Pulsed gradient spin-echo

NMR microimaging may be used to observe the effect of molecular diffusion in the vicinity of a thin wire subjected to current pulses. By this means the pulsed gradient spin echo technique can utilize very large pulsed magnetic field gradients, on the order of 100 Tm '. The quadratic dependence of gradient amplitude on distance from the wire leads to large dynamic range while the distribution of local gradient vectors makes it possible to image anisotropic diffusion. We demonstrate these properties in measurements on polymer solutions and liquid crystals.

Pulsed gradient spin echo (PGSE) measurements on first-year Antarctic sea ice have been performed using a portable Nuclear Magnetic Resonance probe that utilises the terrestrial magnetic field. This probe is a new device that incorporates independent transverse and longitudinal field gradient coils. A depth profile of the self-diffusion coefficient of liquid in brine inclusions has been measured through the 2002 season sea ice sheet in McMurdo Sound, near Cape Evans, Ross Island. The data is comprised of both horizontal and vertical brine diffusivity that have been measured in situ and with reduced invasive impact on the ice sheet, which constitutes a significant refinement of the Earth's field NMR technique. The observation that the depth dependence of the diffusion coefficient matched the expected magnitude with the temperature gradient through the ice sheet both confirmed the validity of our experiment and suggested that the sheet is best modelled as a matrix consisting of liquid inclusions in which the water molecules move in a Brownian manner.

The interaction of polyvinylpyrrolidone (PVP) with an anionic surfactant (sodium dodecyl sulfate, SDS), a nonionic surfactant (pentaethylene glycol monodecyl ether, C 10 E 5), and a zwitterionic surfactant (lauryl amido propyl betaine, LAPB) has been investigated by means of pulsed gradient spin-echo NMR (FT-PGSE NMR), allowing self-diffusion coefficients to be determined. The results confirm the strong interaction prevailing in the PVP/SDS system, whereas no association has been observed in the PVP/C 10 E 5 and PVP/LAPB systems. Mixing PVP with two surfactants, namely SDS and C 10 E 5 or SDS and LAPB, results in the formation of ternary aggregates between the polymer and the mixed micelles.

Direct oxidation fuel cell (DOFC) based on proton conducting membranes (PCM's) represents a promising energy technology, with the vast majority of efforts devoted to methanol as the fuel. The most commonly used PCM is Nafion. However, because of high production cost, high fuel crossover, and sensitivity to metal-ion impurities, alternative materials are being developed. Methanol presents several limitations including low boiling point (Â/65 8C), and reduced cell efficiency due to fuel crossover. We present here a measure of the mass transport represented by the self-diffusion coefficients (D) of methanol, and two alternative fuels */ethylene glycol (EG) and dimethyl oxalate (DMO) in new low-cost nanoporous (NP) PCM's. The NP-PCM's are based on commercial poly(vinylidene fluoride), with variable pore size determined by the addition of nanoscale SiO 2 or TiO 2 particles. Proton NMR pulsed gradient spin-echo (PGSE) self-diffusion measurements were conducted on aqueous solutions of 2 M fuel in 3 M H 2 SO 4 and also on five NP-PCM equilibrated in the aqueous solutions over the temperature range of 30 Á/90 8C. Here the fuel molecular mobility is indicated by the D values of the methyl and methylene peaks. Results indicate that both DMO and EG solutions have lower fuel molecular mobility than methanol. Results for the various NP-PCM also reveal reduced fuel permeability for DMO and EG compared with methanol.

The interaction of polyvinylpyrrolidone (PVP) with an anionic surfactant (sodium dodecyl sulfate, SDS), a nonionic surfactant (pentaethylene glycol monodecyl ether, C 10 E 5), and a zwitterionic surfactant (lauryl amido propyl betaine, LAPB) has been investigated by means of pulsed gradient spin-echo NMR (FT-PGSE NMR), allowing self-diffusion coefficients to be determined. The results confirm the strong interaction prevailing in the PVP/SDS system, whereas no association has been observed in the PVP/C 10 E 5 and PVP/LAPB systems. Mixing PVP with two surfactants, namely SDS and C 10 E 5 or SDS and LAPB, results in the formation of ternary aggregates between the polymer and the mixed micelles.

We derive the formalism to obtain spatial distributions of collisional correlation times for macroscopic particles undergoing granular flow from pulsed gradient spin echo nuclear magnetic resonance diffusion data. This is demonstrated with an example of axial motion in the shear flow regime of a 3D granular flow in a horizontal rotating cylinder at one rotation rate.

