Orientational relaxation in hydrogen-bonded systems: aqueous solutions of electrolytes

Abstract

A model of the rotational motion of water molecules within ion hydration sheaths is proposed in the present paper. This model is based on the concept of a ‘boundary’ ion, according to which the parameters of the structural surroundings, as well as the translational, rotational and librational motions of water molecules within the first sphere, are the same as those for water molecules in water. The wide-band absorption spectra, α(ν)(0 < ν/cm–1 < 400), and the complex dielectric permittivity spectrum measured for the concentrated electrolyte solutions, are described on the basis of the model proposed in this paper. It is shown that the confined rotator/extended diffusion (CR/ED) model, which has been advanced previously for liquid water, is applicable for spectral calculations of the electrolyte solutions studied in the present paper.The typical characteristics of ions having hydrophilic and hydrophobic hydration, as well as the distinctions between the hydration sheaths of ions with negative and positive hydrations, are discussed. A new method is proposed to study the molecular nature of these phenomena at the level of elementary processes. The complex dielectric permittivity of the concentrated LiCl, NaCl, Nal, KCl and KBr solutions has been measured in the range 7–25 GHz at 25 °C. For electrolyte solutions, the wide band (0 ⩽v/cm–1⩽ 1000) dielectric spectra and the parameters of the proposed model are calculated.