Pulsed Gradient Spin-echo NMR Research Articles

A self-diffusion study in aqueous solution and lyotropic mesophases of amphiphilic block copolymers

Water-soluble poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PPO) triblock copolymers are high-molecular-weight nonionic copolymers and form micellar solutions and liquid-crystalline mesophases in water. We studied the temperature dependence of polymer and water self-diffusion in solutions and lyotropic mesophases of the PEO13 PPO30 PEO13/water and PEO21 PPO47 PEO21/water binary systems. The self-diffusion measurements were performed by means of the pulsed field gradient spin-echo NMR method. The analysis of the water mobility was realised using “the obstruction factor” and “the two-site model”, which consider the reduction of the water self-diffusion due to the microstructure of the lyotropic aggregates and to the presence of one part of the solvent bound to the polymer aggregate surfaces. We calculated the water obstruction factors and the hydration numbers as a function both of the polymer composition and of the temperature. The results are compared with the data obtained in mesophases formed by classical surfactants.

NMR microscopy of the uptake, distribution and mobility of dissolution media in small, sub-millimetre drug delivery systems

NMR techniques were used to study the drug release process from small (sub-mm) lipophilic matrix theophylline beads. NMR microscopy was used to follow the ingress of the dissolution medium into the beads. Pulsed gradient spin echo (PGSE) NMR and 3D NMR imaging were used to measure the mobility and distribution of liquid within fully liquid penetrated beads. These techniques were used to rationalise the increase in the drug release rate with increasing proportion of GMS (glycomonosaccharide):paraffin in the matrix composition. A well-defined penetrant front was seen to move towards the centre of the bead during the dissolution process and the rate of liquid uptake showed the same trend with increasing GMS content as the rate of drug release. The total concentration of absorbed liquid increased and its T2 relaxation time decreased with increasing GMS content of the bead matrix. This combined with the interpretation of PGSE results suggested that liquid resides in the matrix material as well as in pores left by the dissolved drug, and that this tendency increases with increasing GMS content. Heterogeneities in liquid distribution within the beads were quantified using percolation analysis and were related to the lipid matrix composition and may be a contributory factor in determining the rate of drug release.

Self-diffusion coefficients of lithium, anion, polymer, and solvent in polymer gel electrolytes measured using 7Li, 19F, and 1H pulsed-gradient spin-echo NMR

Four series of solution and corresponding polymer gel electrolyte systems were studied. The systems were composed of a solvent (γ-butyrolactone (GBL) or propylene carbonate (PC)), (three different concentrations of) a lithium salt (LiBF 4 or lithium bis(trifluromethanesulfonyl)-imide (LiTFSI, LiN(SO 2CF 3) 2)), and poly(ethylene glycol diacrylate) of molecular weight 4000 (PEO), which was cross-linked to form the polymer gels. The self-diffusion coefficients of the lithium, the anions, the solvents and the polymer were obtained independently using 7Li, 19F and 1H pulsed-gradient spin-echo NMR measurements. From the individual diffusion coefficients of each component of the GBL–LiBF 4, GBL–LiTFSI, PC–LiBF 4, and PC–LiTFSI solutions and the corresponding polymer gel electrolytes, the solvation and the solvent-dependent behaviour of the lithium and the anions are clearly shown. Especially in the gels, the lithium binding with the PEO matrix were found to be quite different depending on whether the solvent was PC or GBL.

Taylor dispersion and molecular displacements in Poiseuille flow.

We have used pulsed gradient spin echo (PGSE) NMR to measure longitudinal displacements of octane molecules undergoing Poiseuille flow in a 150 microm diameter pipe, accessing time scales which approach the Taylor dispersion limit. We monitor the change in displacement distribution which occurs as molecules undergoing Brownian motion sample an increasing proportion of the ensemble of streamlines, observing the effects of wall collisions and the gradual transition of the propagator from Poiseuille to Taylor-Aris behavior. The further use of a double PGSE sequence allows the direct measurement of the stochastic part of the motion alone.

Study of the self‐diffusion of poly(ethylene glycol)s in poly(vinyl alcohol) aqueous systems

We have measured the self-diffusion coefficients of a series of oligo- and poly(ethylene glycol)s with molecular weights ranging from 150 to 10,000, in aqueous solutions and gels of poly(vinyl alcohol) (PVA), using the pulsed-gradient spin-echo NMR techniques. The PVA concentrations varied from 0 to 0.38 g/mL which ranged from dilute solutions to polymer gels. Effects of the diffusant size and polymer concentration on the self-diffusion coefficients have been investigated. The temperature dependence of the self-diffusion coefficients has also been studied for poly(ethylene glycol)s with molecular weights of 600 and 2,000. Several theoretical models based on different physical concepts are used to fit the experimental data. The suitability of these models in the interpretation of the self-diffusion data is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2396–2403, 1999