We describe an experimental approach that combines multidimensional NMR experiments with a steadily renewed source of laser-polarized 129 Xe. Using a continuous flow system to circulate the gas mixture, gas phase NMR signals of laser-polarized 129 Xe can be observed with an enhancement of three to four orders of magnitude compared to the equilibrium 129 Xe NMR signal. Due to the fact that the gas flow recovers the nonequilibrium 129 Xe nuclear spin polarization in 0.2 to 4 s, signal accumulation on the time scale of seconds is feasible, allowing previously inaccessible phase cycling and signal manipulation. Several possible applications of MRI of laser-polarized 129 Xe under continuous flow conditions are presented here. The spin density images of capillary tubes demonstrate the feasibility of imaging under continuous flow. Dynamic displacement profiles, measured by a pulsed gradient spin echo experiment, show entry flow properties of the gas passing through a constriction under laminar flow conditions. Further, dynamic displacement profiles of 129 Xe, flowing through polyurethane foams with different densities and pore sizes, are presented.

The global climate is predicted to change in the next century; for the Mediterranean Basin, an increase in air temperature more than 48C and a higher frequency of extreme climatic events such as drought and heat waves are expected. In this work, the response of Cistus salvifolius L. to the rise in winter temperature has been studied. Plants acclimated to winter conditions [outdoor (OUT)] were moved into a greenhouse [indoor (IND)] at higher temperature and eco-physiological behaviour was analysed on leaves after 15 days from plant transferring (IND 15d) and on leaves developed IND. IND leaves were characterized by reduced thickness, higher specific leaf area, higher CO 2 mesophyll conductance and photosynthetic rate, and lower respiratory rate than leaves grown OUT upon current winter conditions. In IND 15d leaves, no improvement of photochemical activity was found. When IND leaves were subjected to a rapid increase in air temperature, the CO 2 fixation was not limited indicating a high thermotolerance of photosynthetic machinery. The results for IND leaves indicate the occurrence of a strategy that merging changes in leaf structure as well as in photosynthetic regulation allow C. salvifolius to maintain an elevated carbon gain in response to temperature increase.

NMR pulse gradient spin echo is the most efficient method for non-invasive elucidation of molecular transport in heterogeneous media. With a proper interpretation of experimental data, the method can also be applied to investigate molecular self-diffusion in pores small enough that the characteristic diffusion times are much shorter than time, needed to build up the spin phase structure by the pulse of magnetic field gradient. This is demonstrated by the analysis of restricted self-diffusion measurement of water molecules trapped in a polyamide membrane. The results are presented as a distribution of spin-relaxation rates and pore sizes in this nanoporous system that also present the method in its true colors as a useful tool in the bio-nanotechology.

UV Ultraviolet v Vitesse (m.s-1) V Volume molaire de solution (L.mol-1) v Volume spécifique partiel (cm 3 .g-1) V e Volume d'élution V s Volume partiel spécifique de soluté (m 3 .kg-1) X,Y Groupements terminaux X et Y d'un polymère z Nombre de charges d'un ion ZQ Zéro quantum * Les signaux POE de l'homopolymère et du copolymère sont confondus sur le spectre proton quantitatif, l'erreur est donc calculée en comparant l'intégrale quantitative à la somme des deux signaux POE renormalisés.

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-protei...

The solution behavior of lysozyme was studied as a function of protein concentration, NaCl concentration, pH, and temperature using pulsed-gradient spin-echo NMR diffusion measurements. The lysozyme solutions clearly exhibited nonideal behavior which was sensitive to both the salt concentration and pH. Lysozyme has an isoelectric point of pH 11, and it is often overlooked that at normal pH it has a net positive charge. Since lysozyme is a charged species, the changes in the diffusion coefficients were interpreted, considering the competing effects of salt-mediated changes in protein interactions (e.g., electrostatic repulsion) and aggregation. The behavior is in agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO)-type modeling, accounting for the attractive and repulsive forces present. The diffusion data was compared with various self-association models, including corrections for the effects of self-obstruction. The diffusion coefficients of the higher oligomers were calculated, assuming that the monomers aggregated as hard spheres. Using an isodesmic association model, the equilibrium constant for the self-association of lysozyme at pH 4.6 and 298 K in the presence of 0.5 M NaCl was estimated to be 118 (12 M-1 .

The novel core-shell polymers were synthesized based on the perfluoroalkyls grafting onto the inner hydroxyl groups of hyperbranched polyglycidol while the outer hydroxyl groups remained unaffected. The samples were characterized with FTIR, NMR, element analysis, and liquid chromatography/mass spectrometry. The various results obtained by different measurements are in good agreements. The conversion ratio of inner hydroxyl groups is about 0.3 and most functionalizations took place in linear 1,4-units (L 1-4). Differential Scanning Calorimetry results suggest the samples are amorphous within the experiment range. Their inner lipophilicity and outer hydrophilism resulted from their unique structures are expected to gain promising applications.

