Pulsed Field Gradient NMR Measurements Research Articles

Ion and Solvent Dynamics in Gel Electrolytes Based on Ethylene Oxide Grafted Acrylate Polymers

Multinuclear pulsed field gradient NMR measurements and rheological viscosity measurements were performed on three series of polymer gel electrolytes. The gels were based on a lithium salt electrolyte swollen into a copolymer matrix comprising an acrylate backbone and ethylene oxide side chains. In each series the side chains differed in length and number, but the acrylate-to-ethylene oxide ratio was kept constant. It was found that the self-diffusion coefficient of the cations was much lower than that of the anions, and that it decreased rapidly when the side chains got longer. In contrast, the self-diffusion coefficient of the anions was found to be independent of chain length. In the gel electrolytes, the diffusion coefficients of the solvent molecules are relatively constant despite an increased viscosity with increasing length of the side chains. However, in salt-free gels made for comparison, the diffusion coefficients of the solvent molecules decreased with increasing length of the side chains, which is consistent with an increased viscosity.

On the diffusion of water in silicalite-1: MD simulations using ab initio fitted potential and PFG NMR measurements

Molecular dynamics simulations of water diffusion in silicalite-1 are reported. The simulations are carried out using an ab initio fitted silicalite-1–water potential based on quantum chemical calculations. In addition, preliminary results of pulsed field gradient (PFG) NMR diffusion measurements of water and small alkane molecules in silicalite-1 samples are presented. Pre-adsorption of water in silicalite-1 samples was found to change the intra-crystalline diffusivities of small alkane molecules in silicalite-1. This is interpreted as an indirect evidence that under our experimental conditions water molecules occupy a significant part of the silicalite-1 channel system. The preliminary results of the PFG NMR diffusion measurements of water in silicalite-1 samples are discussed in terms of the contributions of extra- and intra-crystalline water to the measured signals. An-order-of magnitude agreement between the measured and the simulated intra-crystalline diffusivities of water in silicalite-1 is obtained.

Diffusion in thin films on the surface of a porous solid

A water-wet mono-dispersed glass bead system is saturated with two phases, a wetting phase of water and a non-wetting phase of tetrachloroethylene (no 1H signal). Pulsed field gradient NMR measurements of the one-dimensional probability density distribution P Δ (X) for the diffusive displacements of water molecules in times, Δ, are presented for the whole accessible water saturation range. At lower water contents the distributions show a distinctive shape, which is attributed to the distribution of the aqueous phase in thin surface wetting films connecting pendular rings where the beads are in contact. The data are reproduced well by a computer simulation of a random walk model based on diffusion of molecules within such a structure, allowing determination of the surface film thickness.

Determination of ionic self-diffusion coefficients of lithium electrolytes using the pulsed field gradient NMR

Abstract Pulsed field gradient NMR measurements were carried out to determine the self-diffusion coefficients of the Li ion (DLi) and anion (DF) by 7 Li and 19 F species. The DF of LiCF3SO3 in propylene carbonate (PC) solvent increased with the decrease of molar concentration and larger than DLi in whole concentration region. The DLi also increased with the concentration decrease and then saturated in the lower concentration region. The effective ionic radius of lithium estimated from the DLi was about four times larger than that of naked Li ion. This indicates the solvation effect of PC on dissociated Li ions. In order to enhance the Li ion transport number of electrolytes, two types of acidic materials, trimethoxyborane ((CH3O)3B) and trimethyleneborate ((CH2)3(BO3C3H6)2) were added into PC solution of LiCF3SO3 changing the mixing ratio of the salt to the acid. The DF decreased with the increase of (CH3O)3B content in spite of the decrease of the solution viscosity. The addition of (CH2)3(BO3C3H6)2, on the other hand, induced decrease of both DLi and DF, mainly due to the increase of the solution viscosity. These results suggest that (CH3)3BO3 is effective to increase the transport number due to the acidic trapping effect on anions.

