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

Sensitive and robust electrophoretic NMR: instrumentation and experiments

Although simple as a concept, electrophoretic NMR (eNMR) has so far failed to find wider application. Problems encountered are mainly due to disturbing and partly irreproducible convection-like bulk flow effects from both electro-osmosis and thermal convection. Additionally, bubble formation at the electrodes and rf noise pickup has constrained the typical sample geometry to U-tube-like arrangements with a small filling factor and a low resulting NMR sensitivity. Furthermore, the sign of the electrophoretic mobility cancels out in U-tube geometries. We present here a new electrophoretic sample cell based on a vertically placed conventional NMR sample tube with bubble-suppressing palladium metal as electrode material. A suitable radiofrequency filter design prevents noise pickup by the NMR sample coil from the high-voltage leads which extend into the sensitive sample volume. Hence, the obtained signal-to-noise ratio of this cell is one order of magnitude higher than that of our previous U-tube cells. Permitted by the retention of the sign of the displacement-related signal phase in the new cell design, an experimental approach is described where bulk flow effects by electro-osmosis and/or thermal convection are compensated through parallel monitoring of a reference signal from a non-charged species in the sample. This approach, together with a CPMG-like pulse train scheme provides a superior first-order cancellation of non-electrophoretic bulk flow effects.

Electrophoretic NMR studies of polymer and surfactant systems

The aim of this tutorial review is to introduce to a broader readership the emerging technique of electrophoretic NMR (eNMR). The "electrophoretic" element of the technique refers to the fact that charged particles are induced to flow by the application of an electric field. This flow is measured using pulsed-gradient spin-echo NMR (PGSE-NMR). The great potential of this experimental approach is the fact that NMR is chemically selective and non-invasive. eNMR, especially when combined with the more established PGSE-NMR experiment, may therefore be used to quantify the structure of multi-component systems via the dynamics and charge of each species within a complex mixture. Accordingly, eNMR is likely to be of great significance for colloid scientists, biologists, technologists and formulation scientists.

Diffusion in porous building materials with high internal magnetic field gradients

Measuring the water diffusivity in porous building materials with NMR is hindered by the presence of large internal magnetic field gradients originating from magnetic impurities (Fe). To investigate the diffusion of water in these materials, a stimulated echo NMR technique is applied. A new analytical equation for the long-time signal decay in the presence of spatially varying internal field gradients is derived. This equation is experimentally confirmed by measurements on representative materials with large internal gradients (fired-clay brick and sintered crushed glass) and a material with very small internal gradients (glass filter). The diffusivity is determined in the long time limit, where it is constant and limited by the tortuosity of the pore structure. Tortuosities of different samples derived from the NMR data show an excellent agreement with the macroscopic tortuosities measured by electrochemical impedance spectroscopy. The developed technique can also be applied in unsaturated media, during e.g., drying, water absorption, and concentration changes. The characteristic length scales of the internal field fluctuations estimated from the model are compared with the structural length scales, whereas the magnitude of these fluctuations is compared with results of macroscopic magnetization measurements.