Existence of two concomitant magnetic structures below T(Néel) for the natrochalcite, NaFe(II)2(H3O2)(MoO4)2

A comparative study of the magnetic properties and magnetic structures of the natrochalcite, NaFe(2)(D(3)O(2))(MoO(4))(2) (FeD) to those of the isostructural NaCo(2)(D(3)O(2))(MoO(4))(2) (CoD) and NaNi(2)(D(3)O(2))(MoO(4))(2) (NiD) is presented. The structural change is a shrinking of the unit cell in the order of the ionic radii of the transition metal, FeD > CoD > NiD. While NiD and CoD are canted-antiferromagnets with T(N) = 28 and 21 K, respectively, FeD is an anisotropic 2D-Ising antiferromagnet (T(N) = 17 K) with a spin-flop field of 14 kOe at 2 K and the presence of a hysteresis loop reaching only (1)/(4) of the saturation magnetization in 70 kOe. The critical field decreases almost linearly on warming to T(N). The neutron diffraction patterns of FeD below T(N) display numerous magnetic Bragg peaks which cannot be assigned to any one magnetic structure but fits well to two superposed sets, one with a temperature independent line width and has a propagation vector k(1) = (0, 0, 0) while for the other there is a clear dependence and k(2) = (0, 0, ?). In the k(1) = (0, 0, 0) magnetic structure the moments are parallel to each other within one chain and lie along the a-axis but are antiparallel to those in neighboring chains. In contrast CoD and NiD, for which k = (0, 0, 0), have their moments aligned along the b-axis and ac-plane, respectively. The second magnetic structure, k(2) = (0, 0, ?), is characterized by four sublattices, two per layer, where the moments are in the ab-plane and canted with a resultant along the a-axis which is compensated by those of the adjacent layers. For the k(2) = (0, 0, ?) structure, the scattering coherent length decreases, and the moments tend progressively toward the a-axis upon increasing temperature. The coexistence of two concomitant magnetic structures is unprecedented for compounds containing transition metal moment carriers.     

Kinetic aspects and swelling changes of magnesium and cerium titano-antimonates in aqueous and mixed solvents

Magnesium titano-antimonate (MgTi₅Sb₂O₁₆·12.5H₂O) and cerium titano-antimonate (Ce₂.₇Ti₅Sb₂O₁₉·15.0H₂O) were synthesized as new cation exchangers using the in situ precipitation technique. Physico-chemical investigations showed different behaviors for the obtained materials. The materials have significant stability at high acid concentration and temperature. The ion exchange capacity for Cs⁺ in the presence of different alcoholic solvents was found to increase and generally obey the order C₂H₅OH > CH₃OH > H₂O. Diffusion coefficients (D_i) and thermodynamic parameters of Cs⁺ exchange in both magnesium and cerium titano-antimonates in aqueous and alcoholic solutions were calculated. The swelling ratios of the materials were predicted by applying modified calculations at constant values of D_i. The results showed insignificant swelling behavior in the presence of organic solvents.     

On the Performance of Free Radicals Combined Electrocoagulation Treatment Processes

The electrocoagulation (EC) combined treatment processes have shown promising performances in which the removal of pollutants from industrial wastewater was greatly enhanced. The integration of a free radical producing step with EC has been considered as one of these new combinations that show synergy and improvements in the overall cleaning process performance. This review is devoted to discuss and analyze references on free radicals-assisted EC processes. Different combinations of free radical mechanisms were cited in the literature during the last two decades. They included ozone, advanced oxidation and ultrasound energy-assisted EC. It was noted that most of those studies were lab-scale processes that used synthetic wastewaters rather than real wastewaters. In addition the performances of those combined processes were improved compared to the EC process alone. This review considers the main parameters of free radical EC processes such as mechanisms, kinetic models, scale-up and cost estimation. Many concluding remarks were stated to give insights for possible future investigations. It seems from the results that the ozone-assisted EC is the most efficient combination since its removal efficiency is high in most applications. On the other hand, the combination of ultrasonic energy with EC was reported to reduce electrode passivation.     