Polymer self-diffusion and surfactant binding in aqueous solutions of unmodified and hydrophobically modified polyacrylamide studied by pulsed field gradient NMR and surface tension measurements

Polymer and surfactant self-diffusion have been measured by pulsed field gradient NMR in aqueous solutions of hydrophobically modified polyacrylamide (HM-PAM) and an unmodified polyacrylamide (PAM) analogue in order to investigate the binding of surfactant to polymer. The binding has also been calculated from surface tension measurements. The binding of sodium dodecyl sulfate (SDS) to the modified polymer in semidilute solutions displays a non-cooperative binding mechanism, while no such binding is detected for the PAM reference. The polymer self-diffusion decreases with increasing polymer concentration. At a surfactant concentration of ca. 7 mmolal (7 mm) a minimum for the polymer self-diffusion is observed. Previous rheological and present self-diffusion and surface tension results indicate a value of SDS addition for maximum effects on solution viscosity and polymer self-diffusion of approximately 7 mm.

Polymer self-diffusion and surfactant binding in aqueous solutions of unmodified and hydrophobically modified polyacrylamide studied by pulsed field gradient NMR and surface tension measurements

Polymer and surfactant self-diffusion have been measured by pulsed field gradient NMR in aqueous solutions of hydrophobically modified polyacrylamide (HM-PAM) and an unmodified polyacrylamide (PAM) analogue in order to investigate the binding of surfactant to polymer. The binding has also been calculated from surface tension measurements. The binding of sodium dodecyl sulfate (SDS) to the modified polymer in semidilute solutions displays a non-cooperative binding mechanism, while no such binding is detected for the PAM reference. The polymer self-diffusion decreases with increasing polymer concentration. At a surfactant concentration of ca. 7 mmolal (7 mm) a minimum for the polymer self-diffusion is observed. Previous rheological and present self-diffusion and surface tension results indicate a value of SDS addition for maximum effects on solution viscosity and polymer self-diffusion of approximately 7 mm.

Analysis of the (Trimethylsilyl)propionic Acid−β(12−28) Peptide Binding Equilibrium with NMR Spectroscopy

The binding of a small molecule, (trimethylsilyl)propionic acid (TSP), to a 17-residue peptide, β(12-28), is examined using (1)H NMR spectroscopy. β(12-28) (VHHQKLVFFAEDVGSNK) is a central fragment of the 40-42-residue Alzheimer's-associated Aβ peptide. This peptide has been previously shown to form soluble aggregates in low-pH aqueous solution. The TSP resonance is broadened appreciably in solutions containing relatively high concentrations (∼2 mM) of the peptide. The changes in TSP line width measured by titration of a peptide solution with TSP indicate a 1:1 binding stoichiometry. If the concentrations of both the peptide and TSP are reduced by 1 order of magnitude, the resonances of both species are sharp, suggesting that TSP binds predominately to the aggregated peptide. Nuclear Overhauser effect experiments indicate that the TSP interacts predominately with the side chains of the aliphatic peptide residues Leu(17) and Val(18). Pulsed-field gradient NMR measurements of TSP and peptide diffusion coefficients provide a more quantitative picture of the TSP-peptide binding equilibrium. The measured diffusion coefficients were used to calculate the fractions of the free and bound TSP. These results substantiate the conclusion that the stoichiometry of the TSP-peptide binding equilibrium is essentially 1:1 and further indicate anticooperative behavior in solutions containing an excess of TSP resulting in a dissociation of the peptide aggregates.

Diffusion Dynamics of Hairy-Rod Polymers in Concentrated Solutions

Photon correlation spectroscopy was employed in the polarized geometry in order to investigate the dynamics of solutions of substituted poly(p-phenylenes) at high concentrations. The total concentration fluctuations were found to exhibit a two-step decay. The fast dominant relaxation was the classical cooperative diffusion, controlled by the osmotic pressure of the system, exhibiting a stronger concentration dependence compared to flexible polymers, and discussed in the framework of the wormlike model. On the other hand, the second diffusive process was not related to self-diffusion (as confirmed by independent pulsed field gradient NMR measurements) and could not be accounted for by the existing theories. The self-friction coefficient was found to exhibit a stronger concentration dependence than the cooperative friction coefficient. At high concentrations, a diffusive cluster process was detected, despite the high solubility of these polymers in different solvents. The characteristics of this mode depend...