UV Ultraviolet v Vitesse (m.s-1) V Volume molaire de solution (L.mol-1) v Volume spécifique partiel (cm 3 .g-1) V e Volume d'élution V s Volume partiel spécifique de soluté (m 3 .kg-1) X,Y Groupements terminaux X et Y d'un polymère z Nombre de charges d'un ion ZQ Zéro quantum * Les signaux POE de l'homopolymère et du copolymère sont confondus sur le spectre proton quantitatif, l'erreur est donc calculée en comparant l'intégrale quantitative à la somme des deux signaux POE renormalisés.

Novel para-sulfonatocalix[4]arene (sCX[4]) and polyamidoamine (PAMAM) dendrimer nanocomplexes were evaluated as delivery vehicles for the platinum anticancer agent [(1,10-phenanthroline)(1S,2S-diaminocyclohexane)platinum(II)] chloride (PHENSS). Different ratios of sCX[4] to PHENSS were tested for their compatibility, with a ratio of 6:1 sCX[4]:PHENSS having the best solubility. The loading of sCX[4], and sCX[4]-bound PHENSS, onto three different generations of PAMAM dendrimers (G3.0-5.0) was examined using UV-visible spectrophotometry. The quantity of sCX[4] bound was found to increase exponentially with dendrimer size: G3, 15 sCX[4] molecules per dendrimer; G4, 37; and G5, 78. Similarly, the loading of sCX[4]-bound PHENSS also increased with increasing dendrimer size: G3, 7 PHENSS molecules per dendrimer; G4, 14; and G5, 28.5. The loading of sCX[4]-bound PHENSS molecules is significantly lower when compared with that of sCX[4], which indicates that less than half of the binding sites were occupied (45, 44, and 44%, respectively). By 1 H NMR and UV-vis analysis, the nanocomplex was found to be stable in NaCl solutions at concentrations up to 150 mM. While PHENSS is more active in vitro than cisplatin against the human breast cancer cell line, MCF-7, delivery of PHENSS using the sCX[4]-dendrimer nanocomplexes, regardless of dendrimer generation, had little effect on PHENSS cytotoxicity. The results of this study may have application in the delivery of a variety of small molecule metal-based drugs for which chemical conjugation to a nanoparticle is undesired or not feasible.

Bioresponsive polymers are being developed as synthetic viral mimetics to enhance the intracellular delivery of macromolecular therapeutic agents such as genes, proteins and peptides. In this context we have designed pH-responsive, amphoteric polyamidoamines (PAAs) which change conformation on passing from a neutral pH (extracellular) to an acidic pH (endosomal and lysosomal) environments. PAAs have already demonstrated cytosolic delivery of genes and non-permeant toxins (e.g. gelonin and ricin A chain). The aim of this study was to use small-angle neutron scattering (SANS) to investigate the most likely shape of the hydrochloride salt form of one particular PAA (ISA23) in solution, under pH conditions that mimic those the polymer would be expected to encounter during endocytic internalisation (pH 7.4-3). It was shown that models based on a Gaussian coil representation of the polymer conformation described the SANS data better over this pH range than models based on a rod-like conformation. The conformation of ISA23 at 37 • C was expanded (radius of gyration ∼ 80Å) at pH ∼ 3 but collapsed with an increase in pH (radius of gyration ∼ 20Å at pH 7.4), a conclusion also reached in a model-free analysis of the neutron data. Outside this pH range-at the extremes of high and low pH-the polymer coil collapsed and interpretation of the scattering was slightly complicated by the presence of a very weak structure factor indicating that the polymer coils are highly charged. The PAA concentration did not significantly affect the polymer size over the concentration range 10-50 mg/ml. Characterisation of the dynamics of these polymer solutions-diffusion coefficients and viscosityostensibly suggest a very different conclusion with the polymer expanding as the pH is increased, but this arises due to weak aggregation of the amphoteric polymer coils.

