NMR in gels and porous media

2 Nuclear relaxation of solvent imbibed in porous materials 

Though confinement of fluids within random porous media can lead to drastic changes in their physical properties which are not very well known, the observation of an exponential nuclear relaxation of the longitudinal or transverse magnetization 

Similar equations arise for the transverse relaxation rate l/7 2 . The problem now is to extract the pore distribution, W , from the observed RJ,t). From a mathematical point of view, this could be done by a Laplace inversion of Eqn (28.3). Examples of this method can be found in the review paper of W. P. Halperin et al. Other methods have used a prerequisite distribution that must be verified a posteriori. This has been carried out recently for the nuclear relaxation of and of methanol and nitromethane adsorbed on an organic polymeric resin crosslinked by paramagnetic divalent metal ions (Fig. 28.2). The results have been interpreted with a fractal distribution of categories of quasi-disconnected spherical pores, each being composed of N spherical pores of radius R = RJ f (a 1), with = log(/ o/ n.in)/log Introducing iht fractal dimension Df through the relation N a , leads to 

The continuous line has been calculated with = 10 ms and Tit = 95 ms , (b) Time dependence of the transverse magnetization at 46 MHz for deuterated nitromethane adsorbed on organic polymeric resin crosslinked by paramagnetic VO and diamagnetic Cd ions. The continuous line has been calculated with Tj, = 0.9 ms and Tjb =11 ms . 

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Gibbs, S.J., E.N. Lightfoot, and T.W. Root, Protein diffusion in porous gel filtration chromatographic media studied by pulsed field gradient NMR-spectroscopy. Journal of Physical Chemistry, 1992, 96, 7458-7462. 

Webber, J.B.W., Anderson, R., Strange, J.H., and Tohidi, B., 2007a. Clathrate formation and dissociation in vapour/water/ice/hydrate systems in SBA-15, Sol-Gel and CPG porous media, as probed by NMR relaxation, novel protocol NMR cryoporometry, neutron scattering and ab-initio quantum-mechanical molecular dynamics simulation. Magn. Reson. Imag. 25 533-536. 

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