Selectivity control in the separation of aromatic amino acid enantiomers with sulphated beta-cyclodextrin

Control of selectivity in the enantiomeric separation of three aromatic amino acids (phenylalanine, tyrosine and tryptophan) is demonstrated by electrokinetic capillary chromatography utilising temperature variations coupled with the use of sulphated-beta-cyclodextrin (s-beta-CD) as a pseudostationary phase. The concentration of s-beta-CD and temperature were used as experimental variables to control the observed selectivity. A double-coated capillary was used and proved very robust with reproducibility of migration times being <2.0% R.S.D. between runs and <2.6% on using a new capillary. The system was modelled successfully using an artificial neural network (ANN) comprising one input layer, two hidden layers and one output layer. The model accurately described the observed separations with a correlation coefficient of 0.999 being observed between predicted and observed migration times. Selectivity optimisation was achieved using the normalised resolution product and minimum resolution criteria, with both providing optima at different experimental conditions. The selectivity changes observed also allowed the estimation of electrolyte temperatures within the capillary at high operating currents (>100 microA). Using a 50 microm i.d. capillary and an electrolyte comprising 20 mM phosphate and 15 mM s-beta-CD, a temperature of 52 degrees C was calculated within the capillary at an applied voltage of +30 kV.     

Mixed-mode electrokinetic chromatography of aromatic bases with two pseudo-stationary phases and pH control

The electrokinetic chromatographic (EKC) separation of a series of aromatic bases was achieved utilising an electrolyte system comprising an anionic soluble polymer (polyvinylsulfonic acid, PVS) and a neutral beta-cyclodextrin (beta-CD) as pseudo-stationary phases. The separation mechanism was based on a combination of electrophoresis, ion-exchange interactions with PVS, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. The ion-exchange and the hydrophobic interactions could be varied by changing the PVS and the beta-CD concentrations, respectively. Additionally, mobilities of the bases could be controlled by varying pH, due to their large range of pKa values. The separation system was very robust with reproducibility of migration times being <2% RSD. The two-dimensional parameter space defined by the two variables, [beta-CD] and %PVS, was modelled using a physical model derived from first principles. This model gave very good correlation between predicted and observed mobilities (r2=0.999) for the 13 aromatic bases and parameters derived from the model agreed with the expected ion-exchange and hydrophobic character of each analyte. The complexity of the mathematical model was increased to include pH and this three-dimensional system was modelled successfully using an artificial neural network (ANN). Optimisation of both the two-dimensional and three-dimensional systems was achieved using the normalised resolution product and minimum resolution criteria. An example of using the ANN to predict conditions needed to obtain a separation with a desired migration order between two of the analytes is also shown.     

Adducts of hexa­methyl­ene­tetr­amine with ferrocene­carboxyl­ic acid and ferrocene‐1,1′‐di­carboxyl­ic acid: multiple disorder in space groups Fmm2 and Cmcm

Hexa­methyl­ene­tetr­amine, C₆H₁₂N₄, and ferrocene­carboxyl­ic acid, C₁₁H₁₀FeO₂, form a 1:2 adduct, (I), which is a salt, viz. hexa­methyl­ene­tetramin­ium(2+) bis(ferro­cene­car­box­yl­ate), (C₆H₁₄N₄)[Fe(C₅H₅)(C₆H₄O₂)]₂. The dication in (I) is disordered with two orientations at a site of mm2 symmetry in space group Fmm2, while the anion lies across a mirror plane with its unsubstituted cyclo­penta­dienyl ring disordered over two sets of sites. With ferrocene‐1,1′‐di­carboxyl­ic acid, C₁₂H₁₀FeO₄, hexa­methyl­enetetr­amine forms a 1:1 adduct, (II), in which both components are neutral, viz. hexa­methyl­ene­tetramine–ferro­cene‐1,1′‐di­car­box­ylic acid (1/1), [Fe(C₆H₅O₂)₂]·C₆H₁₂N₄. The amine component in (II) is disordered with two orientations at a site of mm2 symmetry in space group Cmcm, while the acid component is disordered with two orientations at a site of 2/m symmetry. The components in (I) are linked into a finite three‐ion aggregate by a single N—H⃛O hydrogen bond, while the components of (II) are linked into continuous chains by a single O—H⃛N hydrogen bond.     

Dynamics of an electrified confined planar jet in a zero gravity environment

Meccanica - The present work deals with temporal electro-aerodynamics instability properties of an inviscid confined planar jet in a zero gravity environment and the existence of electric field....     

Détection et approximation de défauts non-réguliers

This Note deals with the identifiability of non-smooth defects by boundary measurements. We prove the uniqueness of the detection by two measurements for arbitrary closed sets satisfying quasi-everywhere a conductivity assumption. This assumption is satisfied by a large class of compact sets, including all the sets which can be written as an arbitrary union of continua of positive diameter. The conductivity is a new regularity concept which is related to the thickness of the set and is to be compared to the Wiener regularity. In order to rigorously justify the numerical approach by the finite element method, we provide a stability result without any a priori smoothness assumptions. To cite this article: Z. Belhachmi, D. Bucur, C. R. Acad. Sci. Paris, Ser. I 340 (2005).