Introduction Multiple hydrogen-bond formation has opened the door to a useful approach for assembling individual molecular components into functional organic nanostructures that are held together in a reversible manner. [1] Molecules containing the correct structural information spontaneously assemble into helices, [2] rosettes, [3] cylinders, [4] capsules, [5] dendrimers, [6] polymers, [7] or grids. [8] Urea functionalities have a great tendency to form hydrogen bonds, not only in the solid state, but also in solution. [9] Recently, some of us have proved that the triureas 1, derived from the tribenzylamine skeleton, assemble to form dimeric aggregates (Scheme 1). [10] In these dimers two molecules associate by forming hydrogen bonds between the six ureas, forming a head-to-tail directional array of 12 hydrogen bonds. [10] The result is a capsular aggregate with an internal cavity, that is too small for the encapsulation of organic molecules. [10a,c] Unimolecular capsules could in principle be achieved from a molecule containing two tripodal subunits of tris(ureidobenzyl)amine, linked through their urea residues by means of a flexible tether. This spacer should be long enough to allow the folding of the molecule to form the ring of hydrogen-bonded ureas, but short enough to minimize the increase in entropy brought about by the freely rotating single bonds. A hexamethylene spacer has previously been Abstract: Compounds formed by linking two tris(ureidobenzyl)amine modules with a hexamethylene tether are described. These compounds self-assemble to form bicapsular aggregates featuring two rings of six hydrogenbonded ureas. 1 H and 1 H/ 1 H ROESY NMR spectroscopy, together with pulsed gradient spin echo (PGSE) NMR diffusion measurements, have been used to characterize the dimers in solution. The results have been compared with energy-minimized structures. The new compounds are kinetically stable on the NMR timescale, and their thermodynamic stabilities are comparable to other capsular aggregates derived from tris(ureidobenzyl)amines.

hydrophobic interactions are usually considered the two Competition between mono-and divalent ions in the association most important. Electrostatic binding is a process that has of counterions to the headgroups of amphiphiles was studied in its most famous description in the DLVO theory for the one surfactant system with organic counterions (piperidine / / diffuse electrical double layer (6). The association is favored piperazine 2/ octanesulfonate) and one with inorganic counterions by the energetic attractive contribution coming from the di-(Na / /Ca 2/ octyl sulfate). By conductivity and 13 C NMR chemical rect electrostatic interactions and is disfavored by the deshift measurements the critical micelle concentration (CMC) was crease in entropy when counterions are ordered near the found to decrease drastically when small amounts of divalent surface aggregate. The second mechanism occurs due to counterions were present in the system. Self-diffusion coefficients hydrophobic interactions between the counterion and the agof surfactant ions and organic counterions were measured in the micellar phase by the Fourier transform pulsed-gradient spin-echo gregate. These interactions tend to be more difficult to gener-(FT-PGSE) NMR method. The degree of counterion binding in the alize into a concise theory than the electrostatic interactions. micellar system with piperidine / /piperazine 2/ counterions was A strong dependence on the hydrophobicity is noted in the obtained from FT-PGSE NMR measurements. It was observed literature though, and for instance, the relation between the that the divalent counterions were more strongly bound than the number of CH 2 groups in the counterion and counterion monovalent counterions. The experimental results were compared binding has been given theoretical descriptions (7). with theoretical Poisson-Boltzmann calculations. The cell model The valency of the counterion plays, for electrostatic reawas used to study the electrostatic effects. Good agreement besons, an important role in the association process. In surfactween electrostatic theory and experiment was observed; however, tant systems with multivalent ions usually the Krafft point an attractive force exists between the monovalent piperidine counis increased so that the surfactant will precipitate. The critical terions and the micelle, probably because of hydrophobic interacmicelle concentration (CMC) is often lower than with monotions.

NMR microimaging may be used to observe the effect of molecular diffusion in the vicinity of a thin wire subjected to current pulses. By this means the pulsed gradient spin echo technique can utilize very large pulsed magnetic field gradients, on the order of 100 Tm '. The quadratic dependence of gradient amplitude on distance from the wire leads to large dynamic range while the distribution of local gradient vectors makes it possible to image anisotropic diffusion. We demonstrate these properties in measurements on polymer solutions and liquid crystals.

The electrokinetic behavior of G6.5 carboxylateterminated poly(amidoamine) (PAMAM) starburst dendrimers (8 ± 1 nm diameter) is investigated over a broad range of pH values (3−9) and NaNO 3 concentrations (c ∞ = 2−200 mM). The dependence of nanodendrimer electrophoretic mobility μ on pH and c ∞ is marked by an unconventional decrease of the point of zero mobility (PZM) from 5.4 to 5.5 to 3.8 upon increase in salt concentration, with PZM defined as the pH value at which a reversal of the mobility sign is reached. The existence of a common intersection point is further evidenced for series of mobility versus pH curves measured at different NaNO 3 concentrations. Using soft particle electrokinetic theory, this remarkable behavior is shown to originate from the zwitterionic functionality of the PAMAM-COOH particles. The dependence of PZM on c ∞ results from the coupling between electroosmotic flow and dendrimeric interphase defined by a nonuniform distribution of amine and carboxylic functional groups. In turn, μ reflects the sign and distribution of particle charges located within an electrokinetically active region, the dimension of which is determined by the Debye length, varied here in the range 0.7−6.8 nm. In agreement with theory, the electrokinetics of smaller G4.5 PAMAM-COOH nanoparticles (5 ± 0.5 nm diameter) further confirms that the PZM is shifted to higher pH with decreasing dendrimer size. Depending on pH, a mobility extremum is obtained under conditions where the Debye length and the particle radius are comparable. This results from changes in particle structure compactness following salt-and pH-mediated modulations of intraparticle Coulombic interactions. The findings solidly evidence the possible occurrence of particle mobility reversal in monovalent salt solution suggested by recent molecular dynamic simulations and anticipated from earlier mean-field electrokinetic theory.

