Nuclear magnetic resonance studies of water self-diffusion in porous systems

Abstract: Proton nuclear magnetic resonance (NMR) was used to study the self-diffusion of water in porous systems that respond to a change in water content in order to elucidate the porous structure and the properties of the confined water. In the carbohydrate systems cellulose fibers and starch granules, water is free to move throughout the porous objects, albeit with a rate reduced from the value of the bulk liquid, The reduction is related to the tortuosity of the pore space filled by water. A decrease of water content leads to a contraction of the porous network and an increased tortuosity. The anisotropic arrangement of the structural elements leads to an anisotropic water diffusion which can be quantified using high-quality NMR self-diffusion data and an adequate model for data analysis. Experiments performed at temperatures below the bulk freezing point showed that nonfreezing water exists as object-spanning films with a thickness of at least a few molecular diameters. Cross relaxation between protons in liquid and solid domains was found to be crucial for the interpretation of the diffusion data. A technique to extend the range of experimental time scales for diffusion was demonstrated on a concentrated water-in-oil emulsion. A numerical method to compare experimental frequency-domain and theoretical time-domain apparent diffusion coefficients was proposed. The Fourier relation between the signal obtained with the NMR self-diffusion experiment and the structure of the pore occupied by water was utilized for a model system consisting of a single water film. Methods analogous to the interpretation of X-ray diffraction data was used for the retrieval of the pore shape from the experimental data.