Study of the self-diffusion of poly(ethylene glycol)s in poly(vinyl alcohol) aqueous systems

We have measured the self-diffusion coefficients of a series of oligo- and poly(ethylene glycol)s with molecular weights ranging from 150 to 10,000, in aqueous solutions and gels of poly(vinyl alcohol) (PVA), using the pulsed-gradient spin-echo NMR techniques. The PVA concentrations varied from 0 to 0.38 g/mL which ranged from dilute solutions to polymer gels. Effects of the diffusant size and polymer concentration on the self-diffusion coefficients have been investigated. The temperature dependence of the self-diffusion coefficients has also been studied for poly(ethylene glycol)s with molecular weights of 600 and 2,000. Several theoretical models based on different physical concepts are used to fit the experimental data. The suitability of these models in the interpretation of the self-diffusion data is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2396–2403, 1999

Strategies for Diagnosing and Alleviating Artifactual Attenuation Associated with Large Gradient Pulses in PGSE NMR Diffusion Measurements

The generation of phase-based artifacts resulting from mismatch in the effective areas (i.e., the time integrals) of sequential gradient pulses is discussed in the context of large gradient pulsed-gradient spin-echo (PGSE) NMR diffusion measurements. Such effects result in artifactual attenuation and distortion in the spectra which, in the first instance, are similar to (and commonly mistaken for) the effects of eddy currents. Small degrees of mismatch cause “unphysical” concave downward curvature in PGSE attenuation plots of freely diffusing species. However, larger mismatches can result in artifactual diffraction peaks in the plots which could easily be confused for true restricted diffusion effects. Although “rectangular” gradient pulses are preferable from a theoretical viewpoint, we found that shaped gradient (e.g., half-sine) pulses, which due to their slower rise and fall times were more tractable for the current amplifier, were more sequentially reproducible. As well as generating fewer phase-based artifacts such shaped pulses also decrease the likelihood of vibration problems.

Self-Diffusion of End-Capped Oligo(ethylene glycol)s in Poly(vinyl alcohol) Aqueous Solutions and Gels

We have studied the self-diffusion of a series of end-capped ethylene glycol and oligo(ethylene glycol)s in poly(vinyl alcohol) aqueous solutions and gels by pulsed-gradient spin echo NMR spectroscopy. The end groups of the diffusants include small flexible groups (methyl, ethyl, hexyl) and bulky rigid groups (tert-butyl and aromatic groups). The effect of the size and geometry of the end groups on the self-diffusion coefficients of the derivatives of ethylene glycol and of oligo(ethylene glycol)s is investigated. The diffusion data are analyzed with several physical models of diffusion based on different physical concepts to test their applicabilities. The variation of the parameters used in these diffusion models with the size and geometry of the diffusants is also discussed.

Diffusion in binary liquid n-alkane and alkane-polyethylene blends

We have used the pulsed-gradient spin-echo proton nuclear magnetic resonance (NMR) method to measure two-component self-diffusion D in melts of two monodisperse polyethylenes, as well as in binary blends of n-dodecane and n-hexacontane, and of n-alkanes with the polyethylene (PE) specimens, over the full concentration range at various temperatures. In the n-alkane blends we observe ideal solution behavior at all concentrations, while in the alkane-PE blends the D ratio increases with rising PE content and molecular weight. This behavior is reproduced by standard free-volume theory including chain-end effects and measured variation of density with molecular weight and temperature. In PE melts and blends the theory must include the gradual transition from Rouse to reptative motion above an entanglement molecular weight which depends on PE concentration. Constraint release effects are included, and dilute solution behavior is modeled near infinite PE dilution. All free-volume parameters are taken from our earlier study of n-alkane melts; the extensions of that theory introduce only one additional adjustable parameter.

Diffusion of water and ethanol in ion-exchange membranes: limits of the geometric approach

Diffusion coefficients of water as a solvent and ethanol as a tracer in sulphonated polyethylene (SPE) were measured by the pulsed-gradient spin-echo NMR method. Comparison with the data available for a number of ionic and non-ionic polymers and solvents shows that, except for Nafion, all the materials exhibit a fairly universal dependence of diffusivity on the volume fraction of the solvent. This result is explained on the basis of the geometric obstruction approach and general conditions are formulated for the observed universal dependence. Explanations are proposed for the peculiar behaviour of Nafion and for the low diffusivity of ethanol within the same framework.

