Electrophoretic NMR studies of electrical transport in fluid-filled porous systems

An NMR technique is described which allows the observation of ionic charge carriers moving in the electric field within a porous system saturated with electrolyte solution. This method, which was recently developed in our laboratory, gives experimental access to the study of electric transport in disordered media on a microscopic level and offers new potential for morphology studies. We performed H-1 NMR PFG self-diffusion measurements on ions combined with ionic drift velocity measurements by electrophoretic NMR (ENMR): each as a function of observation time Delta. In this way we obtained time-dependent self-diffusion coefficients D+/- (Delta) and time-dependent electric mobilities mu (+/-)(Delta) of polyatomic cations and anions in porous media. The porous media used were gels and glass bead packs. From the behaviour of D+/- (Delta) and mu (+/-) (Delta) at long observation times the tortuosities T-p (D+/-) and T-p (mu (+/-)) are derived, allowing a direct experimental check of the validity of the Einstein relation (D+/- proportional to mu (+/-)) in a disordered medium. The tortuosities obtained via the diffusivity of ions are compared with those obtained via the diffusivity of water molecules. We also make a first attempt to derive the specific surface SN, from the time-dependence of the ionic mobility at short observation times and discuss possible advantages of those measurements in morphology studies of porous media. (C) 2001 Elsevier Science Inc. All rights reserved.