Pulsed Field Gradient NMR Research Articles

Transport properties in molten carbonates: self-diffusion and conductivity measurements at high temperature

A better understanding and use of molten carbonate fuel cells (MCFCs) requires more detailed consideration on transport properties in melt. The combination of different methodological developments can be one solution to improve our comprehension. Here we present 23Na and 7Li self-diffusion coefficients measured by pulsed field gradient (PFG) NMR technique combined with electrical conductivity obtained by a 4-electrode set up, in the eutectic mixture Li2CO3–Na2CO3 (52:48 %mol) at high temperature (up to 1050 K), and under pure CO2 atmosphere. The results were compared with known experimental data from literature obtained by radiotracers techniques and 2-electrode set up and also with some calculations of the transport properties.

Spectroscopic Investigation of Entropic Canopy-Canopy Interactions of Nanoparticle Organic Hybrid Materials.

Nanoparticle organic hybrid materials (NOHMs) are self-suspended liquid-like nanoparticle-based functional materials consisting of a surface-functionalized inorganic nanocore and oligomeric or polymeric chains. They often exhibit complex intermolecular and intramolecular interactions among their constituents, resulting in versatile physicochemical characteristics that range from glassy solids to solvent-free nanoparticle fluids. A variety of applications involving NOHMs have been investigated thus far, including thermal management fluids, lubricants, magnetic fluids, nanocomposites, electrolytes, water treatment and biomass pretreatment chemicals, and CO2 capture solvents. In particular, NOHMs have recently been recognized as a promising CO2 capture and utilization medium. To capture CO2 more effectively, a variety of specific functional groups of strong chemical affinity to CO2 can be added to the polymeric canopy (enthalpic contribution), and various steric considerations induced by attractive/repulsive interactions among the nanocores and canopies can be introduced (entropic contribution). These occur while maintaining negligible vapor pressure and enhanced thermal stability. Here, we investigated the canopy dynamics of NOHMs with different-sized SiO2 nanocores, aiming to reveal the hidden nature of the entropic interaction occurring in NOHMs. Pulse-field gradient nuclear magnetic resonance spectroscopy (with 1H) was employed to investigate the canopy dynamics of the NOHMs synthesized using 7, 12, and 22 nm SiO2 particles, and these results were compared with those from a ternary mix of all three sizes of SiO2 nanocores. The self-diffusion coefficient and thermal diffusivity were also evaluated.

Effect of PAMAM Dendrimers on Interactions and Transport of LiTFSI and NaTFSI in Propylene Carbonate-Based Electrolytes.

Poly(amidoamine) (PAMAM)-based electrolytes are prepared by dissolving the PAMAM half-generations G1.5 or G2.5 in propylene carbonate (PC), either with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) or sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) salts. The solutions, designed for ion battery applications, are studied in terms of ions transport properties. Raman Spectroscopy reveals information about the interactions between cations and PAMAM dendrimers as well as full dissociation of the salts in all solutions. Pulsed-field gradient Nuclear Magnetic Resonance (PFG NMR), measured as a function of both temperature and PAMAM concentration, are obtained for the cation, anion, solvent, and dendrimer molecules using lithium (7Li), sodium (23Na), fluorine (19F), and hydrogen (1H) NMR, respectively. It was found that lithium diffusion is slow compared to the larger TFSI anion and decreases with PAMAM concentration due to interactions between cation and dendrimer. Comparison of conductivities calculated from diffusion coefficients using the Nernst–Einstein equation, with conductivity measurements obtained from Impedance Spectroscopy (IS), shows slightly higher IS conductivities, caused among others by PAMAM conductivity.

Study on release suppression of cinnamaldehyde from κ-carrageenan gel by HR-MASNMR and pulsed field gradient NMR (PFG-NMR)

Aiming toward the production and characterization of delicious and functional gel foods, this communication studies the flavor release from cinnamon-containing κ-carrageenan gel. Cinnamaldehyde, which provides the flavor of cinnamon, was released in a trace amount from the gel and detected by flame ionization detector gas chromatography. The retention of cinnamaldehyde in κ-carrageenan gel and the interaction between flavor and polysaccharide were investigated by high-resolution magic-angle spinning nuclear magnetic resonance (HR-MAS NMR) and pulsed-field gradient NMR (PFG NMR). The intact cinnamaldehyde in the gel was also observed by HR-MAS NMR. The relative mobility difference of the flavor and polysaccharide molecules was observed from the diffusion-ordered NMR spectrum of PFG NMR.

