Ion size effects on the dynamic and static dielectric properties of aqueous alkali solutions

Abstract

Dielectric spectroscopy studies of aqueous ionic solutions ACl/H, 0 (A = Li, Rb, Cs) were carried out at frequencies between 45 MHz and 20 GHz, using recently developed coaxial measurement techniques. The behavior of the static dielectric constant es0 and the dielectric relaxation time Q-, of the solutions were studied as functions of ion size and concentration. For moderate concentrations both es0 and r, decrease linearly with solution conductivity. While the behavior of gso can be understood in terms of either static or Hubbard-Onsager kinetic polarization models, the experimental results for 7. are at present not understood quantitatively in terms of these models. However we point out the good correlation of the 7. data with empirical viscosity results, which suggests an alternative explanation based upon the solution viscosity, modified by ion size effects, which play an important role in the dielectric response. We also discuss the various length scales relevant to dielectric and conductivity processes in the solutions.Dielectric spectroscopy can be a powerful probe of microscopic processes in a liquid, particularly when carried out over a broad range of frequency. The frequency dependence of the dielectric function e(w) of a liquid (pure, solution or mixture), reflects the dynamic response of the constituents of the liquid, whether ionic or dipolar.'-3 An ionic solution may be considered to consist of 3 constituents: the charged anions and cations, "hydration" solvent molecules near the vicinity of the ions, and "free" solvent molecules. At high concentration, an additional component may exist in the form of ion pairs due to incomplete dissociation.4'5 The distinction between the hydration and free solvent molecules is largely a matter of the time scales involved, and the strength of the ion-solvent interaction. Furthermore, the dynamical processes are affected by the ion size and charge. These aspects are some of the issues addressed in this work.While ionic conductivity and viscosity has been the subject of intense research,6 the dynamic aspects of the dielectric properties of ionic solutions have received less attention. Certain phenomena, such as the (static) dielectric decrement, are well known.' Reliable experimental data on the dynamic aspects, such as the relaxation time TV, have not been easily available, partly because of the higher frequencies required ( -10 GHz) particularly for aqueous solutions. In this paper we present detailed studies and analysis of the dielectric spectra of AC1 (A = Li, Rb, Cs) in H,O. The complex dielectric functions E(O) = E)(w)k" (w) were measured between 45 MHz and 20 GHz at 25 C, using recently developed techniques8*9 which enables the measurement of continuous spectra in this frequency range. E' and (T = E"WE" (E, = 8.85 X 10 -'* F/m) vs frequency for RbCl/H,O for various concentrations are shown in Figs.

Dielectric Constant of Ionic Solutions: A Field-Theory Approach

“…When the Debye length κ −1 D is of the same order of magnitude as a (high salt limit), the last term in ∆ε s starts to dominate and the dielectric decrement becomes smaller until eventually it will reverse the trend and may even cause a dielectric increment as seen in experiments [16] in the high salt limit.…”

“…In dilute solutions the dielectric constant depends linearly on c s , the salt concentration, ε(c s ) = ε w − γc s , where γ is the coefficient of the linear term. The value of γ is ion specific and ranges from 8 M −1 to 20 M −1 , for concentrations up to 1.5 M. At higher c s values, noticeable deviations from linearity are observed and are due to ion-ion interactions [15][16][17].…”

“…(16) with experimental data from Ref. [16], as function of ionic concentration, cs, for various salts. The theoretical prediction (solid line) was calculated using a as a fitting parameter.…”

“…(16), to experimental values [16] for seven different ionic solutions in concentration range of 0-6 M. We separate the seven salts into three subgroup according to the size of the alkaline cations as presented in the three parts of Fig. 2.…”

“…Hence, in order to model the experimentally known dielectric decrement phenomenon [7,[15][16][17], one needs to employ more refined theories, which take into account ion-dipole correlations and fluctuations. The electric decrement stems from the fact that the local electric field around each ion is greater than the external field, and orients the dipolar water molecules in its vicinity.…”