusion of Proteins under Crowding Conditions.

Molecular motion of biopolymers in vivo is known to be strongly influenced by the high concentration of organic matter inside cells, usually referred to as crowding conditions. To elucidate the effect of intermolecular interactions on Brownian motion of proteins, we performed (1)H pulsed-field gradient NMR and fluorescence correlation spectroscopy (FCS) experiments combined with small-angle X-ray scattering (SAXS) and viscosity measurements for three proteins, αB-crystalline (αBc), bovine serum albumin, and hen egg-white lysozyme (HEWL) in aqueous solution. Our results demonstrate that long-time translational diffusion quantitatively follows the expected increase of macro-viscosity upon increasing the protein concentration in all cases, while rotational diffusion as assessed by polarized FCS and previous multi-frequency (1)H NMR relaxometry experiments reveals protein-specific behavior spanning the full range between the limiting cases of full decoupling from (αBc) and full coupling to (HEWL) the macro-viscosity. SAXS was used to study the interactions between the proteins in solution, whereby it is shown that the three cases cover the range between a weakly interacting hard-sphere system (αBc) and screened Coulomb repulsion combined with short-range attraction (HEWL). Our results, as well as insights from the recent literature, suggest that the unusual rotational-translational coupling may be due to anisotropic interactions originating from hydrodynamic shape effects combined with high charge and possibly a patchy charge distribution.

Lithium Garnet Based All-Solid-State Batteries By Spark Plasma Sintering

Solid state electrolytes (SSEs) have a wider electrochemical and thermal stability window than either liquid or polymer electrolytes, making them a desirable component of high voltage lithium ion batteries. The superior thermal stability of SSEs reduces the need for added coolant in a multi-cell battery allowing for more efficient stacking. The SSE also provides structural stability, lending itself to smaller and inherently safer batteries. Recently LISICON sulfates, perovskites and garnets have been investigated because they exhibit a highly mobile sub-lattice of lithium. Among these the lithium garnet Li7La3Zr2O12 is a suitable candidates for use as a SSE as it is stable against lithium metal. Room temperature ionic conductivities as high as 1.3 S/cm have been reported for the cubic phase of this material, however the thermodynamically stable tetragonal phase has an ionic conductivity orders of magnitude bellow the cubic phase. Higher conductivity values and stabilization of the cubic phase may be obtained by chemical substitution. The strategy adopted in this study is the substitution of Li for Al, Ga, In, and Sc; La for Ca, Sr, Ba and the lanthanide series; and Zr for Hf and Ta. The lithium occupation and local environment will be studied in these structures to give insight into the phase transition from the cubic to the tetragonal phase. Preliminary PXRD results have confirmed the cubic phase can be stabilized by substitution of 25% of Zr for Ta.The structure of these garnets will be characterized by NMR and diffraction techniques. To avoid Li volatilization and the formation of undesirable reaction products, the garnets will be consolidated by spark plasma sintering. The ionic conductivities will be determined by pulsed field gradient NMR and impedance spectroscopy. Half cells will be assembled by sintering a layered composition of the electrolyte and active material. The half cells will be cycled against lithium and the electrolyte/electrode interface will be characterized by microscopy techniques. Compatible electrode materials will be determined in this way and an entire solid state cell will be assembled by spark plasma sintering. These cells will be characterized by impedance spectroscopy and an analysis of their charge discharge behavior will be performed.

Self-organization of a water-soluble fullerene derivative studied by pulsed field gradient NMR spectroscopy

The self-organization of a water-soluble sulfur-containing fullerene adduct with captopril in deuterated water was studied by a pulsed field gradient NMR technique. Two types of fullerene particles characterized by the self-diffusion coefficients Ds1 and Ds2 of (4.0±0.5)×10–10 and (6.5±0.3)×10–11 m2 s–1, respectively, were revealed.

