Dielectric properties of aqueous NaCl solutions at microwave frequencies

At 20 degrees C the complex (electric) permittivity of aqueous solutions of sodium chloride has been measured as a function of frequency nu (20 MHz less than or equal to nu less than or equal to 40 GHz) and salt content (0.003 less than or equal to gamma less than or equal to 0.035; gamma, mass fraction of NaCl). The dielectric part of the spectra has been represented by the semiempirical Cole-Cole relaxation spectral function to yield the extrapolated low-frequency (static) permittivity epsilon(0) and the principal relaxation time tau(s) of the solutions. At c > 0.2 mol/L the extrapolated permittivity follows the linear relation epsilon(0) epsilon(w)(0) (1 - 0.19 (mol/L)(-1) c) when epsilon(w)(0) denotes the static permittivity of water at 20 degrees C. This dielectric decrement is related to the effects of dilution of the dipolar solvent, of kinetic depolarization, and of structure saturation. At lower NaCl concentration (c < 0.2 mol/L) small indications for an additional polarization mechanism are found. The principal dielectric relaxation time linearly decreases with the solute molality in. The relative molal shift in tau(s) appears to be stronger at 20 degrees C (-(0.15 +/- 0.06) (mol/kg)(-1)) than at higher temperatures (-(0.08 +/- 0.03) (mol/kg)(-1), 25 degrees C). Mechanisms that might lead to a reduction of the relaxation time on addition of salt are briefly presented.