PFG NMR diffusion studies

Unlike the mobility of imsaturated hydrocarbons, the mobihty of saturated hydrocarbons in Na-X is, in fact, essentially unaffected by the presence of sodium cations. This has been confirmed by PFG NMR diffusion studies with benzene and -heptane in zeohte Na-X and La-X [103,151]. Since the triva-lent lanthanum ions are predominantly localized at positions in the hexagonal prisms and sodalite units, the molecules adsorbed in lanthanum-exchanged zeolites are essentially without contact to the cations. As a consequence of the specific interaction between the cations and the unsaturated hydrocarbon, the benzene mobility in Na-X was found to be two orders of magnitude smaller than in La-X, while the -heptane diffusivities were the same. 

Modification of H-ZSM-5 zeolites by impregnation with H3PO4 has become a common technique to improve their activity and selectivity [190,191]. In parallel with these changes, however, the transport properties of the zeolite crystallites are also changed. As in the cases of hydrothermal treatment and coking discussed above, a combined application of PFG NMR to study both intracrystalline diffusion and intercrystalline molecular exchange may provide information about... 

Adsorption dynamics. IH NMR was employed beyond spectral studies by performing diffusion measurements Applying IH PFG-NMR diffusion experiments to surfactants adsorbed to latex particles in dilute dispersions, a method was developed for the investigation of surfactant adsorption dynamics [23, 26]. Since surfactant molecules were occurring in two sites, i.e. in solution and as adsorbed surfactant, each site exhibited a different diffusion coefficient and was distinguished in a PFG experiment. This offered a convenient way to vary the relevant experimental time scale, which is determined by the spacing of the gradient pulses A... 

Solid polymer electrolytes for lithium batteries applications are commonly prepared by dissolving a lithium salt in poly(ethylene oxide) (PEO)-based materials. Chiappone et al. investigated these systems by a Li and NMR study yielding local dynamics and mass transport by temperature-dependent Ti and PFG-NMR diffusion measurements. 

Fig. 3.1.4 Anisotropic self-diffusion of water in and filled symbols, respectively). The horizon-MCM-41 as studied by PFG NMR. (a) Depen- tal lines indicate the limiting values for the axial dence of the parallel (filled rectangles) and (full lines) and radial (dotted lines) compo-perpendicular (circles) components of the axi- nents of the mean square displacements for symmetrical self-diffusion tensor on the inverse restricted diffusion in cylindrical rods of length temperature at an observation time of 10 ms. / and diameter d. The oblique lines, which are The dotted lines can be used as a visual guide, plotted for short observation times only, repre-The full line represents the self-diffusion sent the calculated time dependences of the...

It is worth noting that within a range of 20 %, five different methods of analyzing the crystallite size, viz., (a) microscopic inspection, (b) application of Eq. (3.1.7) for restricted diffusion in the limit of large observation times, (c) application of Eq. (3.1.15) to the results of the PFG NMR tracer desorption technique, and, finally, consideration of the limit of short observation times for (d) reflecting boundaries [Eq. (3.1.16)] and (e) absorbing boundaries [Eq. (3.1.17)], have led to results for the size of the crystallites under study that coincide. 

Pulsed field gradient NMR has become a standard method for measurement of diffusion rates. Stilbs [272] and others have exploited in particular the FT version for the study of mixtures. An added advantage of PFG-NMR is that it can be employed to simplify complex NMR spectra. This simplification is achieved by attenuation of resonances based on the differential diffusion properties of components present in the mixture. 

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

Lateral diffusion of phospholipids in model membranes at ambient pressure has been studied over the years by a variety of techniques including fluorescence recovery after photobleaching (FRAP), spin-label ESR, pulse field gradient NMR (PFG-NMR), quasielastic neutron scattering (QENS), excimer fluorescence and others.In general, the values reported for the lateral diffusion coefficient (D) range from 10 to 10 cm /s in the... 

Nowak et al. (63) presented a comparative study of the diffusivities of rigid models of methane, ethane, and propane in silicalite. (The details of the calculation are reported in the preceding section.) The calculated diffusion coefficients decreased as the length of the carbon chain increased, and the effect was found to be far more pronounced for ethane than propane. The calculated diffusivities, in units of 108 m2/s, were 0.62, 0.47, and 0.41 for methane, ethane, and propane, respectively. The ethane value is in satisfactory agreement with PFG-NMR measurements [0.38 (77), 0.3 (80), 0.4 (42) for silicalite. The value for propane, however, was calculated to be almost an order of magnitude larger than the NMR results of Briscoe et al. (80). [The agreement with the value of Caro et al. (71) is better, but still an overestimation.]... 

The self-diffusion of benzene in PIB [36], cyclohexane in BR [37] and toluene in PIB [38-40] has been investigated by PFG NMR. In addition more recently Schlick and co-workers [41] have measured the self-diffusion of benzene and cyclohexane mixtures in polyisoprene. In the first reported study of this kind, Boss and co-workers [36] measured the self-diffusion coefficients of benzene in polyisoprene at 70.4 °C. The increase in Dself with increasing solvent volume fraction could be described by the Fujita-Doolittle theory which states that the rate of self-diffusion scales with the free volume which in turn increases linearly with temperature. At higher solvent volume fractions the rate of selfdiffusion deviates from the Fujita-Doolittle theory, as the entanglement density decreased below the critical value. 

PFG NMR studies in porous media have also been carried out using 13C [77], 15N [78], 19F [79] and 129Xe NMR [78]. The lower limit of diffusivities accessible by H PFG NMR is of the order of 10"13 mV. However, such low diffusivities may only be measured under suitable conditions, in particular for large nuclear magnetic relaxation times T and Ti [32]. 

In principle, any of the techniques described for studying self-diffusion may be applied to both single- and multicomponent systems. So far, however, most selfdiffusion measurements of multicomponent systems have been carried out by PFG NMR. Such measurements are possible by applying a set of samples with deuterated compounds with only one species (namely that to be measured) in the protonated form [98], by simultaneously considering different nuclei (e.g. H and 19F [79]), and by Fourier transform PFG NMR [91,92]. 

NMR Sorption Comparison Diffusion of the large tri-ethylamine molecule is sufficiently slow that reliable dif-fusivities can be determined from uptake rate measurements in 50]jm crystals(57) at least over a limited range of conditions. A comparative study, carried out with the same zeolite samples showed good agreement between the sorption and PFG NMR measurements, both as to the magnitude of the diffusivity ( 10 cm2.s- at 445K) and the trends with concentration and temperature(41). 

To study the mobility of Li" " on the gel matrix, we measured the Li diffusion coefficient before and after gelation of the 2-Li (with 5 wt% PAMPSLi) using pfg-NMR. The Li diffusion coefficient in the gel was almost unchanged from the value before gelation, despite the decrease in the ionic conductivity with gelation. Because, as this result suggests, the mobility of Li is not suppressed by gelation, this gel system is favorable for Li" " conductive materials. 

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