Dielectric Properties of Aqueous NaCl Solutions at Microwave Frequencies

At 20 °C the complex (electric) permittivity of aqueous solutions of sodium chloride has been measured as a function of frequency ν (20 MHz ≤ ν ≤ 40 GHz) and salt content (0.003 ≤ y ≤ 0.035; y, 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 ε(0) and the principal relaxation time τs of the solutions. At c > 0.2 mol/L the extrapolated permittivity follows the linear relation ε(0) = εw(0) (1 − 0.19 (mol/L)-1 c) where εw(0) denotes the static permittivity of water at 20 °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 m. The relative molal shift in τs appears to be stronger at 20 °C (−(0.15 ± 0.06) (mol/kg)-1) than at higher temperatures (−(0.08 ± 0.03) (mol/kg)-1, 25 °C). Mechanisms that might lead to a reduction of the relaxation time on addition of salt are briefly presented.