Ammonium recognition by 18-crown-6 in different solutions and at an aqueous interface: a simulation study.

The complexation of alkylammonium RNH3(+) cations by 18-crown-6 (18C6) is studied by molecular dynamics (MD) and potential of mean force (PMF) simulations in different solvents (methanol, chloroform, 90:10 chloroform/methanol mixture, water) and at the chloroform/water interface. The free energies of association ΔGass, obtained with different charge models of 18C6, are compared for PrNH3(+) and K(+) cations yielding, with suitable electrostatic models, a preference for K(+) in the different monophasic solutions, as well as in the gas phase. Furthermore, for a given cation, ΔGass is markedly solvent dependent and decreases in magnitude in the order chloroform ≫ mixture > methanol > water, that is, with the (de)solvation energy of the cation and with the extent of pairing with the counterion (here, picrate, Pic(-)). Despite their macroscopic intermiscibilities at all proportions, chloroform and methanol are found to form, at the microscopic level, an inhomogeneous liquid that displays dual solvation properties toward its solutes. As a result, in the monophasic 90:10 mixture that contains mainly chloroform, the ΔGass energies for PrNH3(+) or K(+) complexation are closer to those in methanol than to those in chloroform. On the other hand, chloroform and water form a biphasic mixture and delineate an interface onto which 18C6 and the tBuNH3(+) and Pic(-) ions, as well as their complex, are found to adsorb, a feature also supported by the different free energy profiles for interface crossing. Interestingly, the complexation energy of tBuNH3(+) is found to be stronger at the interface than in pure water, demonstrating the crucial role of complexation by 18C6 at the interface to promote the cation transfer to the organic phase.

Liquid-liquid extraction of uranyl by TBP: the TBP and ions models and related interfacial features revisited by MD and PMF simulations.

We report a molecular dynamics (MD) study of biphasic systems involved in the liquid-liquid extraction of uranyl nitrate by tri-n-butylphosphate (TBP) to hexane, from "pH neutral" or acidic (3 M nitric acid) aqueous solutions, to assess the model dependence of the surface activity and partitioning of TBP alone, of its UO2(NO3)2(TBP)2 complex, and of UO2(NO3)2 or UO2(2+) uncomplexed. For this purpose, we first compare several electrostatic representations of TBP with regards to its polarity and conformational properties, its interactions with H2O, HNO3, and UO2(NO3)2 species, its relative free energies of solvation in water or oil environments, the properties of the pure TBP liquid and of the pure-TBP/water interface. The free energies of transfer of TBP, UO2(NO3)2, UO2(2+), and the UO2(NO3)2(TBP)2 complex across the water/oil interface are then investigated by potential of mean force (PMF) calculations, comparing different TBP models and two charge models of uranyl nitrate. Describing uranyl and nitrate ions with integer charges (+2 and -1, respectively) is shown to exaggerate the hydrophilicity and surface activity of the UO2(NO3)2(TBP)2 complex. With more appropriate ESP charges, mimicking charge transfer and polarization effects in the UO2(NO3)2 moiety or in the whole complex, the latter is no more surface active. This feature is confirmed by MD, PMF, and mixing-demixing simulations with or without polarization. Furthermore, with ESP charges, pulling the UO2(NO3)2 species to the TBP phase affords the formation of UO2(NO3)2(TBP)2 at the interface, followed by its energetically favorable extraction. The neutral complexes should therefore not accumulate at the interface during the extraction process, but diffuse to the oil phase. A similar feature is found for an UO2(NO3)2(Amide)2 neutral complex with fatty amide extracting ligands, calling for further simulations and experimental studies (e.g., time evolution of the nonlinear spectroscopic signature and of surface tension) on the interfacial landscape upon ion extraction.

Liquid-liquid extraction of uranyl by an amide ligand: interfacial features studied by MD and PMF simulations.

