Etude cinétique de la complexation du cuivre en milieu hydrogénocarbonate par électrochimie

La détermination de la capacité complexante des eaux naturelles est généralement effectuée par titrage ampérométrique du cuivre à pH fixé. Beaucoup, sinon tous les tampons classiques utilisés perturbent les études de spéciation du cuivre par formation de complexes peu stables. Dans ce travail nous nous sommes intéressés à la complexation du cuivre par les ions carbonates généralement présents dans les eaux naturelles. Afin de mener à bien notre étude, nous avons mesuré, dans un milieu synthétique de force ionique et pH contrôlés (KNO3 0,02 M + NaHCO3 2,38.10-3 M) toutes les constantes des divers équilibres prévisibles : produits de solubilité de l'hydroxyde de cuivre Cu(OH)2 et de la malachite Cu2 (OH)2 CO3, constante de formation du carbonate de cuivre dissous CuCO3. A des valeurs de pH suffisamment faibles, la formation du complexe CUOH+ peut étre négligée. Les trois autres réactions ont été suivies race aux mesures combinées, d'une part du pH et, d'autre part, de l'activité en ion Cu2+ libre ou hydraté par ionométrie. Compte tenu des propriétés acido-basiques des diverses espèces, les mesures ont été effectuées à divers pH compris entre 5,5 et 8. Le complexe CuCO30étant l'espèce majoritaire dans ces solutions, sa labilité, pouvant perturber follement les déterminations des capacités complexantes par titrage ampérométrique, a été étudiée par diverses méthodes électrochimiques. La polarographie à tension sinusoïdale surimposée (AC) a indiqué une réduction rapide du cuivre (II) sur électrode de mercure. Deux vagues ont été obtenues en voltampérométrie sur électrode tournante à disque de platine (RDE), ce dédoublement étant d'origine cinétique compte tenu de l'évolution de ces vagues avec la température. Une estimation des valeurs des constantes de vitesse de formation kf et de dissociation kb, a permis de montrer la rapidité des réactions mises en jeu et la labilité du complexe CuCO30.The determination of the complexing capacity of natural waters by amperometric copper (II) titrations must be made in pH-buffered solutions. Most if not all of classical buffers influence copper speciation through the formation of weak complexes. This paper deals with an attempt to use the naturally occurring hydrogenocarbonate buffet. A detailed study of copper (II) chemistry in synthetic solutions at controlled ionic strength and pH (0.02 mol.l-1 KNO3, 2.38 mol.l-1 NaHCO3, controlled Pco2) was carried out. In these solutions copper (Il) chemistry may involve precipitation of copper hydroxide Cu(OH)2, copper hydroxycarbonate Cu2(OH)2 CO3 (malachite) and formation of the soluble complexes CuCO30 and CuOH+. At low enough values of pH CuOH+ formation may be neglected. The three other reactions are studied using copper ionometry with a specific electrode and equilibrium constants determined in the 5.5 to 8.0 pH range. As CuCO30 is a major species in these solutions its lability is of major concern for the validity of the amperometric determination of the complexing capacity by copper (II) titration and bas been studied by several electrochemical approaches. AC polarography indicated a rapid reduction of copper (II) on the mercury electrode. Two waves were obtained for the deposition of copper (Il) by voltamperometry with a rotated platinum disk electrode (RDE) presenting the characteristics of a kinetic control. However the values of the forward (kf) and backward (kb) reaction rate constants for CuCO30 formation or dissociation seem too high for a precise determination

Two-dimensional solitary pulses in driven diffractive-diffusive complex Ginzburg-Landau equations

Two models of driven optical cavities, based on two-dimensional Ginzburg-Landau equations, are introduced. The models include loss, the Kerr nonlinearity, diffraction in one transverse direction, and a combination of diffusion and dispersion in the other one (which is, actually, a temporal direction). Each model is driven either parametrically or directly by an external field. By means of direct simulations, stable completely localized pulses are found (in the directly driven model, they are built on top of a nonzero flat background). These solitary pulses correspond to spatio-temporal solitons in the optical cavities. Basic results are presented in a compact form as stability regions for the solitons in a full three-dimensional parameter space of either model. The stability region is bounded by two surfaces; beyond the left one, any two-dimensional (2D) pulse decays to zero, while quasi-1D pulses, representing spatial solitons in the optical cavity, are found beyond the right boundary. The spatial solitons are found to be stable both inside the stability region of the 2D pulses (hence, bistability takes place in this region) and beyond the right boundary of this region (although they are not stable everywhere). Unlike the spatial solitons, their quasi-1D counterparts in the form of purely temporal solitons are always subject to modulational instability, which splits them into an array of 2D pulses, that further coalesce into two final pulses. A uniform nonzero state in the parametrically driven model is also modulationally unstable, which leads to formation of many 2D pulses that subsequently merge into few ones.Comment: a latex text file and 11 eps files with figures. Physica D, in pres

Two-dimensional solitons on the surface of magnetic fluids

We report an observation of a stable soliton-like structure on the surface of a ferrofluid, generated by a local perturbation in the hysteretic regime of the Rosensweig instability. Unlike other pattern-forming systems with localized 2D structures, magnetic fluids are characterized by energy conservation; hence their mechanism of soliton stabilization is different from the previously discussed gain/loss balance mechanism. The radioscopic measurements of the soliton's surface profile suggest that locking on the underlying periodic structure is instrumental in its stabilization.Comment: accepted for publication by Physical Review Letter

