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 1H 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(+/-) is 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 S/V(p) 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.
Holz
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Institut für Physikalische Chemie, Universität Karlsruhe, D-76128, Karlsruhe, Germany. Manfred.Holz@chemie.uni-karlsruhe.de
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D017136:Ion Transport; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation / Q000379:methods*; D055585:Physical Phenomena; D010825:Physics; D016062:Porosity; D055672:Static Electricity; D013997:Time Factors