Atomistic simulation of hydrocarbon diffusion in silicalite

Molecular dynamics simulation using a burchart-DREIDING force field has been employed to determine the self-diffusion coefficients of methane, ethane, propane, n-butane, isobutane, benzene, and pyridine in silicalite at 300 K. In the case of the alkanes, Einstein diffusion was observed early in a 500-ps simulation. Values of the diffusion coefficient for the alkanes ranged from about 9 × 10 −4 cm 2 s −1 for methane to 2 × 10 −6 cm 2 s −1 for isobutane at a loading of 0.5 molecule per unit cell (mpuc) or 2 molecules per simulation box. For the alkanes, there is good agreement between the results of simulation and experimental values of the self-diffusion coefficient obtained by microscopic techniques such as pulsed-field gradient NMR measurements. In the case of benzene and pyridine, anomalous diffusion was observed for time scales up to 9 ns and apparent diffusion coefficients (range of 10 −11 to 10 −8 cm 2 s −1) calculated form the Einstein relation increased with increased loading (up to 3.75 mpuc) and, as shown for pyridine, with increasing temperature. In the case of benzene where experimental data was available, the apparent diffusion coefficients calculated at high loadings were within an order of magnitude of experimental values. Heats of adsorption were calculated by a canonical Monte Carlo method. For the alkanes (methane, ethane, propane, and n-butane), the magnitude of the heats of adsorption linearly increased with the number of alkane carbons in qualitative and quantitative agreement with experimental data. Heats of adsorption calculated for isobutane and benzene also agreed well with experimental values. No experimental values were available for pyridine for comparison.

Ionic interactions and transport in a low-molecular-weight model polymer electrolyte

AC impedance, FT-Raman and pulsed field gradient (pfg) NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the νS(SO3) and δS(CF3) vibrational band envelopes of the CF3SO3 anion, respectively, the relative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation of associated ionic species into spectroscopically “free” anions with increasing salt concentration in dilute solutions. The relative abundance of associated ionic species increases with increasing temperature. Pfg-NMR measurements show that D−(19F) and D+(7Li) are very similar for all concentrations (i.e., O:Li⩾53:1) and temperatures (25–80 °C) investigated. Most notably, the diffusivity of the oligomer solvent, D(1H), is significantly faster than the self-diffusion coefficients of the dissolved ions in all cases. Predicted values for the ionic conductivity were obtained from the NMR diffusivities, using the Nernst–Einstein relation, and compared with those from direct measurement. We find that the calculated values are higher for all concentrations; the discrepancy increases with decreasing salt concentration and increasing temperature. A good correlation is found between the concentration dependence of the ionic redissociation pattern, as determined from the νS(SO3) Raman band envelope, and an increase in equivalent ionic conductivity with increasing salt concentration in dilute solutions (i.e., O:Li⩾110:1). We suggest that fluctuating, salt-rich heterogeneities of dissolved ions and polymer segments form at low salt concentrations, and that this may be a general behavior of dilute polymer–salt complexes.

A Raman, ac impedance and pfg-NMR investigation of poly(ethylene oxide) dimethyl ether (400) complexed with LiCF 3SO 3

ac impedance, Raman spectroscopy and pulsed field gradient NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether of molecular weight 400 complexed with LiCF 3SO 3 as a function of salt concentration; the ether oxygen to alkali metal cation ratios (O:M) ranging from 53:1 to 563:1. From an analysis of the ν S (SO 3) band envelope of the triflate anion, the relative concentrations of anions in various chemical environments have been calculated. A slight redissociation effect is observed with increasing salt concentration which correlates well with an observed increase in the molar conductivity. Pulsed field gradient NMR measurements show that the 1H, 7Li and 19F self-diffusion coefficients all decrease monotonically with increasing salt concentration. Experimental conductivity data is compared to values calculated from the diffusion coefficients using the Nernst–Einstein relation. We find that the calculated values are higher for all concentrations; the discrepancy increases with decreasing salt concentration.

