Jean-Claude Lestrade

Relaxation de transport de charges et propriétés diélectriques des solutions électrolytiques

The measurements of complex permittivity, e′ — je″, of an electrolytic solution, measurements made in correlation with the frequency, can be analysed by the least square method. This method is illustrated here by an example (LiClO4 in ethyl acetate, 0.6 M), and the discussion shows what objective conclusions can be drawn from the measurements, especially in regard to the existence of an ionic relaxation and the description of this relaxation by a collection of characteristic parameters. The results obtained by solutions of LiClO4 in ethyl acetate of different concentrations from 0.1–1 M are also given.

Very low frequency (down to 3 × 10−3 Hz) impedance measurements on a hanging mercury drop electrode

Abstract The authors describe an experimental set-up, using transfer function analysers, which allows for the impedance of a hanging mercury drop electrode to be measured at frequencies as low as 3 × 10−3 Hz, and for the low frequency limit of that impedance to be compared with the slope of the current-voltage curve. The device is applied to the reduction of the persulfate anion in a 1 M LiCl solution, in a region of potential where the current-voltage curve has a negative slope.

Dielectric properties of electrolyte solutions. Lithium perchlorate solutions in tetrahydrofuran + benzene mixtures

For electrolyte solutions in weakly polar solvents, the collective motion of ions can give rise to an ionic relaxation process in addition to the molecular one. The former is described by a model involving a Brownian linear motion of the ions between collisions. The corresponding part of the complex permittivity is given an analytical form characterized by an asymmetrical distribution of relaxation time. Measurements have been made, between 137 MHz and 34 GHz, on LiClO4 solutions in tetrahydrofuran + benzene mixtures. The data have been analysed according to a new statistical procedure which leads to a straightforward separation between ionic and molecular relaxation. The former supports the above description of the dynamic state of the solute. In particular, the magnitude of the average time between collisions, derived from the dielectric data, compares well with the theoretical value calculated from the viscosity of the solution by means of the Stokes–Einstein formula.

Etude, Par Relaxation Dielectrique, Du Mouvement Brownien des Ions dans les Liquides Simples

The motion of ions in an electrolyte solution can be related to the observed dielectric relaxation by means of correlation functions. When the dynamic variables are chosen as the velocities, the results of the Onsager and Debye-Falkenhagen (ODF) theories can be found. The ionic part of the complex permittivity shows then an asymmetrical distribution of relaxation time, when described phenomenologically in dielectrics language. Unfortunately, these theories are restricted to dilute solutions, too dilute for any experimental verification to be made. If the dynamic variables of the correlation function are chosen as the positions of the ions an a priori separation has to be made between a static conductivity σ(0) and an ionic complex permittivity Δe(ω). Only the latter can be deduced from the corresponding correlation function, and it represents the permittivity of a non-conductive medium. Within this frame, a dielectric model is built. The ions are first assumed to be gathered in complex aggregates which undergo deformations when an electric field is applied. These deformations are then assumed to come from a brownian linear motion of the ions, interrupted by collisions, and can be described by stochastic laws involving a diffusion coefficient and a mean collision time. The magnitude of both quantities can be estimated a priori, as well as the molecular part of the complex permittivity. This model fits fairly well the experimental data obtained between 137 MHz and 34 GHz with solutions of lithium perchlorate (0.6 M l–1) in tetrahydrofuran-benzene mixtures.

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

The complex dielectric permittivity Iµ 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 Iµ 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 Andersons 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 Perrins reorientation time of an ellipsoid 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.

Méthode de détermination absolue de tension superficielle à partir de la masse et du profil d’une goutte formée à l’extrémité d’un capillaire

Les auteurs proposent une relation rigoureuse entre la tension superficielle σ et la masse d’une goutte formee a l’extremite d’un capillaire. L’etablissement de cette relation conduit par ailleurs a une methode de determination absolue de σ ou l’information principale provient d’une mesure de masse, le releve du profil n’intervenant que dans un terme correctif.