Reorientation water-solid interactions

Unfortunately, the usefulness of NMR for the investigation of chemical problems was strictly limited to liquid samples, so solid samples first had to be dissolved or melted. This is because of the anisotropic nuclear interactions which strongly depend on molecular orientation, and are therefore averaged by molecular motion. In liquids, the molecules reorient randomly very quickly a water molecule requires ca. 10 s for complete reorientation. Although certain solids have sufficient molecular motion for their NMR spectra to be obtainable without resorting to special techniques, in the general case of a true solid, there is no such motion, and conventional NMR, instead of sharp spectral lines, yields a broad hump which conceals most information of interest to chemists. For example, the width of the H NMR resonance in the spectrum of water is ca. 0.1 Hz, while the line from a static sample of ice is ca. 100 kHz wide, i.e., a million times broader. Andrew et al. [ 12], and independently Lowe [ 13], had the idea of substituting the insufficient molecular motion in solids for the macroscopic rotation of the sample. 

Mobility of water in cellulose has been studied by solid-state and high-resolution NMR as a function of moisture content within the unfreezable moisture range (0-19% dry basis).Measurements of relative mobilities were based on relative intensities, transverse and longitudinal relaxation times and line shape analysis. At 2-16% moisture content (dry basis), water molecules reoriented anisotropically, suggesting an interaction with cellulose fibers. At moisture content below the monolayer value (2.8%, dry basis), 90% of the protons were immobile and no liquid deuterium signal was detected. A sharp increase in liquid or mobile intensity (accompanied by a decreased LW) and increases in NMR Ti and T2 relaxation times were observed as moisture increased above 9% (dry basis). 

For example, the solid can swell in contact with a certain liquid or even interact by chemical interfacial reactions it can also be partially dissolved. In the case of polymer surfaces, the molecular reorientation in the surface region under the influence of the liquid phase is assumed to be a major cause of hysteresis. This reorientation or restructuring is thermodynamically favoured at the polymer-air interface, the polar groups are buried away from the air phase, thus causing a lower solid-vapour interfacial tension. In contact with a sessile water drop, the polar groups turn over to achieve a lower solid-liquid interfacial tension. Time-dependent changes in contact angles can also be observed (33). 

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