A simplified pore-to-pore hopping model for the two-phase diffusion problem is developed for the analysis of the pulsed gradient spin echo (PGSE) attenuation of water diffusion in the condensed cell suspension systems. In this model, the two phases inside and outside the cells are treated as two different kinds of pores, and the spin-bearing molecules perform hopping diffusion between them. The size and the orientations of those two respective pores are considered, and then the diffraction pattern of the PGSE attenuation may be well simulated. Nevertheless, the intensity of the characteristic peak decreases with increasing membrane permeability, from which the exchange time may be estimated. We then analyze the experimental 1 H PGSE results of the erythrocytes suspension system. The water-residence lifetime in the erythrocyte is obtained to be 10 ms, which is the same as that estimated from the two-region approximation. Furthermore, the PGSE attenuation curve of addition of p-Chloromercuribenzenesulfonate (p-CMBS) is also discussed. It predicts that the alignment of erythrocytes will become normal to the magnetic field direction after the addition of p-CMBS, and inspection using a light microscope confirms that result.

In confined geometries, the MR signal attenuation obtained from single pulsed gradient spin echo (s-PGSE) experiments reflects the dimension of the compartment, and in some cases, its geometry. However, to measure compartment size, high q-values must be applied, requiring high gradient strengths and/or long pulse durations and diffusion times. The angular double PGSE (d-PGSE) experiment has been proposed as a means to extract dimensions of confined geometries using low q-values. In one realization of the d-PGSE experiment, the first gradient pair is fixed along the x-axis, and the orientation of the second gradient pair is varied in the X-Y plane. Such a measurement is sensitive to microscopic anisotropy induced by the boundaries of the restricting compartment, and allows extraction of the compartment dimension. In this study, we have juxtaposed angular d-PGSE experiments and simulations to extract sizes from well-characterized NMR phantoms consisting of water filled microcapillaries. We are able to accurately extract sizes of small compartments (5 lm) using the angular d-PGSE experiment even when the short gradient pulse (SGP) approximation is violated and over a range of mixing and diffusion times. We conclude that the angular d-PGSE experiment may fill an important niche in characterizing compartment sizes in which restricted diffusion occurs.

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20-140 • C. The selectivity of the membrane, defined as (D OH /D CH3 ), decreased from 6 to 2.4 as temperature increased from 20 to 140 • C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 • C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 • C. High pressure NM...

Porous media can be considered as interfacial systems where an internal surface partitions and fills the space in a complex way. Meaningful structural features appear on a length-scale where physical chemistry plays a central role either to impose a specific organisation on the material or to strongly modify the dynamics and the thermodynamics of the embedded fluids. A key issue is to understand how the geometrical and interfacial confinement affects numerous phenomena such as molecular diffusion, excitation relaxation, reaction kinetics, phase transitions, adsorption and capillary condensation. We will first review some experimental techniques able to image the 3D structure of disordered porous media. In the second part, we will analyse the geometrical and particularly some topological properties of a disordered porous material. We will discuss the interest and the limits of several strategies for obtaining 3D representations of various pore networks starting from an incomplete set of morphological characterisations. Finally, connection between geometry and diffusive transport will be presented, with emphasis on the application of pulsed gradient spin echo NMR technique as a tool for a multiscale analysis of transport in a confining geometry.

Specific parameters of the neuronal tissue microstructure, such as axonal diameters, membrane permeability and intracellular water fractions are assessable using diffusion MRI. These parameters are commonly estimated using analytical models, which may introduce bias in the estimated parameters due to the approximations made when deriving the models. As an alternative to using analytical models, a database of signal curves generated by fast Monte Carlo simulations can be employed. Simulated diffusion MRI measurements were generated and evaluated using the two-compartment Kärger model as well as the simulation model based on a database containing signal curves from approximately 60 000 simulations performed with different combinations of microstructural parameters. A protocol based on a pulsed gradient spin echo sequence with diffusion times of 30 and 60 ms and with gradient amplitudes obtainable with a clinical MRI scanner was employed for the investigations. When using the analytical model, a major negative bias (up to approximately 25%) in the estimated intracellular volume fraction was observed for short exchange times, while almost no bias was seen for the simulation model. In general, the simulation model improved the accuracy of the estimated parameters as compared to the analytical model, except for the exchange time parameter.

