C. Petipas

The preparation of a ferrofluid by decomposition of dicobalt octacarbonyl: I. Experimental Parameters

Abstract The experimental parameters which lead to the formation of a stable suspension of metallic cobalt particles in an organic solvent (ferrofluid) were evidenced. The cobalt particles originate from the thermolysis of dicobalt octacarbonyl solution in the presence of a chosen surface active agent. The kinetics of the CO evolution depend on the reaction temperature, the nature of the solvent and of the surfactant, the weight ratio of carbonyl and surfactant, and the initial concentration of the cobalt carbonyl solution. Spherical particles, of a remarkable narrow size distribution, are obtained when the decomposition of Co2(CO)8 is carried out, in an aromatic solvent, above 110°C, in the presence of a surfactant possessing a long hydrocarbon chain and a strong ionic group (sulfonate). The decomposition in toluene, in which ethyl (2 hexyl) sodium sulfo-succinate is dissolved, leads to particles of about 70 A in diameter. The kinetics of the CO evolution are rather complex, but when a ferrofluid is being formed, several sequences are always recorded: an initial and rapid evolution of CO corresponding to the formation of Co4(CO)12. Part of this compound is insoluble in the reaction medium and appears to be a regulating intermediate. After this short initial stage the rate of decomposition of Co4(CO)12 slows down and becomes practically constant. Later the CO formation is accelerated again and finally it decreases as the reaction goes to completion. This S-shaped curve which describes the decomposition of Co2(CO)8 is always observed when a ferrofluid is in progress of formation.

The preparation of a ferrofluid by decomposition of dicobalt octacarbonyl: II. Nucleation and growth of particles

Abstract The formation of a ferrofluid by thermal decomposition of a toluene solution of dicobalt octacarbonyl , in the presence of ethyl (2 hexyl) sodium sulfosuccinate, has been studied. The carbon monoxide evolution, the diameters of the particles, and the number of growing particles have been measured using also small-angle X-ray scattering and magnetic methods. The different sequences of CO evolution are explained. At least two factors are responsible for the formation of particles of very narrow size distribution: the presence of microreactors in the reaction medium and a diffusion controlled growth mechanism.

Measurement of the average velocity of sedimentation in a dilute polydisperse suspension of spheres

An X-ray attenuation technique is used to obtain the local concentration of spherical particles in a polydisperse suspension as a function of vertical position and time. From these experimental data, the average velocity of sedimentation in the homogeneous part of the suspension is derived by considering the variation with time of the total volume of particles located above a given fixed horizontal plane. Measurements have been performed in suspensions of particles which differ from each other in size with a total volume concentration in particles between 0.13% and 2.5%, and also in suspensions of particles which differ from each other both in size and in density, the total volume concentration being 2%. For the first kind of suspension, the experimental hindered settling factor is plotted versus the concentration and a linear regression analysis provides the slope with its 90% confidence limits: S e = −5.3 ± 1.1. This experimental average coefficient of sedimentation is in good agreement with the theoretical average coefficient S t = −5.60 obtained from the results of Batchelor & Wen (1982). The second kind of suspension, for which permanent doublets of spheres may theoretically exist, is not in the range of validity of Batchelor & Wens results. The experimental average coefficient of sedimentation for this case is found to be much larger than the prediction obtained by extrapolating Batchelor & Wens results out of their range of validity. This increased velocity may be experimental evidence of the existence of permanent doublets.

Crystallographic study of thin Pb-Sn lamellar eutectic ribbons

Abstract The study of the crystallographic structure of thin Pb-Sn eutectic specimens by X-ray diffraction has shown that for a single lamellar direction there are many possible crystal directions. The tin phase crystal contains subgrains with approximately ten lamellae and the lead crystal is plastically strained. The orientation relatioships between the tin and lead lamellae in these specimens were determined by the Buerger method.

Role of solvent-head-group interaction on the formation of lyotropic liquid-crystals in structured non-aqueous solvents

Molecules of amphiphiles associate in structured solvents to form flexible interfacial films, which may then join up to form crystals. A considerable variety of structures are observed in such crystals. In most cases, consideration of the geometry of the structures, based on frustration analysis, can account for the sequence of phases observed in these systems, independent of the chemical nature of the amphiphilic molecules. The geometric theory is well validated with structured polar solvents whose properties may be either quite similar or quite distinct from those of water. In surfactant-non-aqueous polar solvent systems, only the theoretical phases are in fact observed. However, in water, intermediate phases not accounted for by the geometric theory may be observed. Their conditions of formation are not well understood, indicating that complex phenomena take place in water, a solvent which is nevertheless regarded as an ideal and model solvent.

Thermotropic liquid-crystalline properties of some novel hexuronic acid derivatives bearing a single or two alkyl chains

We have recently developed a new method forO-glycosidation of totally O-unprotected aldoses and uronic acids. Either pyranosides or the corresponding furanosidic isomers are thus obtained in one step from natural carbohydrates. This paper is concerned with the thermotropic liquid-crystalline phases of alkyl β-d-galacto-(or gluco)-furanosiduronic acids bearing a saturated alkyl chain at the anomeric position and of some dialkylated derivatives. Phase identifications and measurements of phase transition data were carried out by differential scanning calorimetry (DSC), polarizing optical microscopy and small angle X-ray diffraction studies. Uronic derivatives have quite crystalline structures; upon heating, these compounds exhibit a smectic A* transformation.

Études morphologique et cristallographique de l'eutectique In2BiIn solidifié unidirectionnellement en rubans minces

Abstract Unidirectional solidification of very thin specimens of PbSn has been investigated by Racek; thin In 2 Bi In lamellar cutectic ribbons are produced by the same experimental procedure. The lamellar structure of these specimens is perfect in areas of about 0.5–1 mm 2 . The relationship between the interlamellar spacing λ and the growth rate ν has been determined: λ 2,3 ν = 3.47 × 10 −18 MKS. The orientation relationship is: growth direction ‖ ∥101∥ In ‖ ∥010∥ In 2 Bi ; lamellar interface ‖ (12 1 ) In ‖ ( 1 02) In 2 Bi . The PbSn and InIn 2 Bi systems are compared. The results are explained by a large anisotropy and a slow growth kinetic of the In 2 Bi phase. Thermal vibrations of low frequencies of In chains parallel to the c -axis are shown in the In 2 Bi eutectic phase.