Heat stable whey protein stabilised O/W emulsions: Optimisation of the whey protein concentrate dry heat incubation conditions

The industrial application of whey proteins as a food emulsifier is still limited due to its susceptibility towards heating. In this study, dry heat conjugation of whey protein and lactose was investigated to improve the heat stability of WPC-stabilised oil in water (O/W) emulsions. A commercially available whey protein concentrate (WPC), containing 80 % protein and 12 % lactose, was dry heat incubated. Hereby, the effect of incubation temperature (60, 70, and 80 °C), relative humidity (64, 74, and 79 %), and preconditioning pH (4, 6, 8, 10) were determined. Temperature variation showed a pronounced effect on the required duration to obtain conjugates with excellent emulsion heat stabilising capacity. Four hours of incubation at 80 °C was sufficient to retain the original particle size and consistency after heating the emulsions at 80 °C for 20 min. Whereas circular dichroism spectroscopy revealed that this dry heat incubation condition (80 °C, 4 h) maintained the secondary structure of WPC, pulsed-field gradient NMR revealed that about 40 % of the lactose present should become protein-bound to generate conjugates with the desired functionality. The results indicated that a heat stable WPC with about 40 % of bound lactose could be obtained by some hours of dry heat treatment at 80 °C.

Establishing Ultralow Activation Energies for Lithium Transport in Garnet Electrolytes.

Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquid-based electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and inherent inhomogeneous current distribution due to different ion dynamics at grains, grain boundaries, and interfaces. In this study, we use a combination of electrochemical impedance spectroscopy, distribution of relaxation time analysis, and solid-state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary, and interfacial processes play in the ionic transport and electrochemical performance of garnet-based cells. Variable temperature impedance analysis reveals the lowest activation energy for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C, that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the way toward targeted engineering of garnet-type electrolytes and ameliorate their electrochemical performance in order to enable their use in commercial devices.

A Step toward Molecular Evolution of RNA: Ribose Binds to Prebiotic Fatty Acid Membranes, and Nucleosides Bind Better than Individual Bases Do.

A major challenge in understanding how biological cells arose on the early Earth is explaining how RNA and membranes originally colocalized. We propose that the building blocks of RNA (nucleobases and ribose) bound to self-assembled prebiotic membranes. We have previously demonstrated that the bases bind to membranes composed of a prebiotic fatty acid, but evidence for the binding of sugars has remained a technical challenge. Here, we used pulsed-field gradient NMR spectroscopy to demonstrate that ribose and other sugars bind to membranes of decanoic acid. Moreover, the binding of some bases is strongly enhanced when they are linked to ribose to form a nucleoside or - with the addition of phosphate - a nucleotide. This enhanced binding could have played a role in the molecular evolution leading to the production of RNA.

Structural and Li-ion diffusion properties of lithium tantalum phosphate LiTa2PO8

We synthesized a polycrystalline LiTa2PO8 sample at 1323 K by a solid-state reaction. The crystal structure of LiTa2PO8 was examined by selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) techniques. Electrochemical impedance measurements showed that the total conductivity at room temperature was 4.85 × 10−4 S cm−1 and the activation energy was 0.52 eV. The high Li-ion conductive properties were confirmed by 7Li pulse-field gradient (PFG) or pulse-gradient spin-echo (PGSE) NMR spectroscopy. The activation energy of the Li diffusion (0.51 eV) measured by PFG NMR was well consistent with the result of impedance measurement. We believe that the LiTa2PO8 would be as one of the important candidates for the solid electrolyte used in all solid-state lithium batteries.

Dry heat induced whey protein–lactose conjugates largely improve the heat stability of O/W emulsions

Whey protein–lactose conjugates were prepared by dry heat incubation (60 °C, 74% relative humidity) at 1:1 M ratio of protein amino groups to reducing sugar. Pulsed-field gradient-nuclear magnetic resonance (PFG-NMR) was introduced as a new and absolute method to determine the degree of conjugation without any sample pretreatment, based on the different diffusion behaviour of free and protein-bound lactose. Whereas the TNBS method revealed 73% conjugation, PFG-NMR results indicated that only 37% of lactose was bound to the protein after 8 h incubation. Circular dichroism spectra showed no change of WPI secondary structure upon dry heat incubation, whereas the electrophoretic mobility shifted towards more negative values. The emulsifying and, more importantly, heat stabilising properties of the conjugates were subsequently evaluated in oil/water emulsions. Eight hours of incubation was sufficient to produce conjugates that successfully stabilised the emulsion against heating (80 °C, 20 min), regarding the particle size and viscosity.