The diastereoisomeric complexes formed between 2,6-di-O-alkyl-and-cyclodextrins and the arylammonium ions propranolol, ephedrine and amphetamine have been studied by electrode response and NMR methods. Enantioselectivity in binding propranolol is 3.3 : 1 with 2,6-di-O-dodecyl-cyclodextrin in favour of the (؉)-enantiomer as revealed by measurement of the association constant using pulsed-gradient spin-echo (PGSE) NMR methods. In all of the cases of enantiodifferentiation studied here, the (؉)-enantiomer is more strongly bound by the cyclodextrin. Relaxation rate measurements of the host and guest proton NMR resonances highlight the importance of hydrogenbonding in enantiomer discrimination.

Star polymers with a hydrophobic cholane core and four poly(ethylene glycol) (PEG) arms, CA(EG n) 4 , have been synthesized by anionic polymerization. Pulsed-gradient spin-echo NMR spectroscopy was used to study the diffusion behavior of the star polymers, ranging from 1000 to 10,000 g/mol, in aqueous solutions and gels of poly(vinyl alcohol) (PVA) at 23 C. The star polymers have a lower self-diffusion coefficient than linear PEGs at equivalent hydrodynamic radius. In water alone, the star polymers and their linear homologues have a similar diffusion behavior in the dilute regime, as demonstrated by the similar concentration dependence of the self-diffusion coefficients. In the semidilute regime, the star polymers tend to aggregate due to their amphiphilic properties, resulting in lower self-diffusion coefficients than those of linear PEGs. 1 H NMR T 1 measurements at 10e70 C revealed that the PEG arms of the star polymers are more mobile than the core, suggesting the star polymers in solution have a conformation similar to that of poly(propylene imine) dendrimers.

Direct oxidation fuel cell (DOFC) based on proton conducting membranes (PCM's) represents a promising energy technology, with the vast majority of efforts devoted to methanol as the fuel. The most commonly used PCM is Nafion. However, because of high production cost, high fuel crossover, and sensitivity to metal-ion impurities, alternative materials are being developed. Methanol presents several limitations including low boiling point (Â/65 8C), and reduced cell efficiency due to fuel crossover. We present here a measure of the mass transport represented by the self-diffusion coefficients (D) of methanol, and two alternative fuels */ethylene glycol (EG) and dimethyl oxalate (DMO) in new low-cost nanoporous (NP) PCM's. The NP-PCM's are based on commercial poly(vinylidene fluoride), with variable pore size determined by the addition of nanoscale SiO 2 or TiO 2 particles. Proton NMR pulsed gradient spin-echo (PGSE) self-diffusion measurements were conducted on aqueous solutions of 2 M fuel in 3 M H 2 SO 4 and also on five NP-PCM equilibrated in the aqueous solutions over the temperature range of 30 Á/90 8C. Here the fuel molecular mobility is indicated by the D values of the methyl and methylene peaks. Results indicate that both DMO and EG solutions have lower fuel molecular mobility than methanol. Results for the various NP-PCM also reveal reduced fuel permeability for DMO and EG compared with methanol.

Ionic core-shell dendrimers with an octacationic core have been applied as noncovalent supports for homogeneous catalysts. Catalytically active arylpalladium complexes, which bear a tethered sulfato group, were noncovalently attached to the ionic core-shell dendritic supports via a straightforward ionexchange reaction under mild conditions. Diagnostic shifts in 1H NMR and Overhauser contacts show that the sulfato groups of the catalysts are

Water uptake, swelling, 1 H pulsed gradient spin-echo nuclear magnetic resonance (NMR) and variable temperature and pressure complex impedance/electrical conductivity studies have been carried out on sulfonated styrene/ethylenebutylene/styrene (S-SEBS) triblock polymer proton conducting membranes. At the highest water contents, the activation volume calculated from the effect of pressure on the electrical conductivity is negative. Previously reported results for Nafion 117 show the same behavior. In addition, above about 10 wt% water, the diffusion coefficients, D from NMR and D | calculated from conductivity data, are similar for S-SEBS. The same result is obtained for Nafion 117. The conclusion is that proton transport at high water content is by molecular diffusion for both materials. For low water contents, however, the materials are significantly different. For low water content S-SEBS, D and D | are different while they are the same for Nafion 117. In addition, the variation of the conductivity with temperature for S-SEBS is Arrhenius while that for Nafion 117 is not. Finally, the variation of the electrical conductivity with pressure gives rise to activation volumes on the order of 14 cm 3 /mol for S-SEBS while those for Nafion 117 are about four times larger. These results indicate that proton transport in low water content S-SEBS occurs via a thermally activated process (ion motion via energy barriers) that is consistent with the more rigid side chains in that material.

