José Goulon

Droplet model for the analysis of the dielectric properties of critical binary mixtures

We present here some further developments of the droplet model recently proposed by Oxtoby, which appears quite suitable for an extensive phenomenological analysis of the dielectric behavior of binary mixtures in the vicinity of a consolute critical point. Along the critical isochore, one may expect the static permittivity eE(T) to remain finite but to exhibit a cusp, whereas the nonlinear dielectric effect, the Kerr effect, and the Cotton–Mouton effect all are predicted to diverge strongly with the universal exponent Ψ=2γ−dν. These predictions are supported by recent experimental evidence.

Dielectric behaviour of a molecular liquid mixture in the one-phase precritical region

Abstract The temperature dependence of density d , electric permittivity ϵ s and nonlinear high field dielectric variation Δϵ are reported for various benzonitrile-alkane solutions. For the benzonitrile-isooctane mixtures which exhibit a phase separation in the considered temperature range, the phase diagram has been obtained. In the “homogeneous” one phase region, significant deviations from the normal temperature dependence of d , ϵ 0 , Δϵ have been observed and characterize a precritical region of several degrees C above the consolute temperature. These results support the hypothesis of strong precritical orientational interactions or local ordering phenomena. The precritical dielectric behaviour of the benzonitrile-isooctane system is shown to be very different from those of nitrobenzene-alkane or aniline-alkane mixtures.

Dielectric relaxation and far infrared dispersion in pure liquid chloroform

Abstract Accurate measurements of the complex permittivity of pure liquid chloroform are reported for discrete frequencies within the broad range 0 to 112 GHz. The measurements were taken at temperatures of 25 and −60°C. In addition, continuous far infrared dielectric spectra (3.3 tp 140 cm −1 , 100 to 4200 GHz) are presented for room temperature. The data are analysed by means of a Cole-Cole plot. This is found to be re-entrant, in agreement with recent theoretical expectations.

Bridge-stacked polymeric structure of a fluorinated Ga(III) porphyrin deduced from a perturbed difference Fourier analysis of EXAFS spectra

Abstract The aim of the present paper is to check if EXAFS spectroscopy can afford a rapid discrimination between a purely molecular or a bridge-stacked polymeric structure of a fluoro-octamethylporphyrinato-gallium(III) complex (OMP):GaF, A new method, i.e. the perturbed difference Fourier analysis (PDFA), combining the structural information of two independent experiments carried out on (OMP):GaF and (OMP):GaOH was developed and proved to be most appropriate for solving this problem. Consistent results are indeed obtained for the distances Ga...Ga (3.9 4 A), Ga...F (1.97 A) and Ga...N (2.04 A) in (OMP):GaF. Also derived is the axial shift of the metal Δ h = 0.3 g A in the penta-coordinated reference compound (OMP):GaOH.

EXAFS investigation of nickel and cobalt precursors of homogeneous ziegler-type hydrogenation catalysts

Abstract A preliminary EXAFS study is reported of precursors of homogeneous hydrogenation catalysts obtained by addition of AlEt 3 to nickel or cobalt octoate solutions in benzene. For a given nickel octoate solution studied before reduction, monodentate coordination of the nickel cations to at least four carboxylic anions was established and accurate Ni...O 1 , Ni...C 1 distances (i.e. 2.06±0.01 A, 3.01±0.02 A) were determined. The spectra of the reduced solutions are however clearly dominated by intense Ni * ...Ni or Co * ...Co signals at distances comparable to the relevant distances of the bulk metal, and thus suggest the presence of rather disordered, amorphous-like metal clusters. Strong evidence is also produced in favour of additional metal...carbon bonds which were often assumed to play an important role in the catalytic activity of these or related solutions.

High frequency dielectric behaviour of critical binary solutions of benzonitrile-isooctane

Abstract The high frequency dielectric behaviour of critical binary mixtures of benzonitrile-isooctane is described, as a function of temperature, for two concentrations and for a frequency coverage comprising several discrete values between 3 and 112 GHz and a continuous range between 600 and 3600 GHz. For the first time, the precritical region is explored using far infrared spectroscopy and the temperature dependence of the refractive index and the absorption is reported. As the temperature approaches the critical value, the Cole-Davidson diagrams are found to show a progressive flattening, which can be characterised by a decrease in β, the Cole-Davidson coefficient. Both the microwave and the far-infrared properties are strongly anomalous near the consolute temperature and this could be consistent with the assumption of growing microscopic regions within the liquid, near this temperature.

First experimental evidence of circularly polarized X-ray excited optical luminescence (XEOL) from chiral Eu3+ complexes

Abstract The X-ray excited optical luminescence (XEOL) spectra of chiral Eu3+ complexes are shown to exhibit a small degree of circular polarization and a Kuhn asymmetry factor which is quite comparable to what can be measured in conventional CPL spectroscopy. Additional selectivity could be introduced in EXAFS or XANES data by recording CP XEOL excitation spectra. This next step may require more brilliant X-ray sources, but will also imply a far better control of the radiation damages to the sample.

Champ de Reaction Dynamique en Relaxation Dielectrique, Application au Cas du Chloroforme a 25 °C

The formulation of dielectric relaxation using a model of a dipole in a spherical cavity, is reexamined and the approximation of the continuum is discussed. A general formula is established. In the case of chloroform, this formula makes possible the choice of the best function for the continuum associated to a given auto-correlation function and an approximate value of the perpendicular moment of inertia are deduced for chloroform.

Active Intramolecular Motion in Dielectric Relaxation of Pure Liquid Diacetyl

Interferometric measurements of the complex permittivity of pure liquid diacetyl in the range 17 GHz to 300 GHz are reported at four temperatures (5°C; 25 °C; 50 °C; 75 °C), together with far infrared spectra on solutions of diacetyl in cyclopentane, at room temperature. Very short relaxation times have been observed, whereas a very strong and broad absorption appears in the far infrared region. The experimental results may be interpreted as being due to active intramolecular motions involving a considerable change of dipole moment: i.e.-cis/-trans isomerism or inversion of the cis forme. A simple analysis in terms of ‘chemical relaxation processes’ obeying the Curie principle allowed of the evaluation, from the microwave part of Cole-Cole plot, of the kinetic constants (k cis→trans and k trans→cis). The nature of the far infrared absorption has been also discussed.