Diffusion in nanopores: correlating experimental findings with \u201cfirst-principles\u201d predictions

Measurement of molecular diffusion in nanoporous host materials, which are typically inhomogeneous and anisotropic, often involves an intricate web of factors and relations to be taken into account since the associated diffusivities are a function of the diffusion path of the guest molecules during a given observation time. Depending on the observation time, therefore, the result of the experimental measurement can point to completely different conclusions about the underlying diffusion phenomena. The risk of misinterpretation of the experimental data, by correlating them with irrelevant phenomena, may be reduced if there is an option to compare the data with the results of totally independent measurements. The present communication addresses this issue with reference to the particular potentials of pulsed field gradient NMR and microimaging by infrared microscopy as techniques of microscopic diffusion measurement.

A Novel Quaternary Composite Solid State Electrolyte for Li Batteries

All-solid-state batteries continue to grow as an alternative to replace the traditional liquid-based ones not only because they provide increased safety but also higher power and energy density [1]. However, current solid-state electrolytes are either brittle (ceramics) or suffer low ionic conductivity at room temperature (polymers) [1]. Therefore, composite solid electrolytes are considered as a viable option. We have developed flexible and easy film-forming composite solid state electrolytes (CSEs). Our promising CSE is a quaternary composite that consists of the Solid polymer electrolyte (SPE) Poly(ethylene oxide)/ Lithium bis(trifluoromethanesulfonyl)imide (PEO/ LiTFSI), the fast conducting perovskite inorganic ceramic Li0.33La0.557TiO3 (LLTO) and the plasticizer Succinonitrile (SN). Over 40 samples with different (PEO/LiTFSI/LLTO/SN) compositions were prepared and characterized. The optimal mechanical and electrochemical properties was found for the sample 10S which has the following composition: 70%-PEO 12% LiTFSI 9%-LLTO 9%-SN. 10S had the ionic conductivity of ̴ 10-3 S/cm at 55 ̊ C and young’s modulus of around 105 MPa. The all-solid-state battery made by sandwiching 10S between a lithium metal anode and an NMC 532-based cathode achieved specific discharge capacities of 161 mAh/g at C/20. Pulsed-field gradient NMR was performed at different temperatures to measure the diffusion coefficient. The results have shown an Arrhenius-type dependence with a slope switch at around 55 ̊ C which is a similar behavior as the one observed in the Arrhenius plot obtained from conductivity measurements. Further investigation is underway to elucidate the conductivity mechanism in this new type of quaternary composite by advanced NMR and other techniquesReference[1] M. Keller, A. Varzi, S. Passerini, J. Power Sources 2018, 392, 206. Figure 1

High Transference Number of Li Ion in Highly Concentrated Lithium Bis(trifluoromethanesulfonyl)Amide/Dinitrile Liquid Electrolytes

