Importance Of Interfacial Phenomena Research Articles

Surfactant Behavior of “Ellipsoidal” Dicarbollide Anions: A Molecular Dynamics Study

We report a molecular dynamics study of cobalt bis(dicarbollide) anions [(B(9)C(2)H(8)X(3))(2)Co](-) (XCD(-)) commonly used in liquid-liquid extraction (X = H, Me, Cl, or Br), showing that these anions, although lacking the amphiphilic topology, behave as anionic surfactants. In pure water, they display "hydrophobic attractions", leading to the formation of aggregates of different sizes and shapes depending on the counterions. When simulated at a water/"oil" interface, the different anions (HCD(-), MeCD(-), CCD(-), and BrCD(-)) are found to be surface active. As a result, the simulated M(n+) counterions (M(n+) = Na(+), K(+), Cs(+), H(3)O(+), UO(2)(2+), Eu(3+)) concentrate on the aqueous side of the interface, forming a "double layer" whose characteristics are modulated by the hydrophobic character of the anion and by M(n+). The highly hydrophilic Eu(3+) or UO(2)(2+) cations that are generally "repelled" by aqueous interfaces are attracted by dicarbollides near the interface, which is crucial as far as the mechanism of assisted cation extraction to the oil phase is concerned. These cations interact with interfacial XCD(-) in their fully hydrated Eu(H(2)O)(9)(3+) and UO(2)(H(2)O)(5)(2+) forms, whereas the less hydrophilic monocharged cations display intimate contacts via their X substituents. The results obtained with the TIP3P and OPLS models for the solvents are confirmed with other water models (TIP5P or a polarizable 4P-Pol water) and with more polar "oil" models. The importance of interfacial phenomena is further demonstrated by simulations with a high oil-water ratio, leading to the formation of a micelle covered with CCD's. We suggest that the interfacial activity of dicarbollides and related hydrophobic anions is an important feature of synergism in liquid-liquid extraction of hard cations (e.g., for nuclear waste partitioning).

Importance of Interfacial Phenomena and Synergistic Effects in Lanthanide Cation Extraction by Dithiophosphinic Ligands: A Molecular Dynamics Study

According to molecular dynamics simulations, uncomplexed Cyanex-301-like ligands L- and LH (derived from the dithiophosphinic acid R2PS2H) and their EuL3 complexes are surface active. They adsorb at a water/“oil” interface, where “oil” is modeled by chloroform. It is shown that addition of neutral phosphoryl ligands such as TBP (tri-n-butyl phosphate) is necessary to promote the extraction of the complex, via two complementary mechanisms. First, gradual co-complexation of TBP, forming EuL3(TBP)n species, enhances the interfacial activity and the hydrophobicity of the lanthanide complex. Second, high concentrations of TBP are needed to saturate and modify the interface, leading to the migration of the EuL3(TBP)3 complex to the oil phase. The effect of oil/water ratio is also investigated. At high ratio, one observes the formation of water-in-oil microdroplets onto which TBPs adsorb, while the complexes are fully extracted to the oil phase. These features make it possible to better understand synergistic and solvation effects in the assisted liquid−liquid extraction of lanthanide or actinide cations.

Theoretical Studies on Lanthanide Cation Extraction by Picolinamides: Ligand–Cation Interactions and Interfacial Behavior

We report theoretical investigations on the complexation and liquid–liquid extraction of trivalent lanthanide cations by picolinamide ligands L. The relative contributions of pyridine vs. amide moieties of L and the effect of alkyl substituents are investigated in the gas phase by quantum mechanical calculations. The uncomplexed ligands L with different lipophilic substituents and Eu(NO3)3 L 3 complexes are simulated at a water–“oil” interface, where “oil” is modeled by chloroform. The simulations point to the importance of interfacial phenomena in the liquid–liquid extraction of cations by picolinamide ligands and the difference between pyridine vs. amide substituted ligands.

Strong metal-support interactions in nickel/titania catalysts: The importance of interfacial phenomena

Nickel catalysts supported on silica and titania have been prepared by wet impregnation. The uncalcined catalysts, after an initial mild reduction at 623 K, have been treated by heating in hydrogen or argon at successively higher temperatures up to 923 K. The activity and selectivity of the catalysts have been determined under flow conditions at 1 bar pressure for the hydrogenolysis of n-hexane at 548 K, for the hydrogenolysis of ethane over the temperature range 478–673 K, and for the CO/hydrogen reaction at 548 K. Hydrogen adsorption isotherms have been measured and the specific metal surface areas determined using the Langmuir equation for dissociative adsorption to determine the monolayer coverage. Mean crystallite diameters have been determined by X-ray line broadening. Ni/titania catalysts exhibit normal hydrogen chemisorption properties after reduction at 723 K, although adsorption is suppressed after thermal treatment at 923 K. X-Ray line broadening measurements indicate a mean particle size of 8.0 nm after reduction at 723 K, and 13.3 nm after heating to 923 K. Activity measurements, after reduction at 723 K, show excellent agreement between the specific activities of the Ni/silica and Ni/titania catalysts for the hydrogenolysis of n-hexane and ethane. All these results are consistent with literature data for these reactions. However, for the CO/hydrogen reaction the specific activity of the Ni/titania catalyst is a factor of 50 higher than that of the Ni/silica catalyst. After heating at 923 K, there was a large decrease in the activity of the Ni/titania catalysts for all the reactions investigated. It is concluded that the enhanced activity of the Ni/titania catalysts for the CO/hydrogen reaction cannot be due to strong metal-support interactions (SMSI) as defined in the literature. A model of the Ni/titania system is presented which emphasizes the importance of considering the interface between the metal and the partially reduced support. It is proposed that new active sites created at the interface are responsible for the high specific activity of these catalysts for the CO/hydrogen reaction.