Dielectric Relaxation of Aqueous NaCl Solutions

The complex dielectric permittivity of aqueous sodium chloride solutions has been determined in the frequency range 0.2 ≤ ν(GHz) ≤ 20 with a commercial dielectric measurement system based on a vector network analyzer. NaCl solutions 0.1 ≤ m (mol kg^-1) ≤ 5 (mass fraction 0.005 ≤ w ≤ 0.23) were investigated at 5, 20, 25, and 35 °C. An improved calibration procedure of the dielectric measurement system for conducting samples was developed. The complex permittivity spectra have been represented by a Cole−Cole relaxation time distribution. Where possible, the obtained fitting parameters, static permittivity ε and relaxation time τ, and distribution parameter α, are compared with literature data to assess the performance of the instrument, which was found to be comparable to that of time domain and waveguide systems. Effective solvation numbers were deduced from the effect of NaCl concentration on ε. The data suggest that in addition to the irrotational bonding of water molecules by Na⁺ ions, kinetic depolarization under slip boundary conditions determines the solution permittivity. A three-state model is proposed to describe the concentration dependence of τ.