Liquid electrolytes with high concentration of Li salts are receiving increasing attention owing to various unusual physicochemical and electrochemical properties.1 Recently, our group reported that Li+ ion hopping conduction in highly concentrated solutions of lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in dinitrile solvents, namely, succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN), is involved in the ionic conduction.2 In the liquids with composition [LiFSA]/[dinitrile] > 1, the two cyano groups of the dinitrile coordinate to two different Li+ ions and form solvent-bridged structures of Li+−dinitrile−Li+. In addition to the solvent-bridged structures, ionic aggregates (Li+−FSA−−Li+) are formed in these liquids. We report here the phase behaviors, solvate structures and transport properties of dinitrile-based highly concentrated electrolytes composed of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) with higher thermal and chemical stability. LiTFSA/SN and LiTFSA/ADN binary mixtures form stable solvates at the composition of LiTFSA-(Dinitrile)1.5 and keep solid state at room temperature. On the other hands, LiTFSA/GN mixtures become glass forming liquids and maintains liquid state in a wide range of composition. Raman spectra for the liquids of LiTFSA/GN revealed that the average Li+ ion solvation number in the range of 1/8 ≤ [LiTFSA]/[GN] ≤ 1/3 is 3.1. In the highly concentrated solutions with compositions of [LiTFSA]/[GN] ≥ 1/3, the coordination of TFSA− anion to Li+ ion becomes remarkable due to the deficiency of GN and contact ion pairs and ionic aggregates are formed. The self-diffusion coefficients of Li+, TFSA−, and GN measured with pulsed field gradient (PFG) NMR suggested that Li+ ion diffuses faster than the anion in the concentrated electrolytes. This suggests that Li+ ion exchanges the anions in the ionic aggregates and moves forward leaving the anions. In other words, the ion-exchange conduction mode emerges in the high concentration region. We estimated the Li+ transference number in an electrolyte of [LiTFSA]/[GN] = 1/1.5 using the Bruce-Vincent method3 and found that the electrolyte possesses a high transference number of ca. 0.7. Finally, we applied the [LiTFSA]/[GN] = 1/1.5 electrolyte to a lithium sulfur cell. Thanks to the high transference number, the cell showed a relatively high rate capability, regardless of its low ionic conductivity (0.21 mS cm−1) at room temperature. References 1 Y. Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423.2 Y. Ugata, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Li-ion hopping conduction in highly concentrated lithium bis(fluorosulfonyl)amide/dinitrile liquid electrolytes, Phys. Chem. Chem. Phys., 2019, 21, 9759–9768.3 J. Evans, C. A. Vincent and P. G. Bruce, Electrochemical measurement of transference numbers in polymer electrolytes, Polymer, 1987, 28, 2324–2328. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 16H06368, 18H03926, and 19H05813) from the Japan Society for the Promotion of Science (JSPS) and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).

Achieving Stable Cycling of Li-Metal Anodes in Li-O2 Batteries: Optimizing Solvation Environment in Ionic Liquid/Solvent Blend Formulations

The realization of a truly rechargeable, high capacity lithium-oxygen (Li-O2) battery faces serious technological barriers relating to material/intermediate stability, electrochemical reversibility of the cathode discharge processes, and gas supply challenges.1 Furthermore, to maximize energy densities of Li-O2 cells, the utilization of Li-metal anodes is highly desired, thus introducing additional and serious interfacial challenges in the form of electrolyte degradation and susceptibility to dendrite formation on cycling. These factors ultimately create serious safety issues and further contribute to the reduced cyclability of Li-O2 cells that is already limited by the poor reversibility of Li2O2 formation at the cathode. Without introducing artificial anode protection or isolation of Li from a cathoylte compartment, the electrolyte for a high-performance Li-O2 device must exhibit complementary stability at both reactive interfacial regions of the two different electrodes. In many cases, the solvents that mediate good performance at the Li-O2 cathode exhibit poor stability at Li-metal and, consequently, new electrolyte formulations geared towards good stability and performance at both the cathode and the anode are required.In this work, we report on the development of blend electrolytes of ionic liquids (ILs) and molecular solvents for improving cell performance in Li-O2 batteries with considered focus on enhancing stability at the Li-metal anode. Through combined experimental and theoretical investigations, we present how the electrochemical stability of Li-O2-cathode relevant molecular solvents at the Li-metal anode can be regulated by tuning formulation properties. Wide formulation ranges, including dilute to highly concentrated Li-salt mixtures are explored within the ternary formulations of IL, molecular solvent and Li-salt. Good performance in symmetrical Li|electrolyte|Li cells is achieved through improving the Coulombic efficiency of the electroplating/stripping processes, as quantified in asymmetric Li|Cu cells. In lieu of strong SEI forming additives, these observed stability enhancements are found to be highly reliant on Li+-solvent solvation interactions within the liquids. Shifts in vibrational modes associated with changes in coordination, as measured by Raman and IR spectroscopy, provide insights into the nature of these important solvent and salt interactions. These experimental insights are supported by computational modelling of local solvation environments in select exemplary mixtures. Pulsed-field gradient NMR techniques are also utilized to understand what information the diffusional motion of differing species within the IL/solvent blends further reveals about the nature of these stabilizing coordination interactions. The implications of complete formulation design towards achieving mutual stability at anode and cathode interfaces is demonstrated within full cell (Li-O2) testing using composite air-cathodes. 1. D. Aurbach, B. D. McCloskey, L. F. Nazar, and P. G. Bruce, Nat. Energy, 1 16128 (2016).

