Nuclear Magnetic Resonance Methods to Study Soft Matter Dynamics and Microstructure
Abstract: Soft matter is of great importance for food, hygiene and pharmaceutical products. Gels are an example of soft matter and are found in e.g. diapers, that are able to take up large amounts of liquids. Translational motion such as diffusion and/or flow is the fundamental mechanism for transport of solutes or solvents e.g. within gels and is strongly dependent on their microstructure. In addition, interactions e.g. solutes binding to the gel surface, may alter diffusion in soft matter. In order to tailor-made materials, it is necessary to have a detailed knowledge of how microstructure and interactions influences mass transport in soft matter. Different nuclear magnetic resonance methods were applied to monitor mass transport in a variety of microstructures during steady and non-steady state conditions. Water dynamics were monitored during capillary formation in alginate gels using 1H magnetic resonance imaging. This method allowed the water at the gelation front, where the growth of capillaries occurred, to be followed as a function of time. It was found that there are enhanced water dynamics at the gelation front and that the rate of gel formation occurs faster and produces a denser gel in the case of capillary formation. 1H and 19F chemical shift imaging was applied to study the dynamics of water and fluoride ions dynamics during gelation of a non-aqueous cellulose solution, which forms a gel upon addition of water. The rate of gelation was followed tracking the water signal as a function of time and position. 3D 1H diffusion tensor imaging combined with 3D 2H spectroscopic information mapped the microstructure of shear-induced multi-lamellar vesicles as a function of time. Furthermore, the half-life time could be estimated in a spatially resolved manner and it was found that shear-induced multi-lamellar vesicles seems to decay faster close to surfaces. The interaction of water with silica surfaces was studied with 17O relaxation measurements as a function of amount silica and salt. Upon the addition of salt, a gel is formed and a reduced amount of surface “bound” water is reported. The information obtained was then used to rationalize the diffusion of water in this systems in the framework of the cell model.
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