Heterogeneous systems, nuclear magnetic resonance

Analytical techniques are conveniently discussed in terms of the excitation-system-response parlance described earlier. In most cases the system is some molecular entity in a specific chemical environment in some physical container (the cell). The cell is always an important consideration however, its role is normally quite passive (e.g., in absorption spectroscopy, fluorescence, nuclear magnetic resonance, electron spin resonance) because the phenomena of interest are homogeneous throughout the medium. Edge or surface effects are most often negligible. On the other hand, interactions between phases are the central issue in chromatography and electrochemistry. In such heterogeneous techniques, the physical characteristics of the sample container become of critical... 

Stemlicht H, Kenyon GL, Packer EL, Sinclair J, Carbon-13 nuclear magnetic resonance studies of heterogeneous systems. Amino acids bound to cationic exchange resins, J. Am. Chem. Soc., 93 199-209, 1971. 

To determine the nature of the silicon moieties in a polymer, clearly the easiest method would be a technique that provides a direct observation of the silicon atom and meaningful, interpretable information on the atom. Nuclear magnetic resonance spectroscopy tuned to the Si isotope ( Si NMR) is a tool of this nature it can directly probe the state of the silicon atom, and with it one can often readily determine the extent to which Si-O-Si crosslinks (fi-om silanol condensation), have formed. One can observe spectra of silicon-containing compounds either dissolved in a solvent or in the solid state. Liquid-state Si NMR, while the most sensitive, cannot be used quantitatively on heterogeneous systems such a latex formulations. Therefore, one must separate the liquid and solid portions of the latex (without heat, which would promote hydrolysis and condensation) and use the solid residue for the Si NMR experiments. 

Both the above-described assumptions seem to provide a qualitatively satisfactory description of the order parameter s continuous temperature profile in LCEs. It is likely that, in real systems, the two mechanisms go hand in hand. However, the remaining questions are which one prevails and can the properties of LCEs be tailored so that either the heterogeneous or the supercritical nature is promoted Only recently, has this ambiguity been satisfactorily resolved by combining nuclear magnetic resonance and high-resolution calorimetry [3,4]. It is the purpose of this chapter to discuss the approach used in these studies, with particular emphasis on an analysis of the experimental NMR data. 

Previously we discussed the findings obtained by the method of nuclear magnetic resonance and dielectric relaxation, related to heterogeneous systems with organic fillers. 

Measurements of spin-lattice relaxation rate as a function of the magnetic field/resonance frequency are commonly referred to as nuclear magnetic relaxation dispersion, NMRD. Measurements of this kind, when performed over a broad range, are an invaluable source of information on frequency-resolved molecular motions in complex biological or colloidal systems. Several years ago, Halle proposed a theory for relaxation of quadrupolar spins in dynamically heterogeneous systems... 

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