A novel proton conducting ionogel electrolyte based on poly(ionic liquids) and protic ionic liquid

As opposed to the traditional hydrogels and organogels, ionogels are a new class of gel family where the liquid phase, percolating through the solid phase, consists of an ionic liquid (IL). The properties inherent in the ionic liquids, including high ionic conductivity, low-volatility and wide electrochemical stability window, are imparted to the ionogels. In this work, a novel proton conducting ionogel polymer electrolyte (iGPE) based on a poly (ionic liquid) Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide, ([PDADMA][TFSI]) and protic ionic liquid, N,N-diethylmethylammonium triflate ([DEMA][Tf]) is prepared. The influence of the protic ionic liquid content on the physical, electrochemical, and ion transport properties of the iGPE has been investigated by differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and multi-nuclear solid-state and pulse-field gradient nuclear magnetic resonance. Upon addition of 60 wt% protic IL, the conductivity of the iGPE can be enhanced to 5.8 mS/cm at 100 °C, which is signifiantly higher than that of Nafion at 120 °C (0.2 mS/cm). Solid-state and PFG NMR measurements show that although the incorporation of [PDADMA][TFSI] leads to decreased ion dynamics, it promotes proton dissociation, resulting in a lower activation energy for ion diffusion compared to the neat protic IL. Evidence of proton activity in the iGPEs was obtained by cyclic votammetry, making this material a promising candidate as a proton conducting membranes in fuel cell applications.

Influence of the Headgroup on the Interaction of Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymers with Lipid Bilayers.

The lipid headgroup plays an important role in the association of polymers with lipid bilayer membranes. Herein, we report how a glycerol headgroup versus a choline headgroup affects the interaction of poly(ethylene oxide)-b-poly(propylene oxide) (PEO-PPO) block copolymers with lipid bilayer vesicles. Unilamellar vesicles composed of phosphatidylcholine and phosphatidylglycerol at various molar ratios were used as model membranes. The interactions between the block copolymers and lipid bilayers were quantified by pulsed-field gradient nuclear magnetic resonance (PFG-NMR) based on the distinctly different mobilities of free and bound polymers. All the investigated polymer species showed significantly higher binding with 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) sodium salt (POPG) liposomes than with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes, indicating stronger association with the glycerol headgroup compared to the choline headgroup. This effect did not become significant until the composition of mixed POPC/POPG liposomes contained more than 20 mol % POPG. A plausible explanation for the enhanced polymer binding with POPG invokes the role of hydrogen bonding between the glycerol headgroup and the ether moieties of the polymers.

Controlling Gas Selectivity in Molecular Porous Liquids by Tuning the Cage Window Size

Control of pore window size is the standard approach for tuning gas selectivity in porous solids. Here, we present the first example where this is translated into a molecular porous liquid formed from organic cage molecules. Reduction of the cage window size by chemical synthesis switches the selectivity from Xe-selective to CH4-selective, which is understood using 129Xe, 1H, and pulsed-field gradient NMR spectroscopy.

Highly concentrated LiN(SO2CF3)2/dinitrile electrolytes: Liquid structures, transport properties, and electrochemistry.

Liquid structures, transport properties, and electrochemical properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and dinitrile solvents [succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN)] were investigated. In the LiTFSA/SN and LiTFSA/ADN systems, the stable crystalline solvates of LiTFSA-(SN)1.5 [melting point (Tm): 59 °C] and LiTFSA-(ADN)1.5 (Tm: 50 °C) were formed, respectively. In contrast, the LiTFSA/GN mixtures of a wide range of compositions were found to be glass-forming liquids at room temperature. Raman spectroscopy of LiTFSA/GN liquid mixtures revealed that increasing the LiTFSA concentration results in the formation of the solvent-bridged network structure Li+-GN-Li+. In addition, the considerable formation of contact ion pairs and ionic aggregates was observed in highly concentrated electrolytes. In the liquids, the Li+ ion dynamically exchanged ligands (GN and TFSA) and higher LiTFSA concentrations led to an increase in the ratio of the self-diffusion coefficients of Li+ and TFSA-, DLi/DTFSA, as determined by pulsed field gradient NMR spectroscopy. The Li+ transference number (tLi+ ) of the [LiTFSA]/[GN] = 1/1.5 electrolyte in an electrochemical cell under anion-blocking conditions was estimated to be as high as 0.74. Furthermore, electrochemical measurements revealed that the reductive stability of the LiTFSA/GN electrolyte increases with increasing LiTFSA concentration. A [LiTFSA]/[GN] = 1/1.5 electrolyte is stable against the Li metal electrode, provided that the polarization is relatively small. Owing to high tLi+ , a Li-S battery with the [LiTFSA]/[GN] = 1/1.5 electrolyte showed a high rate discharge capability despite its low ionic conductivity (0.21 mS cm-1) at room temperature.