Transport properties of catalyst supports studied by pulsed field gradient (PFG) and 2D exchange (EXSY) NMR spectroscopy

2D-EXSY and PFG 129Xe NMR provide a powerful means for probing tortuosity and pore connectivity in bimodal alumina catalysts.

Li+ ion transport in ionic liquid-based electrolytes and the influence of sulfonate-based zwitterion additives

The influence of sulfonate-based zwitterion (ZI) additives on the thermodynamic properties, ion transport properties and electrochemical properties of ionic liquid/Li-TFSI mixtures was studied by combining a number of experimental methods, namely DSC, conductivity measurements, viscosity measurements, 7Li NMR spectroscopy, pulsed field gradient NMR spectroscopy (PFG-NMR), Raman spectroscopy and cyclic voltammetry. We find that the addition of ZI leads to a suppression of crystallization processes in the ternary mixtures. The ion transport properties were analysed in a modified Walden plot with a newly defined reference line. The comparison of experimental data with the reference line points to simple liquid behaviour at high temperatures around 100°C and to a dynamic decoupling of faster ionic species from slower ionic species at lower temperatures. Thus, the addition of ZI increases the amount of decoupling in IL/Li salt mixtures. This interpretation is confirmed by the results of the PFG-NMR measurements, which indicate the existence of faster TFSI− anions and slower Li+/ZI complexes in the ternary mixtures. Raman spectra reveal strong changes in the coordination environment of the TFSI− anion upon addition of the ZIs. The cyclic voltammograms point to a cathodic shift of the Li+ reduction potential caused by the strong binding between Li+ ions and ZI.

Validity of the Stokes-Einstein Relation in Soft Colloids up to the Glass Transition.

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering, and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q,t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Monomeric nature of dengue virus NS3 helicase and thermodynamic analysis of the interaction with single-stranded RNA.

Dengue virus nonstructural protein 3 (NS3) is a multifunctional protein formed by a superfamily-2 RNA helicase linked to a protease domain. In this work, we report results from in vitro experiments designed to determine the oligomeric state of dengue virus NS3 helicase (NS3h) and to characterize fundamental properties of the interaction with single-stranded (ss)RNA. Pulsed field gradient-NMR spectroscopy was used to determine the effective hydrodynamic radius of NS3h, which was constant over a wide range of protein concentrations in the absence and presence of ssRNA. Size exclusion chromatography-static light scattering experiments showed that NS3h eluted as a monomeric molecule even in the presence of ssRNA. Binding of NS3h to ssRNA was studied by quantitative fluorescence titrations using fluorescein-labeled and unlabeled ssRNA oligonucleotides of different lengths, and the effect of the fluorescein label on the interaction parameters was also analyzed. Experimental results were well described by a statistical thermodynamic model based on the theory of non-specific interactions of large ligands to a one-dimensional lattice. We found that binding of NS3h to ssRNA oligonucleotides and to poly(A) is characterized by minimum and occluded binding site sizes both of 10 nucleotides and by a weak positive cooperativity between adjacent proteins.

Diffusion of water inside carbon nanotubes studied by pulsed field gradient NMR spectroscopy.

Diffusion dynamics of guest molecules in nanopores has been studied intensively because diffusion is center on a number of research fields such as separation, drug delivery, chemical reactions, and sensing. In the present work, we report an experimental investigation of the self-diffusion of water inside carbon nanotube (CNT) channels using a pulsed field gradient (PFG) NMR method. The dispersion of CNTs homogeneously in water and cooling to temperatures below the melting point of bulk water allow us to probe the translational motion of confined water molecules. The results demonstrate that the self-diffusion coefficient of water in CNTs is highly dependent on the diffusion time and CNT diameter. In particular, the diffusivity of water in double-walled carbon nanotubes (DWNTs) with an average inner diameter of 2.3 ± 0.3 nm is twice that in multiwalled carbon nanotubes (MWNTs) with an average inner diameter of 6.7 ± 0.8 nm in the temperature range of 263-223 K. In addition, the effective self-diffusion coefficient in DWNTs is 1 order of magnitude higher than that reported for mesoporous silica materials with a similar pore size. The faster diffusivity of water in CNTs could be attributed to the ordered hydrogen bonds formed between water molecules within the confined channels of CNTs and the weak interaction between water and the CNT walls.

