Nuclear magnetic resonance studies of evolving porous media

Abstract

Porous media are inescapable, occurring naturally as soil, rocks and plants, as well as in human-made materials including cement, ceramics and paper. Broadly, porous media describes systems with holes or voids in a solid-like matrix, which are inherently heterogeneous and often opaque, making characterization difficult. The structure of porous media may also change with time or with environmental exposure. For example, ice crystals in snow coarsen with age, and above 32°F snow melts. To characterize changes in porous media, measurements often need to be rapid and nondestructive. Time domain 1 H nuclear magnetic resonance (NMR) provides a noninvasive, nondestructive and potentially rapid approach to characterize porous media. NMR relaxometry measures heterogeneity in 1 H molecular environments through T 1 and T 2 relaxation distributions. Diffusometry measures diffusion coefficients of 1 H populations. Emergence of benchtop NMR spectrometers has expanded accessibility of these techniques, yet their full potential is often underutilized. This work advances porous media characterization methodologies, studying two evolving porous media systems with NMR. First, NMR relaxometry and diffusometry were applied to study changing water populations from adsorption and processing in biomass. Baseline water adsorption profiles of corn stover were established with both high and low, i.e. benchtop, magnetic field strength NMR spectrometers. Corn stover was then pretreated to improve processability, which was characterized as a function of drying. Next, the structural breakdown of corn stover during processing was observed. Finally, saturated biomass was monitored for 10+ days to characterize evolution due to hydration and to assess stability as biocomposite additives in high moisture environments. Time domain NMR also characterized phase change materials (PCMs) during melting. Alkane waxes can be used for thermal energy storage but in PCM preparation these materials are microencapsulated. This restriction on the solid to liquid phase transition was studied by comparing NMR data on these restricted PCMs to unrestricted waxes. A broadening of the melting temperature range was measured in the PCMs. This research demonstrates novel application of NMR relaxometry and diffusometry to evolving porous media systems. This work builds on NMR methodologies to study hydration, degradation and phase change transitions in porous media, particularly biomass systems.