Solid water dielectric relaxation

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

Thus, specific interactions directly determine the spectroscopic features due to hydrogen bonding of the water molecules, while unspecific interactions arise in all or many polar liquids and are not directly related to the H-bonds. Now it became clear that the basis of four different processes (terms) used in Ref. [17] and mentioned above could rationally be explained on a molecular basis. One may say that specific interactions are more or less cooperative in their nature. They reveal some features of a solid state, while unspecific interactions could be understood in terms of a liquid state of matter, if we consider chaotic gas-like motions of a single polar molecule, namely, rotational motions of a dipole in a dense surroundings of other molecules. The modem aspect of the spectroscopic studies leads us to a conclusion that both gas-like and solid-state-like effects are the characteristic features of water. In this section we will first distinguish between the following two mechanisms of dielectric relaxation ... 

In addition to the more usual application to solids, dielectric relaxation or dispersion measurements are also used on solutions (and pure liquids). Cook (425) related the relaxation mechanism in water-dioxane mixtures to the rupture of H bonds. Hasted and co-workers (890) found that water-dioxane mixtures had longer relaxation times as the dioxane proportion increased or the temperature was lowered. Both trends are explained by formation of a H bonded complex. Yasumi (2219) found similar effects when large amounts of hexane... 

Fig. 4.8 Plot of 8out against 8 using dielectric relaxation data for water in the frequency range 60 10 GHz [G5]. The solid line shows the contribution from the low-frequency relaxation process.

Solids and dipole relaxation of defects in crystals lattices Molecules which become locked in a solid or rigid lattice cannot contribute to orientational polarization. For polar liquids such as water, an abrupt fall in dielectric permittivity and dielectric loss occur on freezing. Ice is quite transparent at 2.45 GHz. At 273 °K, although the permittivity is very similar (water, 87.9 ice, 91.5) the relaxation times differ by a factor of 10 (water, 18.7 x 10 s ice, 18.7 x 10 s). Molecular behavior in ordinary ice and a feature which may be relevant to a wide variety of solids has been further illuminated by the systematic study of the dielectric properties of the nu-... 

Jonquieres, A. and Fane, A. 1998. Modified BET models for modeling water vapor sorption in hydrophilic glassy polymers and systems deviating strongly from ideality. J. Appl. Polym. Sci. 67 1415—1430. Jonscher, A.K. 1983. Dielectric Relaxation in Solids. London, U.K. Chelsea Dielectric Press. 

NMR is another useful means of measuring molecular mobility of freeze-dried formulations. The spin-lattice relaxation time in the laboratory frame (Ti) and the spin-spin relaxation time T2) of H, H, or have been used to represent the mobility of water and polymer molecules in freeze-dried cakes or aqueous polymer solutions [18-22]. In contract to DSC or dielectric relaxation spectrometry, NMR allows identification of the origin of molecular motion. Determining molecular mobility for each drug and excipient in a freeze-dried formulation is therefore possible when high-resolution solid-state NMR is used. 

Extraneous molecules in solid phase polymer systems are not limited to plasticizer molecules or even exclusive to substances deliberately added. Impurities wdien present often affect the dielectric behaviour of pol mers and water in particular often has very significant effects on the dielectric spectrum. Poly(niethyl methacrylate) poly(oxymethylene) , and nylons to mention a few are influenced by moisture in this way. The influence of moisture on dielectric relaxation can be the result of interfacial polarization as well as dipolar mechanism. Further, this complication is not restricted to additives such as water but may occur whenever a combination of phase boundary and bulk or sur ce conductivity to or over the botmdaiy can take place. The proof that a relaxaticu is the result of interfacial polarization is not easy to establish, but there is evidence that mie of the relaxations in nylons and pol3 urethanes) are of this type. As expected, conductive fillers will introduce interfacial polarization and this effect has been well documented, especially in carbon filled rubbers . Indeed, as we shall disci later, electronic conductance when localized by interfacial boundaries does result in a form of interfacial polarization. Here, because of its large magnitude the phenomenon has been termed hyperelectronic polarization. 

Figure 41. Typical dielectric spectra of 20 mol% of glycerol—water mixtures at (a) 185 K (supercooled state) and (b) 218 K (frozen state), where solid and dashed curves show the real and imaginary parts of complex dielectric permittivity. Each relaxation process in the frozen state was fitted by (114) and by Cole-Cole and Debye relaxation functions, respectively, in order to separate the main process, the process due to interfacial water, and the process due to ice. (Reproduced with permission from Ref. 244. Copyright 2005, American Chemical Society.)...

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