The electric double layer has a life of its own

Using molecular dynamics simulations with recently developed importance sampling methods, we show that the differential capacitance of a model ionic liquid based double-layer capacitor exhibits an anomalous dependence on the applied electrical potential. Such behavior is qualitatively incompatible with standard mean-field theories of the electrical double layer, but is consistent with observations made in experiment. The anomalous response results from structural changes induced in the interfacial region of the ionic liquid as it develops a charge density to screen the charge induced on the electrode surface. These structural changes are strongly influenced by the out-of-plane layering of the electrolyte and are multifaceted, including an abrupt local ordering of the ions adsorbed in the plane of the electrode surface, reorientation of molecular ions, and the spontaneous exchange of ions between different layers of the electrolyte close to the electrode surface. The local ordering exhibits signatures of a first-order phase transition, which would indicate a singular charge-density transition in a macroscopic limit

European security in the 1990s and beyond : the implications of the accession of Cyprus and Malta to the European Union

For the last decade, the dramatic events in eastern and central Europe have (rightly) dominated the security debate in Europe and, indeed, the wider world. One of the consequences of this has been that the traditional neglect of the Mediterranean region has been compounded. However, there are now signs-notably the recent Barcelona conference at which the European Union's Mediterranean policy was relaunched and extended (to incorporate the grand design of a Mediterranean free trade area) - that the Mediterranean is, at last, receiving some of the attention it deserves and justifies.peer-reviewe

Specific Interaction between Uranium Anionic Complexes and the Cations of Bis(trifluoromethylsulfonyl)imide Based Ionic Liquids

The redox properties of uranium(IV) hexachloro complex have been investigated with a glassy carbon electrode in four bis(trifluoromethylsulfonyl)imide (Tf2N-) based ionic liquids (ILs): the 1-butyl-3-methylimidazolium ([BuMeIm]+), the 1-butyl-2,3-dimethylimidazolium ([BuMe2Im]+), the N-butylmethylpyrrolidinium ([BuMePyr]+), and the tributyl-methylammonium ([MeBu3N]+). The cyclic voltammetric analysis has revealed two main redox systems: UVCl6-/UIVCl62- around 0.2 V and UIVCl62-/UIIICl63- around −2 V. The formation of UVCl6-, a non-dioxo uranium(V) species, can then be observed at the electrode in [Tf2N]− based ILs. This work also provides evidence of a specific interaction between the uranium anionic species and the IL cations because the standard potentials of both redox couples depend on the IL. The interaction extent has been evaluated by comparison of the IL cation number associated with the uranium anionic complex. For that purpose, the standard potentials of both systems have been measured in the less interacting medium [MeBu3N][Tf2N] as a function of the [BuMeIm]+, [BuMe2Im]+, and [BuMePyr]+ concentration. Predominance diagrams for uranium hexachloro complexes, analogous to the Pourbaix diagram, have then been built in [MeBu3N][Tf2N] depending on the IL cations concentration. The exploitation of these diagrams leads to the conclusion that the interaction is function of the charge of the uranium hexachloro complex (UVCl6- < UIVCl62- < UIIICl63-) and the IL cation ([BuMe2Im]+ < [BuMePyr]+ < [BuMeIm]+). The influence of the IL cation could be correlated to the size and the electropositivity of the IL cation. The association would occur by H-bonding and electrostatic interaction. Ab initio calculations were also carried out to evaluate the strength of the interaction between the anionic uranium(IV) chloro complex and the IL cations. The results show that [BuMeIm]+ interacts the most and [MeBu3N]+ the least with UIVCl62-, and the magnitude of the interaction is comparable for [BuMe2Im]+ and [BuMePyr]+

Complexation of protactinium( v ) with nitrilotriacetic acid: a study at the tracer scale

International audienc

Electrochemical behaviour of tetrachloro and tetrabromo uranyl complexes in room temperature ionic liquids

