Dielectric relaxation in non aqueous solutions. Part 6.—Effect of dipole-dipole interactions on the dielectric relaxation time of solutions of tri-(n-butyl)ammonium picrate in p-xylene

Abstract

A simple electrostatic theory has been developed, which attempts to explain the concentration dependence of dielectric relaxation times (τ) in terms of dipole-dipole forces. The theory has been used to interpret some experimental dielectric dispersion data obtained in the frequency range 0.3 to 3.5 GHz with solutions of tri-(n-butyl)ammonium picrate in p-xylene. Various concentrations of salt between 0.05 and 0.5 mol dm^–3 were studied and measurements were made at four temperatures between 12.5 and 67°C.

It is necessary to make provision for the effect of further association into quadrupoles in calculating the dipole moment(µ) of the ion pairs from the dispersion amplitudes (ε_s–ε_∞), where ε_s and ε_∞ are the low and high frequency limiting permittivities deduced from the experimental data. When this correction is applied, values of µ are independent of concentration and temperature and have a mean of 12.1 ± 0.28 D.

Values of τ₀ corresponding to an infinitely dilute solution have been calculated from the experimental data by a non-linear least squares method and these are in reasonable agreement with the average values of the reduced relaxation time (τ*), where τ*=τ(ε_∞+ 2)/(ε_s+ 2). In addition the apparent distance of closest approach of two ion pairs has been estimated to be 1.10 ± 0.04 nm. The temperature dependence of τ₀ is adequately described by the equation τ₀=τ⁰₀ exp (H₀/RT) with τ⁰₀=(3.37 ± 0.14 )× 10^–12 s and H₀= 10.42 ± 0.10 kJ mol^–1.

Viscosity data for the relevant solutions at 25°C are also included.