Actual Chimique Actual Chimique

Ion extraction to supercritical CO2. Contribution from the molecular dynamics simulations Supercritical fluid carbon dioxide (SC-CO2) With its moderate critical constants, nonflammable nature and low cost provides an attractive alternative for replacing organic solvents traditionally used in liquid/liquid processes. A recently developed technique uses SC-CO2 as organic phase with suitable complexant molecules to extract metallic cations from water. We illustrate the contribution from molecular dynamics simulations to describe, at the molecular level, the solvation, complexation and migration of various species involved in the supercritical fluid extraction of cesium by calixarene, and of uranyl nitrate by TBP. Both systems are important in the context of nuclear waste partitioning with > solvents. The simulations demonstrate the importance of the interfacial phenomena in ion extraction to CO2. Water and CO2 do not mix and display an interface where the ligands adsorb and the complexation takes place. In the case of the uranyl extraction, spontaneous complexation and extraction has been observed in the simulations, which show the importance of the concentration in extractant and nitric acid. At high TBP and acid concentrations, the interface evolves from a well-defined border to a > where CO2, TBP, acid and complexes mix, allowing the final extraction of uranyl

J Phys Chem A J Phys Chem A

We present a molecular dynamics study of the solvation properties of the tetrahedral AsPh4+ and BPh4- ions in water and chloroform solutions. According to the "extrathermodynamic" TATE (telraphenylarsonium tetraphenylborate) hypothesis, these nearly isosterical ions have identical free energies of solvation in any solvent, as the latter are generally assumed to display little dependence on the details of the charge repartition, provided that the total +/- charge is delocalized and that the ion's periphery is relatively inert. We compare eight different sets of charges obtained consistently for both ions and find that the anion is always better hydrated than the cation, as evidenced by ion-solvent interaction energies and changes in free energies of ion charging. This is explained by specific OH-Jr bridging interactions in the anion and the positive electrostatic potential at the center of the fictitious AsPh40 and BPh40 all-neutral species. With all models, the cation is also predicted to be more easily transferred from water to dry chloroform. The conclusions obtained with standard solvent models (TIP3P water and OPLS chloroform) are validated by tests with the polarizable Wallqvist and Berne water model and the Chang et al. chloroform model, and with computer simulations on a "wet chloroform" solution. The recently developed TIP5P water model yields, however, much closer hydration energies of AsPh4+ and BPh4-. The importance of "long-rangre" electrostatic interactions on the charge discrimination by solvent is demonstrated by the comparison of standard vs corrected methods to calculate the Coulombic interactions. These results are important in the context of the "TATB hypothesis" and for our understanding of solvation of large hydrophobic ions in pure liquids or in heterogeneous liquid environments.378tn Times Cited:29 Cited References Count:6

J Phys Chem A J Phys Chem A

We present a molecular dynamics study of the solvation properties of the tetrahedral AsPh4+ and BPh4- ions in water and chloroform solutions. According to the "extrathermodynamic" TATE (telraphenylarsonium tetraphenylborate) hypothesis, these nearly isosterical ions have identical free energies of solvation in any solvent, as the latter are generally assumed to display little dependence on the details of the charge repartition, provided that the total +/- charge is delocalized and that the ion's periphery is relatively inert. We compare eight different sets of charges obtained consistently for both ions and find that the anion is always better hydrated than the cation, as evidenced by ion-solvent interaction energies and changes in free energies of ion charging. This is explained by specific OH-Jr bridging interactions in the anion and the positive electrostatic potential at the center of the fictitious AsPh40 and BPh40 all-neutral species. With all models, the cation is also predicted to be more easily transferred from water to dry chloroform. The conclusions obtained with standard solvent models (TIP3P water and OPLS chloroform) are validated by tests with the polarizable Wallqvist and Berne water model and the Chang et al. chloroform model, and with computer simulations on a "wet chloroform" solution. The recently developed TIP5P water model yields, however, much closer hydration energies of AsPh4+ and BPh4-. The importance of "long-rangre" electrostatic interactions on the charge discrimination by solvent is demonstrated by the comparison of standard vs corrected methods to calculate the Coulombic interactions. These results are important in the context of the "TATB hypothesis" and for our understanding of solvation of large hydrophobic ions in pure liquids or in heterogeneous liquid environments.378tn Times Cited:29 Cited References Count:6