Redox-active ferrocene-modified Cowpea mosaic virus nanoparticles

A naturally occurring nanoparticle, the plant virus Cowpea mosaic virus, can be decorated with ferrocene derivatives, of various linker lengths with amine and carboxylategroups, on the external surface using a range of conjugation strategies. The multiple, organometallic, redox-active ferrocene moieties on the outer surface of the virus are electrochemically independent with reduction potentials that span a potential window of 0.16 V that are dependent on the site of modification and the nature of the ferrocene derivative. The number of ferrocenes coupled to each virus ranges from about 100 to 240 depending upon the conjugation site and the linker length and these redox active units can provide multielectron reservoirs

Two and three-dimensional oscillons in nonlinear Faraday resonance

We study 2D and 3D localised oscillating patterns in a simple model system exhibiting nonlinear Faraday resonance. The corresponding amplitude equation is shown to have exact soliton solutions which are found to be always unstable in 3D. On the contrary, the 2D solitons are shown to be stable in a certain parameter range; hence the damping and parametric driving are capable of suppressing the nonlinear blowup and dispersive decay of solitons in two dimensions. The negative feedback loop occurs via the enslaving of the soliton's phase, coupled to the driver, to its amplitude and width.Comment: 4 pages; 1 figur

Is the Unitarity of the quark-mixing-CKM-matrix violated in neutron β\beta-decay?

We report on a new measurement of neutron $\beta$-decay asymmetry. From the result \linebreak $A_0$ = -0.1189(7), we derive the ratio of the axial vector to the vector coupling constant $\lambda$ = ${\it g_A/g_V}$ = -1.2739(19). When included in the world average for the neutron lifetime $\tau$ = 885.7(7)s, this gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix $V_{ud}$. With this value and the Particle Data Group values for $V_{us}$ and $V_{ub}$, we find a deviation from the unitarity condition for the first row of the CKM matrix of $\Delta$ = 0.0083(28), which is 3.0 times the stated error

Practical computational toolkits for dendrimers and dendrons structure design

Dendrimers and dendrons offer an excellent platform for developing novel drug delivery systems and medicines. The rational design and further development of these repetitively branched systems are restricted by difficulties in scalable synthesis and structural determination, which can be overcome by judicious use of molecular modelling and molecular simulations. A major difficulty to utilise in silico studies to design dendrimers lies in the laborious generation of their structures. Current modelling tools utilise automated assembly of simpler dendrimers or the inefficient manual assembly of monomer precursors to generate more complicated dendrimer structures. Herein we describe two novel graphical user interface (GUI) toolkits written in Python that provide an improved degree of automation for rapid assembly of dendrimers and generation of their 2D and 3D structures. Our first toolkit uses the RDkit library, SMILES nomenclature of monomers and SMARTS reaction nomenclature to generate SMILES and mol files of dendrimers without 3D coordinates. These files are used for simple graphical representations and storing their structures in databases. The second toolkit assembles complex topology dendrimers from monomers to construct 3D dendrimer structures to be used as starting points for simulation using existing and widely available software and force fields. Both tools were validated for ease-of-use to prototype dendrimer structure and the second toolkit was especially relevant for dendrimers of high complexity and size.Peer reviewe

Cymantrene–Triazole "Click" Products: Structural Characterization and Electrochemical Properties

We report the first known examples of triazole-derivatized cymantrene complexes (η5-[4-substituted triazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I), obtained via a “click” chemical synthesis, bearing a phenyl, 3-aminophenyl, or 4-aminophenyl moiety at the 4-position of the triazole ring. Structural characterization data using multinuclear NMR, UV–vis, ATR-IR, and mass spectrometric methods are provided, as well as crystallographic data for (η5-[4-phenyltriazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I) and (η5-[4-(3-aminophenyl)triazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I). Cyclic voltammetric characterization of the redox behavior of each of the three cymantrene–triazole complexes is presented together with digital simulations, in situ infrared spectroelectrochemistry, and DFT calculations to extract the associated kinetic and thermodynamic parameters. The trypanocidal activity of each cymantrene–triazole complex is also examined, and these complexes are found to be more active than cymantrene alone

Wave patterns generated by an axisymmetric obstacle in a two-layer flow

Gravity waves generated by a moving obstacle in a two-layer stratified fluid are investigated. The experimental configuration is three-dimensional with an axisymmetric obstacle which is towed in one of the two layers. The experimental method used in the present study is based on a stereoscopic technique allowing the 3D reconstruction of the interface between the two layers. Investigation into the wave pattern as a function of the Froude number, Fr, based on the relative density of the fluid layers and the velocity of the towed obstacle is presented. Specific attention is paid to the transcritical regime for which Fr is close to one. Potential energy trapped in the wave field patterns is also extracted from the experimental results and is analyzed as a function of both the Froude number, Fr, and the transcritical similarity parameter Γ. In particular, a remarkable increase in the potential energy around Fr = 1 is observed and a scaling allowing to assemble data resulting from different experimental parameters is proposed