Direct oxidation fuel cell (DOFC) based on proton conducting membranes (PCM's) represents a promising energy technology, with the vast majority of efforts devoted to methanol as the fuel. The most commonly used PCM is Nafion. However, because of high production cost, high fuel crossover, and sensitivity to metal-ion impurities, alternative materials are being developed. Methanol presents several limitations including low boiling point (Â/65 8C), and reduced cell efficiency due to fuel crossover. We present here a measure of the mass transport represented by the self-diffusion coefficients (D) of methanol, and two alternative fuels */ethylene glycol (EG) and dimethyl oxalate (DMO) in new low-cost nanoporous (NP) PCM's. The NP-PCM's are based on commercial poly(vinylidene fluoride), with variable pore size determined by the addition of nanoscale SiO 2 or TiO 2 particles. Proton NMR pulsed gradient spin-echo (PGSE) self-diffusion measurements were conducted on aqueous solutions of 2 M fuel in 3 M H 2 SO 4 and also on five NP-PCM equilibrated in the aqueous solutions over the temperature range of 30 Á/90 8C. Here the fuel molecular mobility is indicated by the D values of the methyl and methylene peaks. Results indicate that both DMO and EG solutions have lower fuel molecular mobility than methanol. Results for the various NP-PCM also reveal reduced fuel permeability for DMO and EG compared with methanol.

NMR signal phase variation caused by macroscopic motion of an object during application of the diffusion gradient is a well-known effect in diffusion-weighted imaging (DWI) using the standard pulsed gradient spin-echo sequence (PGSE). This phase error causes severe ghost artifacts in the output image when phase encoding techniques, such as two dimensional Fourier transform (2DFT) imaging, are used. One possible way to eliminate the motion effects is the navigator echo technique. The method is based on estimating the phase error from the navigator echo and using it for the correction of the image echo. The phase errors (zero and first order) for the phase correction of the image echo are usually evaluated from the navigator echo after Fourier transform (FT) in the readout direction, correcting for both translation and rotation. We present here a simple algorithm which enables evaluation and correction in the time domain of phase errors induced by motion. This approach has the advantage of improved 'correction of motional artifacts and minimized sensitivity to noise and inaccurate setting up of the experiment.

Porous media can be considered as interfacial systems where an internal surface partitions and fills the space in a complex way. Meaningful structural features appear on a length-scale where physical chemistry plays a central role either to impose a specific organisation on the material or to strongly modify the dynamics and the thermodynamics of the embedded fluids. A key issue is to understand how the geometrical and interfacial confinement affects numerous phenomena such as molecular diffusion, excitation relaxation, reaction kinetics, phase transitions, adsorption and capillary condensation. We will first review some experimental techniques able to image the 3D structure of disordered porous media. In the second part, we will analyse the geometrical and particularly some topological properties of a disordered porous material. We will discuss the interest and the limits of several strategies for obtaining 3D representations of various pore networks starting from an incomplete set of morphological characterisations. Finally, connection between geometry and diffusive transport will be presented, with emphasis on the application of pulsed gradient spin echo NMR technique as a tool for a multiscale analysis of transport in a confining geometry.

Porous media can be considered as interfacial systems where an internal surface partitions and fills the space in a complex way. Meaningful structural features appear on a length-scale where physical chemistry plays a central role either to impose a specific organisation on the material or to strongly modify the dynamics and the thermodynamics of the embedded fluids. A key issue is to understand how the geometrical and interfacial confinement affects numerous phenomena such as molecular diffusion, excitation relaxation, reaction kinetics, phase transitions, adsorption and capillary condensation. We will first review some experimental techniques able to image the 3D structure of disordered porous media. In the second part, we will analyse the geometrical and particularly some topological properties of a disordered porous material. We will discuss the interest and the limits of several strategies for obtaining 3D representations of various pore networks starting from an incomplete set of morphological characterisations. Finally, connection between geometry and diffusive transport will be presented, with emphasis on the application of pulsed gradient spin echo NMR technique as a tool for a multiscale analysis of transport in a confining geometry.