NMR measurements of ionic mobility in model polymer electrolyte solutions

19F and 7Li pulsed field gradient (PFG) NMR measurements have been undertaken to determine anion and cation diffusion coefficients of tetraethylene glycol dimethyl ether/LiCF 3SO 3 and N,N dimethyl formamide/LiCF 3SO 3 solutions over a range of temperatures and concentration. The diffusion of the solvent molecules has also been determined by 1H PFG NMR measurements. In addition, viscosity and ionic conductivity measurements have been undertaken. The ionic diffusion and ionic conductivity data are discussed in terms of the Nernst-Einstein equation and the relationship between ionic diffusion and viscosity are considered in terms of the Stokes-Einstein equation. Finally, NMR T 1 data are considered in terms of diffusion and rotation of solvent molecules.

Dynamics of Hairy-Rod Polymers: Semidilute Regime

Photon correlation spectroscopy was employed in both the polarized (VV) and depolarized (VH) geometries in order to investigate the dynamics of concentration and orientation fluctuations of model hairy-rod poly(p-phenylenes) in semidilute solution. These materials possess a large inherent optical anisotropy, which allowed the unambiguous detection of orientation relaxation. By probing the dynamics in two solvents of different qualities, chloroform, a good solvent, and toluene, it was possible to distinguish nearly molecular from aggregate processes. Two modes were detected in the VV correlation function: the fast cooperative diffusion, Dc, which increased nearly linearly with concentration (Dc/D0 − 1 ∼ c0.9), and a slow mode, Dslow, which slowed with concentration (Dslow ∼ c-1). The latter process was assigned to the self-diffusion of small aggregates, as confirmed by independent pulsed field gradient NMR measurements. Broad VH correlation functions were obtained at high concentrations and revealed the presence of a dominant mode exhibiting a decay rate, ΓVH, which decreased with increasing q and concentration (ΓVH ∼ c-0.25). This mode was assigned to orientational correlations in space, due to some local ordering of parts of the molecules. These findings cannot be accounted for by the existing theories.

Evaluation of Transport Properties of Packed Beds of Microparticulate Porous and Nonporous Silica Beads by Means of Pulsed Field Gradient NMR Spectroscopy

The pulsed field gradient (PFG) NMR method is applied to study molecular diffusion in beds of spherical silica particles. From the observed transport properties it may be concluded that the internal pore system of the silica particles gives rise to two different modes of molecular migration: restricted motion with molecular mean square displacements of the order of 700 nm, and unrestricted motion with effective diffusivities decreasing with increasing loading over nearly 2 orders of magnitude. In PFG NMR measurements with nonporous silica, the molecular mobility in the space between silica particles and the topology of the space between these particles were investigated. In these experiments a slowly diffusing macromolecule, poly(dimethylsiloxane) (PDMS), is shown to be a most effective probe molecule for dynamic imaging.

Pulsed field gradient NMR measurements of probability distribution of displacement under flow in sphere packings

In this work we used the PFG NMR displacement technique to investigate the transport of water particles under flow through a model porous media. Using different sizes of sphere for monodisperse glass sphere packings, we measured the probability distribution of displacement for mean displacements ranging from 0.08 to 7.3 times the characteristic length of the porous media. We observed, therefore, the transition between the velocity probability distribution and the Gaussian-shaped distribution of classical dispersion.

Measuring protein self-association using pulsed-field-gradient NMR spectroscopy: Application to myosin light chain 2

At the millimolar concentrations required for structural studies, NMR spectra of the calcium-binding protein myosin light chain 2 (MLC2) showed resonance line widths indicative of extensive self-association. Pulsed-field-gradient (PFG) NMR spectroscopy was used to examine whether MLC2 aggregation could be prevented by the zwitterionic bile salt derivative 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). PFG NMR measurements indicated that CHAPS was capable of preventing MLC2 self-association, but only at concentrations well above the critical micelle concentration of approximately 7.5 mM. CHAPS was most effective at a concentration of 22.5 mM, where the apparent molecular mass of MLC2 corresponded to a protein monomer plus seven molecules of bound detergent. The resolution and sensitivity of 2D 15N-1H HSQC spectra of MLC2 were markedly improved by the addition of 25 mM CHAPS, consistent with a reduction in aggregation following addition of the detergent. The average amide nitrogen T2 value for MLC2 increased from approximately 30 ms in the absence of CHAPS to approximately 56 ms in the presence of 25 mM CHAPS. The results of this study lead us to propose that PFG NMR spectroscopy can be used as a facile alternative to conventional techniques such as analytical ultracentrifugation for examining the self-association of biological macromolecules.