Lithium ion diffusion in Li β-alumina single crystals measured by pulsed field gradient NMR spectroscopy

The lithium ion diffusion coefficient of a 93% Li β-alumina single crystal was measured for the first time using pulsed field gradient (PFG) NMR spectroscopy with two different crystal orientations. The diffusion coefficient was found to be 1.2 × 10(-11) m(2)/s in the direction perpendicular to the c axis at room temperature. The Li ion diffusion coefficient along the c axis direction was found to be very small (6.4 × 10(-13) m(2)/s at 333 K), which suggests that the macroscopic diffusion of the Li ion in the β-alumina crystal is mainly two-dimensional. The diffusion coefficient for the same sample was also estimated using NMR line narrowing data and impedance measurements. The impedance data show reasonable agreement with PFG-NMR data, while the line narrowing measurements provided a lower value for the diffusion coefficient. Line narrowing measurements also provided a relatively low value for the activation energy and pre-exponential factor. The temperature dependent diffusion coefficient was obtained in the temperature range 297-333 K by PFG-NMR, from which the activation energy for diffusion of the Li ion was estimated. The activation energy obtained by PFG-NMR was smaller than that obtained by impedance measurements, which suggests that thermally activated defect formation energy exists for 93% Li β-alumina single crystals. The diffusion time dependence of the diffusion coefficient was observed for the Li ion in the 93% Li β-alumina single crystal by means of PFG-NMR experiments. Motion of Li ion in fractal dimension might be a possible explanation for the observed diffusion time dependence of the diffusion coefficient in the 93% Li β-alumina system.

Self-assembly of symmetrical and dissymmetrical dicationic surfactants in the solid phase and in solution

The effect of the spacer structure (linear, cyclic, bicyclic) and dissymmetry of alkyl fragment in a series of dicationic gemini surfactants on the ability to form thermotropic liquid crystals and on the micellar properties was studied. Transitions crystal-thermotropic liquid crystals-isotropic solution were studied using differential scanning calorimetry and optical microscopy. The self-diffusion coefficients of micelles and monomers of the dicationic dialkyl derivatives of 1,4-diazabicyclo[2.2.2]octane in water were determined by pulsed field gradient NMR spectroscopy. The effect of the surfactant structure on the values of critical micelle concentrations, hydrodynamic radii, and aggregation numbers of micelles was analyzed.

Metal Organic Frameworks for Natural Gas Storage in Vehicles

AbstractWith the increasing demand for alternative fuels the storage of natural gas (NG) in adsorbents like metal organic frameworks (MOFs) will become more important. In order to use MOFs as storage media in fuel delivery systems, the optimization of mass and energy transfer of the system is crucial. For rapid NG filling of a tank, molecules need to reach the adsorption sites within a reasonable time while the heat of adsorption should be dissipated to the environment. In this article, mass transfer in shaped bodies of MOFs was determined by permeability measurements and pulsed field gradient (PFG) NMR spectroscopy. The heat dissipation was also experimentally measured and both data sets were used to set up a theoretical density function theory model to predict the behavior of MOFs for NG storage.

Diffusion-Weighted PFGNMR Study of Molecular Level Interactions of Loops and Direct Bridges of HEURs on Latex Particles

Viscosity building in latex coatings to provide desirable shear thinning rheological properties is a key property commercially achieved with hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers (RMs). Prior studies focused on the aqueous solution properties of HEURs, resulting in the well-known transient network model that describes solution rheology reasonably well. Relatively fewer studies have probed the molecular level interactions between the hydrophobe groups of HEUR and latex surfaces under conditions of realistic latex volume fractions and HEUR concentration. The presence of ubiquitous surfactant and oligomer molecules in the latex aqueous phase makes it difficult to detect these interactions directly for any off-the-shelf (industrial) materials. In this work, we outline the use of pulsed field gradient (PFG) NMR spectroscopy as diffusion-weighted filter to remove the signals of low molecular weight species in order to detect hydrophobe end groups and urethane linkers. This i...

