Dielectric relaxation of tri-n-butylammonium picrate in benzene solutions

Abstract

The complex dielectric permittivity ε of tri-n-butylammonium (TBA) picrate solutions in benzene has been measured in the frequency range 8 MHz-35 GHz, at a number of concentrations between 0.02 and 1.4 mol dm^–3, at 25°C. The frequency dependence of ε cannot be described by a single relaxation time as previously proposed from measurements in a narrower frequency range. From an analysis of the conductivity data, the “Free” ions concentration has been estimated and their contribution to the relaxation spectrum (Debye–Falkenhagen effect) is shown to be negligible. Several assumptions are examined as to the possible origin of the small but significant observed distribution of relaxation times, among them, a proton jump process describing the exchange between two H-bonded molecular forms which is treated by Anderson's model. Within the crude assumptions of this model, the molar fraction of the more polar form (H-bonded ion pair) is found to be much lower, at high dilution, than expected from literature data. The main relaxation time, which characterizes the low frequency part of the spectrum, when extrapolated to zero concentration, is found to be Perrin's reorientation time of an ellipsoíd whose axis along the dipole moment is twice the length of the other axes, in agreement with the description of the H-bonded ion pair given in the literature.