Ion transport in gel electrolytes

Gel electrolyte films were prepared by a redox initiated copolymerization of oligo(ethylene glycol) 23 dimethacrylate and acrylonitrile in the presence of oligo(ethylene glycol) 11 dimethyl ether as a plasticizer and LiCF 3SO 3. The influence of the polar comonomer acrylonitrile on the conductivity and the cationic transference number was investigated. The materials show conductivities between 10 −4 and 10 −5 S cm and transference numbers in the range of 0.5 at room temperature. Pulsed field gradient-NMR (PFG-NMR) measurements were carried out to determine the diffusion coefficients of the plasticizer by 1H-NMR and of LiCF 3SO 3 by 19F-NMR.

Effectiveness of window blocking in zeolite NaY by strongly coadsorbed molecules

The effectiveness of window blocking in reducing the diffusion of a sorbate inside a three dimensionally connected zeolite pore system is investigated using pulsed field gradient (PFG) NMR. 1H PFG NMR measurements were used to study the diffusion of methane in two binary sorbate systems at varying cosorbate loading: (i) methane and benzene coadsorbed in NaY, and (ii) methane and ethylene coadsorbed in NaY. In both cases (i.e. benzene and ethylene), D blocking molecule D methane was less than ∼3 × 10 −2, and we are able to test percolation theory predictions. These measurements were compared with previously determined variable loading single-component methane self-diffusion coefficients in NaX (Kärger et al., 1980, J. Chem. Soc. Faraday Trans. I 76, 717–737). Large crystals of NaY (∼30 μm) were essential for this study to ensure the accuracy of the diffusion measurements. A constant base methane loading of ∼2 molecules per NaY supercage was employed in all the samples used in this study. The methane self-diffusion coefficient in the presence of coadsorbed benzene and ethylene was reduced in a manner qualitatively consistent with concepts from percolation theory applied to a diamond lattice (representing the connectivity of the NaY supercages). Coadsorbed benzene though was more effective in “blocking” the methane diffusion. Excellent agreement was found between the measured methane self-diffusion coefficients in the presence of benzene and the prediction based on the effective medium approximation (EMA) to the percolation theory. Suggestions are made about the prospects of applying other models to blocking by more mobile cosorbates like ethylene.

Unusual Sintering Behavior of Porous Chromatographic Zirconia Produced by Polymerization-Induced Colloid Aggregation

The effects of sintering temperature and duration on the pore structure of chromatographic zirconia particles produced by the controlled polymerization-induced aggregation of 1,000 {angstrom} colloids are studied with an eye toward optimally strengthening the aggregates and eliminating small pores while preserving large pores. Nitrogen adsorption and mercury porosimetry are used to estimate the surface area, pore volume, and pore size distribution. Pulsed field gradient NMR measurements of solvent diffusion are used to estimate the diffusion tortuosity of the pore space. Initially of course, the pore volume and surface area decrease significantly, the decrease being more pronounced at higher temperatures. With prolonged sintering, the pore size, pore volume, and surface area change much more slowly, but the diffusion tortuosity seems to be minimized at a sintering temperature and time at which pores are allowed to redistribute so as to optimize large pores. The aggregates synthesized by this aggregation method apparently produce metastable large pores which are not easily collapsed.

Pore size distributions, pore coupling, and transverse relaxation spectra of porous rocks

The properties of porous rocks make NMR measurements difficult to interpret. Broad distributions of pore sizes give rise to magnetization decays characterized by broad distributions of relaxation times. The pore space, which is well coupled with respect to steady state fluid flow and low frequency electrical conductivity, appears disjoint on time scales probed by NMR. Pulsed field gradient NMR measurements show that pores are elongated and that relaxation times measure the minimum dimension. Diffusion in magnetic field gradients affects the transverse relaxation spectrum in complicated ways, but in the surface dominated regime there is a simple relationship between T 1 and T 2.