Combined Measurement of Translational and Rotational Diffusion in Quaternary Acyclic Ammonium and Cyclic Pyrrolidinium Ionic Liquids

The translational self-diffusion coefficients (D(T)) for a series of tetra-alkyl acyclic ammonium and cyclic pyrrolidinium ionic liquids (ILs) were measured using (1)H pulse field gradient (PFG) NMR spectroscopy over various temperatures. These NMR diffusion results were combined with previously measured rotational diffusion coefficients (D(R)) obtained from (14)N NMR relaxation measurements for the same ILs (Alam, T. M.; et al. J. Phys. Chem. A 2011, 115, 4307- 4316). The D(T)/D(R) ratio was then used to estimate the effective hydrodynamic radius and corresponding volumes without the need to directly measure the viscosities of the ILs. The generality, validity, and performance of using this D(T)/D(R) ratio is discussed and compared to the effective hydrodynamic volumes obtained using classic D(T)/viscosity and D(R)/viscosity relationships. The temperature variation observed for the molecular volumes obtained using the D(T)/D(R) ratio is argued to be a signature for the breakdown or decoupling of the Stokes-Einstein and Stoke-Einstein-Debye relationships in these neat IL systems, consistent with recent molecular dynamic simulations.

Analysis of the mesh size in a supramolecular hydrogel by PFG-NMR spectroscopy

Pulsed field gradient NMR (PFG-NMR) spectroscopy has been used to determine the network mesh size in stable hydrogels formed upon addition of Ca2+ to solutions of naphthalene diphenylalanine (2FF). At pH 12, the solutions at 0.55 wt% 2FF comprise worm-like micelles. Addition of Ca2+ results in cross-linking of these micelles. The self-diffusion of dextran guests of nominal 6, 40, 70, 100, 500, 670, 1400 and 2000 kDa, possessing hydrodynamic diameters (2Rh) similar to the expected pore sizes in these systems, was studied both in the precursor micellar solutions and in the hydrogels. The diffusivity of probes with 2Rh < 40 nm is restricted to a similar extent in both types of network with diffusion coefficients scaling as ∼Mr−0.5, where Mr is the nominal mass of the probe, consistent with relatively unrestricted diffusion. Diffusion coefficients fit well the equation Dn/Do = exp(−Rh/ξ), where Dn and Do are the diffusion coefficients in the presence and absence of network respectively and ξ is the mesh size, giving a mesh size of approximately 40 nm. The heaviest ca. 10% of the nominal 2000 kDa dextran fraction having approximate mass and hydrodynamic diameter 3300 kDa and 84 nm respectively was almost immobilised by the gel, consistent with this estimate of the mesh size. The restriction was much weaker in the micellar solution, which is attributed to the transient nature of this micellar network in the absence of Ca2+. Finally, the mesh size for micellar solutions prepared at 1.1 wt% 2FF is smaller than that of micellar solutions prepared at lower concentrations of 2FF. However, the corresponding gels have a larger mesh size than those prepared at lower concentrations of 2FF. We attribute this to increased fibre aggregation at the higher 2FF concentration. This correlates with lower rheological moduli at higher 2FF concentrations.

