Probing water-solid interactions

Probing Water-Solid Interactions in Crystalline and Amorphous Systems Using Vibrational Spectroscopy... 

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

WISE) solid-state NMR technique has been used to probe water-starch interactions on the molecular distance scale (Kulik et al., 1994). 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

The value of the NMR spin-lattice relaxation time in each of the pixels of an image may be converted to a pore size by the adoption of a relaxation model. For a liquid imbibed in a pore space the relaxation rate is enhanced. This is believed to be due to interactions between a thin layer of liquid and the solid matrix at the solid/liquid interface increasing the relaxation rate. There is then also difllisional exchange between this surface-affected layer and the rest (bulk) of the liquid in the rest of the pore. In the case here where the pore sizes are several orders of magnitude smaller than the rms displacement of the probe water molecules employed, the "two-fraction fast-exchange" model of Brownstein and Tarr [7] will be used, where the overall measured value of T, is given by ... 

Heat Capacity. Measurements of the heat capacity of protein systems are particularly interesting for several reasons 1) they reflect solvation of nonpolar elements in addition to other parts of the protein surface and thus can be viewed as the most complete thermodynamic probe for water-protein interactions. 2) Heat capacity can be measured conveniently for both solution and solid samples thus the two categories of protein hydration studies, those on solutions and those on solid samples, can be correlated. 3) There is a substantial literature on the heat capacities of small molecules and on additivity relationships, which appear to be more accurate for heat capacity than for other thermodynamic functions. 

By their nature, and in contrast with microscopic and scattering techniques that are used to elucidate long-ranged structure, spectroscopic methods interrogate short-range structure such as interactions between fixed ions in side chains and counterions, main chain conformations and conformational dynamics, and the fundamental hopping events of water molecules. The most common methods involve infrared (mid-IR and to a much lesser extent near- and far-IR) and solid-state NMR spectroscopies, although other approaches, such as molecular probes, have been utilized. 

Solubilization of water. Detergency is defined as the ability of surfactant molecules to solubilize water molecules or polar substances in soft-core and hardcore RMs. Thus, micellization and solubilization are competitive processes. Any solubilized probe molecule causes a decrease in the CMC. Solubilization describes the dissolution of a solid, liquid or gas by an interaction with surfactant molecules. Addition of water has a dramatic effect on surfactant aggregation in hydrocarbons because hydrogen bonding has an appreciable stabilizing effect on reverse micelles. Solubilization for reverse micelles is phenomenologically similar to the adsorption processes (Eicke and Christen, 1978 Kitahara, 1980 Kitahara et al., 1976 Singleterry, 1955). 

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