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

A simple model of dielectric response due to the three-dimensional motion of ions inside a spherical ideally reflecting sheath is suggested. The present model termed the sphere-confined ionic (SCI) model is combined with the so called hybrid model, previously used to describe dipolar orientational relaxation [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) spectra 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 polar H2O molecules. A modification of this model is also suggested, in which the walls may also vibrate. The parametrisation of the ionic model is made taking account of the experimental value sigma (s) of the static conductivity; the ionic contributions sigma (+/-)(s) due to anions and cations are calculated, as well as ionic self-diffusion coefficients and mobilities. The positive (for NaCl) and negative (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 relatively low electrolyte concentration. If the frequency omega is much less than the ionic plasma frequency omega (p), then the real part sigma' of the 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.