We report a molecular dynamics study of biphasic systems involved in the liquid-liquid extraction of uranyl nitrate by a monoamide ligand (L = N,N-di(2-ethylhexyl)isobutyramide, DEHiBA) to hexane, from pH neutral or acidic (3 M nitric acid) aqueous solutions. We first describe the neat interfaces simulated with three electrostatic models, one of which including atomic polarizabilities. The free energy profiles for crossing the water/hexane interface by L or its UO2(NO3)2L2 complex are then investigated by PMF (potential of mean force) calculations. They indicate that the free ligand and its complex are surface active. With the polarizable force field, however, the complexes have a lower affinity for the interface than without polarization. When DEHiBA gets more concentrated and in acidic conditions, their surface activity diminishes. Surface activity of UO2(NO3)2L2 complexes is further demonstrated by demixing simulations of randomly mixed DEHiBA, hexane, and neutral or acidic water. Furthermore, demixing of randomly mixed solvents, L molecules, UO2(NO3)2 salts, and nitric acid shows in some cases complexation of L to form UO2(NO3)2L2 and UO2(NO3)2L complexes that adsorb at the aqueous interfaces. These features suggest that uranyl complexation by amide ligands occurs "right at the interface", displaying marked analogies with the liquid-liquid extraction of uranyl by TBP (tri-n-butyl phosphate). Regarding the positive effect of nitric acid on extraction, the simulations point to several facets involving enhanced ion pairing of uranyl nitrate, decreased affinity of the complex for the interface, and finally, stabilization of the complex in the organic phase.

Oil-soluble and water-soluble BTPhens and their europium complexes in octanol/water solutions: interface crossing studied by MD and PMF simulations.

Bistriazinyl-phenantroline "BTPhen" ligands L display the remarkable feature to complex trivalent lanthanide and actinide ions, with a marked selectivity for the latter. We report on molecular dynamics studies of tetrasubstituted X(4)BTPhens: L(4+) (X = (+)Et(3)NCH(2)-), L(4-) (X = (-)SO(3)Ph-), and L(0) (X = CyMe(4)) and their complexes with Eu(III) in binary octanol/water solutions. Changes in free energies upon interface crossing are also calculated for typical solutes by potential of mean force PMF simulations. The ligands and their complexes partition, as expected, to either the aqueous or the oil phase, depending on the "solubilizing" group X. Furthermore, most of them are found to be surface active. The water-soluble L(4+) and L(4-) ligands and their (L)Eu(NO(3))(3) complexes adsorb at the aqueous side of the interface, more with L(4-) than with L(4+). The oil soluble ligand L(0) is not surface active in its endo-endo form but adsorbs on the oil side of the interface in its most polar endo-exo form, as well as in its protonated L(0)H(+) and complexed (L(0))Eu(NO(3))(3) states. Furthermore, comparing PMFs of the Eu(III) complexes with and without nitric acid shows that acidifying the aqueous phase has different effects, depending on the ligand charge. In particular, acid promotes the Eu(III) extraction by L(0) via the (L(0))(2)Eu(NO(3))(2+) complex, as observed experimentally. Overall, the results point to the importance of interfacial adsorption for the liquid-liquid extraction of trivalent lanthanide and actinide cations by BTPhens and analogues.

Acid extraction to a hydrophobic ionic liquid: the role of added tributylphosphate investigated by experiments and simulations.

We have studied the extraction of four HA acids (HNO(3), HReO(4), HClO(4), HCl) to a hydrophobic ionic liquid (IL) 1-butyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)amide (BMI(+) Tf(2)N(-)) at room temperature, in a wide range of acidic concentrations in water. The effect of tributylphosphate (TBP) as co-solvent is investigated. According to experimental observations, water dragging to the IL phase increases with added TBP and/or acids. Acid extraction is found to be weak, however, for the four acids except for concentrated HNO(3) (>3 M). Molecular dynamics simulations on model biphasic systems show that TBP is not surface active, but well dissolved in the IL. They also reveal the importance of HA acid model (either totally or half dissociated) and of the TBP content on acid extraction to the IL. Furthermore, they show that "the proton" can be extracted by TBP (H(3)O(+)(TBP)(3)"complex") without its A(-) conjugated base, via a cation transfer mechanism (BMI(+) transfer to water). Experiments and simulations show that TBP plays an important role in the mutual solubility between water and ionic liquid, by different amounts, depending on the HA acid. On the other hand, both approaches indicate that a HTf(2)N containing aqueous solution completely mixes with the [BMI][Tf(2)N] IL that contains the same Tf(2)N(-) anion.

Basicity, complexation ability and interfacial behavior of BTBPs: a simulation study.

