Method using pulsed field gradient

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... 

While the nuclear magnetic resonance (NMR) technique has widely been used to study diffusion processes of normal liquids, solids, or colloidal systems, there are only a few applications to molten salts. The spin echo self-diffusion method with pulsed field gradients was applied to molten salts by Herdlicka et al. "" There is no need to set up or maintain a concentration gradient. 

Figure 4 Methods of selectivity enhancement using pulsed field gradients employing (a) selective excitation pulses, (b) a selective refocusing pulse and (c) a double piUsed field gradient spin echo (DPFGSE) sequence with inversion pulses...

Measurement of diffusion using pulsed field gradient NMR (PFG-NMR) is a powerful analytical tool because it combines the high specificity and information content of NMR spectroscopy with the size selectivity of diffusion coefficients. PFG-NMR employs timescales of tens of ms and has a displacement sensitivity of the order of 100 nm. PFG-NMR can determine molecular self-diffusion coefficients in liquid phases down to a lower limit of 10 " m s Due to the combination of experimental convenience and straightforward interpretation, PFG-NMR has become the method of choice for studying translational diffusion. PFG-NMR experiments have been reported using H, H, Li, C, F and other nuclei. The time A over which PFG-NMR measurements are possible is limited. 

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

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. 

In this chapter we will focus on molecular ordering and confinement effects in pores. Diffusion experiments with the pulse-field gradient method ([162-165] and references therein) and characterization of the surface properties using NMR of noble gases such as 129Xe ([166-171] and references therein), or 83Kr [172], will be omitted due to excellent reviews that have appeared quite recently in these areas. 

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