Diffusion measurements nuclear magnetic relaxation

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]. 

X type zeolites provide favourable conditions for nmr self-diffusion measurements, since in comparison with most other zeolite types, the open pore structure leads to higher molecular mobilities, as well as to larger (transverse) nuclear magnetic relaxation times, which -in turn - allow the application of larger observation times a. 

Comparative investigations between the conventional adsorption/desorption method and PFG NMR have been carried out with aromatics in zeolite NaX. It was pointed out in Table 2 that there is still some divergence between the data obtained by both methods on intracrystalline diffusion. Table 3 compares the values for Tjnira and Tjn,ra determined by the NMR methods [143,175,176]. H PFG NMR measurements of these systems are complicated by the rather short transverse nuclear magnetic relaxation times, which range over milliseconds and lead to mean errors up to 50%. However, as with the n-paraffins in NaX, there is no indication of a significant enhancement of Tjn,ra 0 comparison with Tin,ra° " as... 

The diffusion properties of zeoUtes may be significantly influenced by their content of exchangeable cations. As an example, by both nuclear magnetic relaxation [1,143] and PEG NMR [145] measurements, the molecular mobility of aromatics in zeolite Na-X was found to be larger than in Na-Y. Since the adsorbate-adsorbent interaction of unsaturated hydrocarbons is dominated by the interaction between the n electrons of the double bonds and... 

Diffusion measurements fall into two broad classes. Under macroscopic equilibrium, i.e. if the overall concentration within the sample remains constant, molecular diffusion can only be studied by following the diffusion path of the individual molecules ( microscopic measurement by quasielastic neutron scattering (QENS) [48,183,184], nuclear magnetic relaxation and line-shape analysis, PFG NMR) or by introducing differently labelled (but otherwise identical) molecules into the sample and monitoring their equilibration over the sample ( macroscopic measurements by tracer techniques) [185,186]. The process of molecular movement studied under such conditions is called self-diffusion. 

The diffusion coefficient can also be determined from measurements of other phenomena that are controlled by the activated motion of atoms. These indirect methods include internal friction measurements, nuclear magnetic resonance spectra, and some magnetic relaxation phenomena (in ferromagnetic substances). These techniques are advantageous in allowing the measurement of D at lower temperatures than are practicable by the conventional methods. 

The above properties and phenomena can be assessed with great sensitivity and precision by the measurement of rotational diffusion usually based upon the combined use of polarized excitation and deactivation processes. The faster motions alluded to above are particularly well adapted to the techniques of nuclear magnetic relaxation and fluorescence depolarization, the formalisms for which are extensively documented (references 2 and 3, respectively, and citations therein other chapters in this volume). Optical anisotropy decay measurements with longer time resolution have been very effective in studies of biological and model membrane systems (reviewed in 4-6). 

A second steady-state method involves the analysis of the broadening of the nuclear magnetic resonance spectra of phospholipids in bilayers containing low concentrations of spin-labeled phospholipids. A theoretical analysis of the relation between this line broadening and diffusion rates has been given by Brulet and McConnell.3 [In this paper (6) is not correct the subsequent equations are nonetheless correct. For an alternative derivation, see Brulet.2] In this paper it is shown that a number of measurements of nuclear relaxation rates T71 of nuclei in phospholipids are consistent with lateral diffusion constants in the range 10 7 to 10 R cm2/s. 

Abstract We use Nuclear Magnetic Resonance relaxometry (i.e. the frequency variation of the NMR relaxation rates) of quadrupolar nucleus ( Na) and H Pulsed Gradient Spin Echo NMR to determine the mobility of the counterions and the water molecules within aqueous dispersions of clays. The local ordering of isotropic dilute clay dispersions is investigated by NMR relaxometry. In contrast, the NMR spectra of the quadrupolar nucleus and the anisotropy of the water self-diffusion tensor clearly exhibit the occurrence of nematic ordering in dense aqueous dispersions. Multi-scale numerical models exploiting molecular orbital quantum calculations, Grand Canonical Monte Carlo simulations, Molecular and Brownian Dynamics are used to interpret the measured water mobility and the ionic quadrupolar relaxation measurements. 

The dynamic characteristics of adsorbed molecules can be determined in terms of temperature dependences of relaxation times [14-16] and by measurements of self-diffusion coefficients applying the pulsed-gradient spin-echo method [ 17-20]. Both methods enable one to estimate the mobility of molecules in adsorbent pores and the rotational mobility of separate molecular groups. The methods are based on the fact that the nuclear spin relaxation time of a molecule depends on the feasibility for adsorbed molecules to move in adsorbent pores. The lower the molecule s mobility, the more effective is the interaction between nuclear magnetic dipoles of adsorbed molecules and the shorter is the nuclear spin relaxation time. The results of measuring relaxation times at various temperatures may form the basis for calculations of activation characteristics of molecular motions of adsorbed molecules in an adsorption layer. These characteristics are of utmost importance for application of adsorbents as catalyst carriers. They determine the diffusion of reagent molecules towards the active sites of a catalyst and the rate of removal of reaction products. Sometimes the data on the temperature dependence of a diffusion coefficient allow one to ascertain subtle mechanisms of filling of micropores in activated carbons [17]. 

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