Self-Diffusion Studies of Water and Poly(ethylene glycol) in Solutions and Gels of Selected Hydrophilic Polymers

To test the effect of the matrix polymer on diffusion, we have measured the self-diffusion coefficients of water and poly(ethylene glycol) of a molecular weight of 600 (PEG-600) in aqueous systems of selected polymers using the pulsed-gradient spin-echo NMR technique. The polymers used in this study include poly(vinyl alcohol) (PVA), hydroxypropyl methyl cellulose (HPMC), poly(N,N-diethylacrylamide) (PNNDEA), and poly(N-isopropylacrylamide) (PNIPA). The polymer concentrations varied from 0 to 0.38 g/mL. The effect of the polymer network on the self-diffusion coefficients of the solvent (water) and a solute (PEG-600) was investigated by analyzing the diffusion data with the use of the free volume model of Yasuda et al. [Yasuda, H.; Lamaze, C. E.; Ikenberry, L. D. Makromol. Chem. 1968, 118, 19], the diffusion model proposed by Phillies [Phillies, G. D. J. Macromolecules 1986, 19, 2367], and the model of Petit et al. [Petit, J.-M.; Roux, B.; Zhu, X. X.; Macdonald, P. M. Macromolecules 1996, 29, 6031]. The re...

Pulsed gradient spin echo nuclear magnetic resonance measurements of hydrodynamic instabilities with coherent structure: Taylor vortices

Pulsed gradient spin echo (PGSE) nuclear magnetic resonance (NMR) is applied to the characterization of hydrodynamic instabilities. It is demonstrated theoretically and experimentally that for Taylor vortex flow in a Couette cell the PGSE NMR data is coherently modulated in an interference pattern dependent upon the vortex size, or wavelength, and velocity intensity. Spatially resolved NMR velocity images of all three velocity components for water in supercritical Taylor number flow and NMR velocity image data of the axial disturbance velocity for pentane at three supercritical values of the Taylor number are presented. For the short column used the NMR velocity data clearly show axial asymmetry with maximum velocities and the center (eye) of the vortex drawn toward the radial outflow boundary.

An ESR and PGSE-NMR evaluation of the molecular accessibility of poly(vinyl alcohol) hydrogels

Electron spin resonance spectroscopy (ESR) and pulsed-gradient-spin-echo nuclear magnetic resonance (PGSE-NMR) measurements on poly(vinyl alcohol) (PVA) hydrogels reveal that nanostructure is not appreciably affected by the number of freezing–thawing cycles.

Determination of the diffusion coefficient of 1-(2′-pyridylazo)-2-naphthol in ethanol–water solutions using flow injection and nuclear magnetic resonance techniques

The values of the diffusion coefficient of PAN (1-(2′-pyridylazo)-2-naphthol) in various ethanol–water solutions at 298 K were determined using both flow injection (FI) and nuclear magnetic resonance (NMR) techniques. The transient FI concentration curves monitored by the detector were described by the axially dispersed plug flow model with an axial dispersion coefficient obeying Taylor's theory. The diffusion coefficient of PAN determined in 40–100% (v/v) ethanol solutions in water by both curve-fitting and the statistical moments method varied in the range from 2.43×10 −10 to 8.31×10 −10 m 2 s −1. As expected, the diffusion coefficient values obtained by curve-fitting were found to be more reliable. A pulsed field gradient (PFG) spin echo NMR experiment was also used to measure the diffusion coefficient of PAN in the same solutions. The NMR results were found to follow the same trend as the FI results though they were from 5% to 16% lower in value. This deviation was attributed to the association effects facilitated by the experimental conditions under which the NMR measurements were taking place, i.e. in quiet solutions and at considerably higher concentrations than those used by the FI technique. The diffusion coefficient of PAN in pure water was calculated as 2.21×10 −10 m 2 s −1 by extrapolating the FI results to zero ethanol concentration. The results reported in the present investigation can be used for studying the electrochemical properties of PAN in ethanol–water solutions as well as for elucidating the sensing mechanism of an optode based on immobilisation of PAN into Nafion membranes.

A pulsed field gradient NMR study of poly(oxyethylene) diffusion in aqueous solutions and gels of ethyl (hydroxyethyl) cellulose–sodium dodecyl sulphate systems

The tracer diffusion of poly(oxyethylene) (POE) with molecular weights ( M) of 120 000 and 963 000, respectively, in an aqueous polymer matrix composed of ethyl (hydroxyethyl) cellulose (EHEC) and the anionic surfactant sodium dodecyl sulphate (SDS) has been studied using pulsed field gradient spin-echo (PGSE) NMR. The tracer diffusion of POE has been studied in both the sol and gel states, which appear at elevated temperatures, for a number of matrix concentrations in the range 1–4 wt.%. The SDS concentration was kept constant at 4 mmolal. Time-resolved PGSE experiments show that in general the tracer diffusion of POE chains with M=120 000 is Fickian, except at the highest matrix concentration in the gel state, where non-Fickian diffusion is detected. For the POE tracer with M=963 000 the diffusion in the sol state gradually departs from Fickian diffusion and becomes anomalous as the matrix concentration increases. In the gel state, anomalous diffusion is observed at matrix concentrations above 1 wt.%. Obstruction effects are determined for the two different tracer chains in the sol state as a function of the matrix concentration. At 1 wt.% EHEC, obstruction effects increase with tracer molecular weight. At higher matrix concentrations, these effects are found to be nearly identical.