In Situ Determination of Carbon Number Distributions of Mixtures of Linear Hydrocarbons Confined within Porous Media Using Pulsed Field Gradient NMR.

Pulsed field gradient (PFG) NMR measurements, combined with a novel optimization method, are used to determine the composition of hydrocarbon mixtures of linear alkanes (C7-C16) in both the bulk liquid state and when imbibed within a porous medium of mean pore diameter 28.6 nm. The method predicts the average carbon number of a given mixture to an accuracy of ±1 carbon number and the mole fraction of a mixture component to within an average root-mean-square error of ±0.036 with just three calibration mixtures. Given that the method can be applied at any conditions of temperature and pressure at which the PFG NMR measurements are made, the method has the potential for application in characterizing hydrocarbon liquid mixtures inside porous media and at the operating conditions relevant to, for example, hydrocarbon recovery and heterogeneous catalysis.

Diffusion of Ions in Phosphonium Orthoborate Ionic Liquids Studied by 1H and 11B Pulsed Field Gradient NMR.

Non-halogenated boron-based ionic liquids (ILs) composed of phosphonium cations and chelated orthoborate anions have high hydrolytic stability, low melting point and exceptional properties for various applications. This study is focused on ILs with the same type of cation, trihexyltetradecylphosphonium ([P6,6,6,14]+), and two orthoborate anions, such as bis(salicylato)borate ([BScB]−) and bis(oxalato)borate ([BOB]−). We compare the results of this study with our previous studies on ILs with bis(mandelato)borate ([BMB]−) and a variety of different cations (tetraalkylphosphonium, dialkylpyrrolidinium and dialkylimidazolium). The ion dynamics and phase behavior of these ILs is studied using 1H and 11B pulsed-field-gradient (PFG) NMR. PFG NMR is demonstrated to be a useful tool to elucidate the dynamics of ions in this class of phosphonium orthoborate ILs. In particular, the applicability of 11B PFG NMR for studying anions without 1H, such as [BOB]−, and the limitations of this technique to measure self-diffusion of ions in ILs are demonstrated and discussed in detail for the first time.

Protein structural changes characterized by high-pressure, pulsed field gradient diffusion NMR spectroscopy

Pulsed-field gradient NMR spectroscopy is widely used to measure the translational diffusion and hydrodynamic radius (Rh) of biomolecules in solution. For unfolded proteins, the Rh provides a sensitive reporter on the ensemble-averaged conformation and the extent of polypeptide chain expansion as a function of added denaturant. Hydrostatic pressure is a convenient and reversible alternative to chemical denaturants for the study of protein folding, and enables NMR measurements to be performed on a single sample. While the impact of pressure on the viscosity of water is well known, and our water diffusivity measurements agree closely with theoretical expectations, we find that elevated pressures increase the Rh of dioxane and other small molecules by amounts that correlate with their hydrophobicity, with parallel increases in rotational friction indicated by 13C longitudinal relaxation times. These data point to a tighter coupling with water for hydrophobic surfaces at elevated pressures. Translational diffusion measurement of the unfolded state of a pressure-sensitized ubiquitin mutant (VA2-ubiquitin) as a function of hydrostatic pressure or urea concentration shows that Rh values of both the folded and the unfolded states remain nearly invariant. At ca 23 Å, the Rh of the fully pressure-denatured state is essentially indistinguishable from the urea-denatured state, and close to the value expected for an idealized random coil of 76 residues. The intrinsically disordered protein (IDP) α-synuclein shows slight compaction at pressures above 2 kbar. Diffusion of unfolded ubiquitin and α-synuclein is significantly impacted by sample concentration, indicating that quantitative measurements need to be carried out under dilute conditions.