The enzymatic activity of Lipase VII from Candida rugosa was studied in the quaternary water-in-oil microemulsion cetyltrimethylammonium bromide (CTAB)/water/pentanol (POH)/hexane (C 6 ). The enzyme-catalyzed hydrolysis of p-nitrophenylbutirate (p-NPB) was found essentially independent from the water/surfactant ratio. On the other hand, the enzyme kinetics was strongly affected by the cosurfactant/surfactant ratio. The interfacial composition and the reverse micelles size, as well as the substrate partition between organic bulk and surfactant aggregates, have been determined through pulsed gradient spin-echo NMR measurements. The correlation of the microemulsion structure with enzyme activity has therefore been possible. The lipase affinity for p-nitrophenylbutirate is lower in microemulsion compared to water because the hydrophobic effect is weaker in the former system. The turnover number is unaffected from the reverse micelles size and from the interfacial bromide concentration, while is strongly dependent on the interfacial composition. The data presented indicate a marked influence of the extent of interfacial surface on the catalytic efficiency of lipase.

The enzymatic activity of Lipase VII from Candida rugosa was studied in the quaternary water-in-oil microemulsion cetyltrimethylammonium bromide (CTAB)/water/pentanol (POH)/hexane (C 6 ). The enzyme-catalyzed hydrolysis of p-nitrophenylbutirate (p-NPB) was found essentially independent from the water/surfactant ratio. On the other hand, the enzyme kinetics was strongly affected by the cosurfactant/surfactant ratio. The interfacial composition and the reverse micelles size, as well as the substrate partition between organic bulk and surfactant aggregates, have been determined through pulsed gradient spin-echo NMR measurements. The correlation of the microemulsion structure with enzyme activity has therefore been possible. The lipase affinity for p-nitrophenylbutirate is lower in microemulsion compared to water because the hydrophobic effect is weaker in the former system. The turnover number is unaffected from the reverse micelles size and from the interfacial bromide concentration, while is strongly dependent on the interfacial composition. The data presented indicate a marked influence of the extent of interfacial surface on the catalytic efficiency of lipase.

We have extended the utility of NMR as a technique to probe porous media structure over length scales of ~ 100 -2000 µm by using the spin 1/2 noble gas 129 Xe imbibed into the system's pore space. Such length scales are much greater than can be probed with NMR diffusion studies of water-saturated porous media. We utilized Pulsed Gradient Spin Echo NMR measurements of the time-dependent diffusion coefficient, D(t) of the xenon gas filling the pore space to study further the measurements of both the surface area-pore volume ratio, S/V p , and the tortuosity (pore connectivity) of the medium. In uniform-size glass bead packs, we observed D(t) decreasing with increasing t, reaching an observed asymptote of ~ 0.62 -0.65D 0 , that could be measured over diffusion distances extending over multiple bead diameters. Measurements of D(t)/D 0 at differing gas pressures showed this tortuosity limit was not affected by changing the characteristic diffusion length of the spins during the diffusion encoding gradient pulse. This was not the case at the short time limit, where D(t)/D 0 was noticeably affected by the gas pressure in the sample. Increasing the gas pressure, and hence reducing D 0 and the diffusion during the gradient pulse served to reduce the previously observed deviation of D(t)/D 0 from the S/V p relation. The Pade approximation is used to interpolate between the long and short time limits in D(t).

We report initial NMR studies of i) xenon gas diffusion in model heterogeneous porous media, and ii) continuous flow laser-polarized xenon gas. Both areas utilize the Pulsed Gradient Spin Echo techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients -a brief overview of this area is provided in the introduction.

We report initial NMR studies of continuous flow laser-polarized xenon gas, both in unrestricted tubing, and in a model porous media. The study uses Pulsed Gradient Spin Echo-based techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational selfdiffusion coefficients. Pulsed Gradient Echo studies of continuous flow laser-polarized xenon gas in unrestricted tubing indicate clear diffraction minima resulting from a wide distribution of velocities in the flow field. The maximum velocity experienced in the flow can be calculated from this minimum, and is seen to agree with the information from the complete velocity spectrum, or motion propagator, as well as previously published images. The susceptibility of gas flows to parameters such as gas mixture content, and hence viscosity, are observed in experiments aimed at identifying clear structural features from echo attenuation plots of gas flow in porous media. Gas-phase NMR scattering, or position correlation flow-diffraction, previously clearly seen in the echo attenuation data from laserpolarized xenon flowing through a 2 mm glass bead pack is not so clear in experiments using a different gas mixture. A propagator analysis shows most gas in the sample remains close to static, while a small portion moves through a presumably near-unimpeded path at high velocities.