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

Abstract

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 Δ. In this way we obtained time-dependent self—diffusion coefficients D ± (Δ) and time-dependent electric mobilities μ ± (Δ) of polyatomic cations and anions in porous media. The porous media used were gels and glass bead packs. From the behaviour of D ± (Δ) and μ ± (Δ) at long observation times the tortuosities T p (D ±) and T p (μ ±) are derived, allowing a direct experimental check of the validity of the Einstein relation (D ± ∝ μ ±) 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.