The redox properties of tetrahalo uranyl(VI) complexes ([UVIO2X4]2− with X=Cl or Br) have been investigated in the ionic liquids [BuMeIm][Tf2N] and [MeBu3N][Tf2N], where [BuMeIm]+ and [MeBu3N]+ are respectively the 1-butyl-3-methylimidazolium and the tributylmethylammonium cations, both associated with the bis(trifluoromethylsulfonyl)imide anion [Tf2N]−. The results suggest that the reduction of [UVIO2X4]2− occurs via an Electron transfer - Chemical reaction - Electron transfer mechanism (ECE mechanism). We have shown by voltammetry that the first electron transfer is quasi-reversible and involves one electron, leading to the tetrahalo uranium(V) complex. This compound undergoes a chemical reaction and its stability depends both on the halide ligand and on the ionic liquid. The most stable complex is [UVO2Cl4]3− in [BuMeIm][Tf2N]. We have assumed that the [BuMeIm]+ cation interacts more strongly by H-bonding than the [MeBu3N]+ cation with the uranium complex, as it is the case for other anionic uranium species. Moreover, in presence of excess chloride ions, the consecutive chemical reaction is not observed anymore by cyclic voltammetry

Spectroscopic and Electrochemical Studies of U(IV)-Hexachloro Complexes in Hydrophobic Room-Temperature Ionic Liquids [BuMeIm][Tf2N] and [MeBu3N][Tf2N]

RADIOCHThe behavior of U(IV) octahedral complexes [cation]2[UCl6], where the [cation]+ is [BuMeIm]+ and [MeBu3N]+, is studied using UV/visible spectroscopy, cyclic staircase voltammetry, and rotating disk electrode voltammetry in hydrophobic room-temperature ionic liquids (RTILs) [BuMeIm][Tf2N] and [MeBu3N][Tf2N], where BuMeIm+ and MeBu3N+ are 1-butyl-3-methylimidazolium and tri-n-butylmethylammonium cations, respectively, and Tf2N- is the bis(trifluoromethylsulfonyl)imide anion. The absorption spectra of [cation]2[UCl6] complexes in the RTIL solutions are similar to the diffuse solid-state reflectance spectra of the corresponding solid species, indicating that the octahedral complex UCl62- is the predominant chemical form of U(IV) in Tf2N--based hydrophobic ionic liquids. Hexachloro complexes of U(IV) are stable to hydrolysis in the studied RTILs. Voltammograms of UCl62- at the glassy carbon electrode in both RTILs and at the potential range of -2.5 to +1.0 V versus Ag/Ag(I) reveal the following electrochemical couples: UCl6-/UCl62- (quasi-reversible system), UCl62-/UCl63- (quasi-reversible system), and UCl62-/UCl6(Tf2N)x-(3+x) (irreversible reduction). The voltammetric half-wave potential, Ep/2, of the U(V)/U(IV) couple in [BuMeIm][Tf2N] is positively shifted by 80 mV compared with that in [MeBu3N][Tf2N]. The positive shift in the Ep/2 value for the quasi-reversible U(IV)/U(III) couple is much greater (250 mV) in [BuMeIm][Tf2N]. Presumably, the potential shift is due to the specific interaction of BuMeIm+ with the uranium-hexachloro complex in ionic liquid. Scanning the negative potential to -3.5 V in [MeBu3N][Tf2N] solutions of UCl62- reveals the presence of an irreversible cathodic process at the peak potential equal to -3.12 V (at 100 mV/s and 60 C), which could be attributed to the reduction of U(III) to U(0)

Etude de l'hydratation du ciment phospho-magnésien au jeune age influence de l'acide borique

National audienc

Role of fluoride ions on the uranium oxidation mechanism in highly alkaline solutions

International audienceThis research is focused on understanding uranium oxidation to predict the behavior of uranium contaminated Mg alloy waste in confinement matrices. The environment of the matrix proposed for the magnesium encapsulation, based on geopolymer material, imposes basic conditions and the presence of fluoride ions (corrosion inhibitor of the Mg alloy). This work demonstrates the opposite behavior of the hydroxide and fluoride ions on the uranium oxidation: the fluoride ions induce the corrosion of uranium, whereas the hydroxide ions protect the metal against corrosion by making an anionic barrier at the uranium/solution interface. Two mechanisms are proposed, depending on the [OH-]/[F-] ratio. In absence of fluoride ions, or if [OH-]/[F-] > 1, the mechanism corresponds to a single step, the charge transfer characteristic of the UO2/UO2+x system. Uranium metal is thus protected from corrosion by the oxides layer. By contrast, if [OH-]/[F-] < 1, uranium is corroded, the fluoride ions diffuse through the oxides layer and cause its desquamation. This phenomenon has to be taken into account for the confinement of uranium contaminated Mg waste in geopolymers matrices containing fluoride ions