Hydrogen network fluctuations and dielectric spectrometry of liquids

Complex dielectric spectra of associating liquids are presented and are discussed considering their relaxation properties. Particular emphasis is given to water and aqueous solutions and to alcohols and mixtures of alcohols with dipolar and non-polar solvents. The relaxation properties of the associating systems are examined in the light of a wait-and-switch model of dielectric relaxation in which the relaxation time is governed by the period for which a given ensemble of hydrogen bond partners within the hydrogen network has to wait until favorable conditions for a reorientation of a molecular permanent dipole exist. These conditions are provided by an additional molecule or group which firstly tends to lower the potential energy barrier for reorientation and which secondly, at the same time, offers a site for the formation of a new hydrogen bond. Some experimental findings are discussed in terms of the wait-and-switch model, among them the reduced dielectric relaxation time of water tinder hydrostatic pressure, effects of negative and hydrophobic hydration of ions, and the influence that water, on the one hand, and n-alkanes, on the other hand, act on the relaxation of alcohols. (C) 2002 Elsevier Science B.V. All rights reserved.