Study of Self-Diffusion of Molecules in Polymer Gel by Pulsed-Gradient Spin-Echo 1H NMR. 3. Stearyl Ιtaconamide/N,N-Dimethylacrylamide Copolymer Gels

Intermolecular hydrophobic interactions between poly(ethylene glycol) oleyl ether (PEG-Ole) as probe polymer and network polymer in stearyl itaconamide (SIA)/N,N-dimethylacrylamide (DMAA) copolymer gel swollen with water, in which the micelle of SIA is immobilized by copolymerization, have been studied through the diffusion coefficient (D) determined by the pulsed-gradient spin-echo 1H NMR method with the field gradient strength of 22.3 T m-1 and 1H spin−lattice relaxation time (T1) and 1H spin−spin relaxation time (T2) by the 1H pulse NMR method. From these experimental results, it was found that in the diffusion process PEG-Ole in SIA/DMAA copolymer gel takes the mobile and immobile states, and 1H T2 of PEG-Ole in SIA/DMAA copolymer gel is shorter than that of PEG. This shows that the comicellization of PEG-Ole and SIA is induced by hydrophobic interaction between the oleyl group in PEG-Ole and the stearyl group in SIA. Furthermore, to elucidate kinetics of the incorporation of PEG-Ole into the micelle ...

Applications of in situ magnetic resonance techniques in chemical reaction engineering

NMR spectroscopy is now a well‐established technique for the in situ study of surface chemistry and the chemical processes occurring during catalytic reactions. Developments in probe design are making the sample environments ever closer to the operating conditions of the catalyst in industrial use. In parallel with these advances there is an increasing interest in the application of field gradient magnetic resonance techniques, namely pulsed gradient spin echo (PGSE) NMR and magnetic resonance imaging (MRI), to in situ studies of mass transport processes in catalysts and reactors. An overview of the recent developments in in situ NMR spectroscopy, PGSE NMR and MRI studies in application to catalysis and reaction engineering is presented and the potential of these techniques in the numerical modelling of catalytic processes and reactor design is highlighted.

A pulsed gradient spin echo NMR study of guest encapsulation by hydrogen-bonded tetraurea calix[4]arene dimers

Diffusion coefficients, as obtained by the PGSE NMR technique, were used to probe encapsulation of guests in self-assembled tetraurea calix[4]arene dimers in organic solvents and to demonstrate their destruction by DMSO.

Characterization of the solution properties of Pichia farinosa killer toxin using PGSE NMR diffusion measurements.

The solution behaviour with respect to pH and NaCl concentration of the tertiary structure and propensity for aggregation of salt- mediated killer toxin (SMKT) from Pichia farinosa was examined using pulsed-gradient spin-echo NMR diffusion measurements. It was found that in 0.15m NaCl the tertiary structure of SMKT was constant below pH 5.0, with the native SMKT existing as an unaggregated heterodimer containing the beta-subunit in a compactly folded form. However, above pH 5.0 the beta-subunit dissociated and lost its compact structure, becoming a random coil with an approximately 37% increase in effective hydrodynamic radius. To determine the effects of NaCl concentration on the tertiary structure of SMKT, diffusion measurements were performed at pH 3.5 and NaCl concentrations up to 2M. Both the tertiary structure and aggregation state of SMKT were found to be insensitive to the salt concentration which indicates that the activity of the toxin is not a direct result of salt-protein interactions.

High Magnetic Field Gradient PGSE NMR in the Presence of a Large Polarizing Field

A description is given of pulsed gradient spin echo (PGSE) NMR experiments in which large pulsed magnetic field gradients may be required. The design contraints are discussed and, in particular, the problem of the use of large pulsed magnetic field gradients in conjunction with large polarizing fields is considered. Issues addressed concern probe mechanical assembly, current supply requirements, and pulse shape design. We describe a quadrupolar coil with a gradient amplitude of 1.65 T m−1A−1which has been used successfully up to a maximum gradient of around 40 T m−1. A diffusion coefficent of 7.5 × 10−16m2s−1has been measured using this system, the lowest yet achieved by PGSE NMR methods.