Pulse field gradient -NMR methods

Johnson et al. [186] measured diffusion of fluorescein-labeled macromolecules in agarose gels. Their data agreed well with Eq. (85), which combined the hydrodynamic effects with the steric hindrance factors. Gibbs and Johnson [131] measured diffusion of proteins and smaller molecules in polyacrylamide gels using pulsed-field gradient NMR methods and found their data to fit the stretched exponential form... 

Zawodzinski et al. [64] have reported self-diffusion coefficients of water in Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) equilibrated with water vapor at 303 K, and obtained results summarized in Fig. 36. The self-diffusion coefficients were deterinined by pulsed field gradient NMR methods. These studies probe water motion over a distance scale on the order of microns. The general conclusion was the PFSA membranes with similar water contents. A, had similar water self-diffusion coefficients. The measured self-diffusion coefficients in Nafion 117 equilibrated with water vapor decreased by more than an order of magnitude, from roughly 8 x 10 cm /s down to 5 x 10 cm /s as water content in the membrane decreased from A = 14 to A = 2. For a Nafion membrane equilibrated with water vapor at unit activity, the water self-diffusion coefficient drops to a level roughly four times lower than that in bulk liquid water whereas a difference of only a factor of two in local mobility is deduced from NMR relaxation measurements. This is reasonably ascribed to the additional effect of tortuosity of the diffusion path on the value of the macrodiffusion coefficient. For immersed Nafion membranes, NMR diffusion imaging studies showed that water diffusion coefficients similar to those measured in liquid water (2.2 x 10 cm /s) could be attained in a highly hydrated membrane (1.7 x 10 cm /s) [69]. 

Advances in experimental techniques, including pulsed-field gradient NMR, and theoretical methods, including volume averaging, macrotransport, and variational methods, that may lead to the resolution of a number of the fundamental issues in gel electrophoresis and to improvements in the practical application of electrotransport in polymeric media... 

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. 

Pulsed field gradient NMR (PFG-NMR) spectroscopy has been successfully used for probing interactions in several research fields.44-53 The method was developed by Stejskal and Tanner more than 40 years ago54 and allows the measurement of self-diffusion coefficient, D, which is defined as the diffusion coefficient in absence of chemical potential gradient. 

Pulsed-field-gradient NMR and NMR imaging are still in their early stages of development Rapid development in methods and apparatus is expanding the field for potential applications. 

Fleisher et al. [12] studied the self-diffusion of oil and water in rape seeds. The selfdiffusion of oil was found to be completely restricted. The experiments could be explained in toms of the model of diffusion within spherical droplets and a Gaussian mass distribution of the droplet radii. At the same time Van den Enden et al. [9] introduced the technique described above. It is a rapid method for the determination of water droplet size distributions in spreads by using low resolution pulsed field gradient NMR. Their method was based on the recognition that a set of echo attenuation values (R) as a function of the field gradient pulsed width, obtained under conditions where R is independent of the time allowed for diffusion, contains all the necessary information on the water droplet size distribution (see above). A log-normal distribution of water droplet sizes was assumed. 

The pulsed field gradient NMR technique can be readily used for the determination of the water droplet size distribution in W/O emulsions or the oil droplet size distributions in O/W emulsions. Important advantages are the non-invasive nature, the ease of sample preparation, and the feet that pulsed field gradient NMR measures the droplet size distribution of the bulk in contrast with microscopic methods which estimate the size distribution of the surface. Both the proposed matrix method and the iterative curve fitting procedure can be successfully applied in a factory environment. The method can be implemented on a high as well as on a low resolution NMR spectrometer. 

The diffusion of small molecules in rubbers is of both theoretical and practical importance. The theories of diffusion based on consideration of free volume can be tested by measurement of self-diffusion using methods such as pulsed field gradient NMR. Selfdiffusion of small molecules must be understood for applications of rubbers as seals in contact with solvents, and for example for diffusion of plasticisers and other small molecules. 

In most cases, structural characterization of carbosilane dendrimers is accomplished by multinuclear one-dimensional NMR spectroscopy (1H, 13C and 29Si). However, as larger dendrimers are characterized standard spectroscopic methods become less useful due to the overlap of signals. This problem has been elegantly circumvented as described in a recent paper by Tessier, Rinaldi and coworkers56. In this paper the researchers described the use of 1 H/13C/29Si triple resonance, 3D and pulse field gradient NMR techniques to... 

Mesoporous structures are commonly characterized with diffraction, electron microscopy methods [14], and gas sorption techniques. The ensemble diffusion behavior of small molecules has been examined with pulsed-field gradient NMR spectroscopy [15] and neutron scattering [16]. Here, we are interested in techniques which give a more direct access to the real structure of the mesoporous host and to the dynamics on a single-molecule basis, and thus reveal structural and dynamic features which are not obscured by ensemble or statistical averaging as in conventional techniques. 

A new method for droplet size measurement, using a bench-top pulsed-field-gradient NMR spectrometer operating in the time domain, has been reported (18). The continuous water phase is successfully suppressed by gradient pulses in order to measure the dispersed oil phase. Simulations show that for most common oil/water food emulsions the influence of droplet diffusion is negligible due to a rather large droplet size or a high viscosity of the continuous water phase. 

Fig. 5.4.4 Pulsed-field gradient NMR. (a) The Hahn echo is attenuated by translational diffusion during the time interval A between two short gradient pulses applied in the dephasing and in the rephasing periods of the echo (top). By use of flie gradient pulses initial position T and final position Tj of the magnetization are labelled to identify migrating magnetization components (bottom), (b) The sensitivity of the method towards slower processes can be increased if the stimulated echo is used in place of the Hahn echo. Adapted from [Cal2] with permission from Oxford University Press.

Hagslatt, H., Jonsson, B., Nyden, M., and Soderman, O. Predictions of pulsed field gradient NMR echo-decays for molecules diffusing in various restrictive geometries. Simulations of diffusion propagators based on a finite element method, /. Magn. Reson., 161,138, 2003. 

Attempts to obtain transport number information by various methods such as pulsed field gradient NMR [62], radio tracer diffusion [77], and potentiostat-ic polarization technique [46] have suggested that both cation and anion mobilities are important for the total ionic conductivity seen. In general, however, the nature of charge carriers in polymer electrolytes is quite complex and ion aggregates such as triple ions have been implicated in conductivity [78-79]. 

Some information can be obtained on porous media from conventional NMR spectroscopy, and this is discussed in Section 2. Relaxation time measurements have been widely used to characterize porous solids, and this technique is discussed in Section 3. Pulsed field gradient (PFG) methods may be used to probe the local structure of the pore space and to characterize transport within it, and these are discussed in Section 4. Magnetic resonance imaging (MRI) techniques can also be used to characterize the pore space and to measure transport, and applications are discussed in Section 5. The bulk of this review will be concerned with mesoporous and macroporous materials, as it is for these systems that NMR is particularly useful in characterizing the pore space. However, some applications of NMR techniques to probe the pore space and transport within microporous materials will be mentioned in Section 6. Finally, some general conclusions are given in Section 7. 

To prove the formation of vesicles a number of indirect techniques can be used such as dynamic light scattering, the use of fluorescent probes and pulsed field gradient NMR self-diffusion measurements. Some more direct techniques such as freeze-fracture and negative staining electron microscopy are less biased by the interpretation of the scientist, but also these methods have their limitations. Cryo-electron microscopy, as introduced by Dubochet in the 80s, is the method of choice when it comes to visualization of small colloidal structures. Recent developments in the vitrification of specimens make it now possible to observe vesicles and other aggregated structures artifact free. 

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