Calculation of the microwave and far-infrared spectra of aqueous electrolyte solutions from a molecular model. On the ionic-conductivity dispersion sigma(omega)

A simple model of dielectric response due to the three-dimensional motion o f ions inside a spherical ideally reflecting sheath is suggested. The prese nt model termed the sphere-confined ionic (SCI) model is combined with the so called hybrid model, previously used to describe dipolar orientational r elaxation [see Dielectric Relaxation and Dynamics of Polar Molecules by V. I. Gaiduk (World Scientific, Singapore, 1999)]. The wideband (up to 1000 cm (-1)) complex permittivity epsilon(omega) and absorption alpha(omega) spect ra of NaCl-water and KCl-water diluted solutions are calculated as the sum of the contributions due to cations and anions and to reorientation of pola r H2O molecules. A modification of this model is also suggested, in which t he walls may also vibrate. The parametrisation of the ionic model is made taking account of the experi mental value sigma (s) of the static conductivity; the ionic contributions sigma (+/-)(s) due to anions and cations are calculated, as well as ionic s elf-diffusion coefficients and mobilities. The positive (for NaCl) and nega tive (for KCl) hydration are described in terms of respectively increasing and decreasing tau (C-M) concentration dependencies of the lifetimes fitted for water molecules. The frequency dependence of the electric conductivity is estimated for a re latively low electrolyte concentration. If the frequency omega is much less than the ionic plasma frequency omega (p), then the real part sigma' of th e complex ionic conductivity is close to its static value sigma (s), while the imaginary part sigma " is close to zero. In the high-frequency region ( at millimetre/submillimetre wavelengths) the (C) 2000 Elsevier Science B.V. All rights reserved.