J Phys Chem B J Phys Chem B

We report a molecular dynamics study of the solvation of UCl6-, UCl62-, and UCl63- complexes in the [BMI][Tf2N] and [MeBu3N][Tf2N] ionic liquid cations based on the same anion (bis(trifluoromethylsulfonyl)imide (Tf2N-)) and the butyl-3-methyl-imidazolium(+) (BMI+) or methyl-tri-n-butyl-ammonium (MeBu3N+) cation, respectively. The comparison of two electrostatic models of the complexes (ionic model with -1 charged halides versus quantum mechanically derived charges) yields similar solvation features of a given solute. In the two liquids, the first solvation shell of the complexes is positively charged and evolves from purely cationic in the case of UCl63- to a mixture of cations and anions in the case of UCl6-. UCl63- is exclusively "coordinated" to BMI+ or MeBu3N+ solvent cations that mainly interact via their CH aromatic protons or their N-Me group, respectively. Around the less charged UCl6- complex, the cations interact via the less polar moieties (butyl chains of BMI+ or MeBu3N+) and the anions display nonspecific interactions. In no case does the uranium atom further coordinate solvent ions. According to an energy components analysis, UCl63- interacts more attractively with the [BMI][Tf2N] liquid than with [MeBu3N][Tf2N], while UCl6- does not show any preference, suggesting a significant solvation effect of the redox properties of uranium, also supported by free energy perturbation simulations. The effect of ionic liquid (IL) humidity is investigated by simulating the three complexes in 1:8 water/IL mixtures. In contrast to the case of "naked" ions (e.g., lanthanide(3+), UO22+, alkali, or halides), water has little influence on the solvation of the UCl6n- complexes in the two simulated ILs, as indicated by structural and energy analysis.163va Times Cited:30 Cited References Count:7

Acs Sym Ser Acs Sym Ser

In relation with the liquid-liquid extraction of uranyl nitrate from an acidic aqueous phase to supercritical CO(2), we present a series of molecular dynamics (MD) simulations on the "interfacial" systems involving UO(2)(NO(3))(2) species and high concentrations of TBP and nitric acid. We compare the distribution of solvent and solutes at the interface which forms upon the demixing of "chaotic mixtures" of water / CO(2) solutions. The simulations highlight the importance of interfacial phenomena in uranyl extraction to CO(2). In most cases, demixing leads to separation of aqueous and CO(2) phases which form an interface. At low concentrations, TBP and the neutral form HNO(3) of the acid adsorb at the interface, while the uranyl salt and ionic species sit m water. Spontaneous complexation of uranyl salts by TBP is observed, leading to UO(2)(NO(3))(2)(TBP)(H(2)O) and UO(2)(NO(3))(2)(TBP)(2) species of 1:1 and 1:2 stoichiometry, respectively, which adsorb at the interface. As the TBP concentration is increased, the proportion of 1:2 species, more hydrophobic than the 1: 1 species, increases, following the Le Chatelier principle. Nitric acid competes with the uranyl complexation by TBP which forms hydrogen bonds with H(3)O(+) or HNO(3) species. Thus, at high acid:TBP ratio, the concentration of 1:1 and 1:2 complexes decreases. Also noteworthy is the evolution of the interface from a well-defined border at low acid and TBP concentrations, to a mixed microscopic "third phase" containing some 1:2 complexes which can be considered as "extracted". We believe that such heterogeneous microphase is important for the stabilization and extraction of uranyl complexes by TBP and, more generally, in the extraction of highly hydrophilic cations (e.g. lanthanides or actinides) to organic media.Bx55a Times Cited:17 Cited References Count:54 ACS Symposium Serie

New J Chem New J Chem

We report a molecular dynamics study on a nitric acid water supercritical CO(2) interface. Three extreme models are compared, where the acid is either all neutral HNO(3), completely dissociated into NO(3)(-) and H(3)O(+), or represented by a 1 : 1 mixture of both forms. The ionic species are found to be repelled by the neat interface, while the neutral HNO(3) molecules are highly surface active. Similar features are observed with SC-CO(2) or with chloroform as the organic phase, pointing to the generality of interfacial activity of the acid.521by Times Cited:16 Cited References Count:4