Solution Behavior of Double-Hydrophilic Block Copolymers in Dilute Aqueous Solution

The self-assembly of double-hydrophilic poly(ethylene oxide)–poly(2-methyl-2-oxazoline) diblock copolymers in water has been studied. Isothermal titration calorimetry, small-angle X-ray scattering, and analytical ultracentrifugation suggest that only single polymer chains are present in solution. In contrast, light scattering and transmission electron microscopy detect aggregates with radii of ca. 100 nm. Pulsed field gradient NMR spectroscopy confirms the presence of aggregates, although only 2% of the polymer chains undergo aggregation. Water uptake experiments indicate differences in the hydrophilicity of the two blocks, which is believed to be the origin of the unexpected aggregation behavior (in accordance with an earlier study by Ke et al. [Macromolecules2009, 42, 5339–5344]). The data therefore suggest that even in double-hydrophilic block copolymers, differences in hydrophilicity are sufficient to drive polymer aggregation, a phenomenon that has largely been overlooked or ignored so far.

Aqueous mixtures of di-n-decyldimethylammonium chloride/polyoxyethylene alkyl ether: Dramatic influence of tail/tail and head/head interactions on co-micellization and biocidal activity

Mixed aggregate formation and synergistic interactions of binary surfactant mixtures of di-n-decyldimethylammonium chloride, [DiC10][Cl], with polyoxyethylene alkyl ethers, CiEj (i=10, 12, j=4, 6, 8), have been investigated for various [DiC10][Cl]/CiEj ratios. The critical aggregation concentration of the binary mixtures has been determined by tensiometry, and the aggregate characteristics (i.e., size and composition, free ammonium concentration) have been estimated using the pulsed field gradient NMR spectroscopy and a [DiC10]-selective electrode. Diffusion coefficient measurements of micelles confirmed the synergistic interaction between the surfactants. It is thus shown that the formation of surface monolayers and mixed aggregates from [DiC10][Cl]/C10Ej mixtures is driven by both tail/tail and head/head interactions, whereas [DiC10][Cl]/C12Ej co-aggregation is mainly driven by tail/tail interactions. As a consequence, the co-aggregation phenomenon notably influences the biocidal activity of [DiC10][Cl] on the Candida albicans fungi. In the presence of C12Ej, the biocidal activity of the ammonium salt is inhibited due to the trapping of the cationic surfactants in the mixed aggregates, whereas in the presence of C10Ej, the biocidal activity of the surfactant mixture is maintained. The mode of action is also confirmed by a faster increase in the zeta potential of a C. albicans suspension in the presence of [DiC10][Cl]/C10E8 than in the presence of [DiC10][Cl]/C12E8. Therefore, a judicious adjustment of the alkyl (i) and polyoxyethylene (j) chain lengths of CiEj avoids its antagonistic effect on the biocidal activity of [DiC10][Cl].

Theoretical and experimental studies of water interaction in acetate based ionic liquids

Water interactions in 1-ethyl-3-methylimidazolium acetate ([emim][CH(3)COO]) were studied utilizing classical and ab initio simulation methods. The self-diffusivities for water and the ionic liquid (IL) were studied experimentally using pulse field gradient NMR spectroscopy and correlated with computational results. Water forms hydrogen bonding networks with the ionic liquid, and depending on the concentration of water, there are profound effects on the self-diffusivities of the various species. Both simulation and experiments show that the self-diffusivities for species in the water-[emim][CH(3)COO] system exhibit minima at 40-50 mol% water. Water interaction with the [CH(3)COO](-) anion predominates over the water-water and water-cation interactions at most water concentrations. Simulations further indicate that decreasing water-[CH(3)COO](-) interaction will increase the IL and water self-diffusivities. Self-diffusivities in the water-IL systems are dependent upon the cation in a complex way. Water interactions with [P(4444)][CH(3)COO] are reduced compared to [emim][CH(3)COO]. The [P(4444)](+) cation is bulkier than the [emim](+) cation and has a smaller self-diffusivity, but when water was introduced to [P(4444)] [CH(3)COO], the water-[CH(3)COO](-) hydrogen bonding network in the [P(4444)][CH(3)COO] was much smaller than the one observed in [emim][CH(3)COO].