BTBPs represent an important class of tetradentate heterocyclic ligands with N-donor binding sites that have been recently developed to separate trivalent actinides from lanthanides. We first investigate by QM calculations the conformational properties, basicity and complexation energies with Eu(NO(3))(3), comparing BTBP derivatives with alkyl substituents on the pyridinyl or triazinyl moieties to their conformationally cis-locked BTPhen analogues. The latter, preorganized for protonation and complexation, are found to be more basic and to afford more stable complexes. We next explore the interfacial behavior of CyMe(4)BTBP in its neutral versus protonated states and of 1:1 Eu(NO(3))(3)(CyMe(4)BTBP) complexes at the aqueous interface with an octanol-hexane mixture. The neutral BTBP ligand displays no visible surface activity, whereas protonated and complexed ligands are surface active. Taken together, the QM and MD results suggest that Eu(III) extraction by BTBPs occurs at the interface, via the protonated form of the ligand in acidic conditions, explaining why the extraction kinetics is slow and why BTPhen ligands are more efficient than BTBPs.

Strontium nitrate extraction to ionic liquids by a crown ether: a molecular dynamics study of aqueous interfaces with C4mim+- vs C8mim+-based ionic liquids.

In order to gain microscopic insights into the extraction mechanism of strontium cations by 18-crown-6 (18C6) to room temperature ionic liquids (ILs), we simulated by molecular dynamics (MD) strontium complexes in neat ionic liquids and at their interfaces with water. We compared two ILs, based on the PF(6)(-) anion and either 1-butyl-3-methylimidazolium (C(4)mim(+)) or 1-octyl-3-methylimidazolium (C(8)mim(+)) cations. Regarding the complexes, two states were considered: charged [Sr⊂18C6](2+) vs neutral [Sr⊂18C6,(NO(3))(2)], where the nitrates are either fully dissociated or coordinated to Sr. In "dry" or "humid" [C(4)mim][PF(6)] and in "dry" [C(8)mim][PF(6)] IL, the neutral complex is found to be the most stable one. In the binary IL/water solutions, the charged complexes mostly partition to the aqueous phase, whereas the neutral [Sr⊂18C6,(NO(3))(2)] complexes are more concentrated in the interfacial domain. The aqueous solutions in contact with the ionic liquids contain C(4)mim(+), but almost no C(8)mim(+) ions, supporting a classical extraction mechanism to [C(8)mim][PF(6)] and an ion exchange mechanism to [C(4)mim][PF(6)]. Furthermore, remarkable events occurred during the dynamics, where complexes were extracted to the IL phases. When compared to the interfacial landscapes obtained with the same solutes at a classical organic liquid (chloroform)/water interface, those with ILs allow us to better understand specific features of liquid-liquid extraction to ILs.

BTP-based ligands and their complexes with Eu(3+) at "oil"/water interfaces. A molecular dynamics study.

Heterocyclic N-donor ligands based on the bistriazinylpyridine (BTPs) skeleton have been recently developed to separate trivalent actinides from lanthanides by liquid-liquid extraction from nuclear solutions. In this paper, we report molecular dynamics investigations on BTPs in water-"oil" biphasic systems (oil = hexane + octanol vs. hexane vs. nitrobenzene vs. chloroform) and compare different BTP derivatives, their neutral vs. protonated forms, and their 1 : 3 complexes with Eu((III)). Neutral BTPs are found to be weakly surface active and to display multiple orientations at the interface, depending on time and on their lateral and para substituents. This contrasts with their protonated forms that strongly adsorb at interfaces with neutral or acidic water. Remarkably, the protonated cyMe(4)-BTPH(+) and, to a lesser extent, (i)PrBTPH(+) ligands adopts at the interface an "inversed orientation", where NH(+) points towards oil, instead of water. The [Eu(BTP)(3)](3+) complexes are also found to be highly surface active: in spite of Eu((III)) shielding by the three ligands, these complexes remain strongly attracted by water at the aqueous side of the interface. Taken together, the MD results suggest that ion complexation by BTPs occurs right at the interface, from the protonated BTPH(+) forms. They may explain why extraction is improved upon increase of the aqueous phase acidity, though with slow kinetics. They also open perspectives for designing new derivatives for efficient separation of trivalent actinides from lanthanides.

Competitive complexation of nitrates and chlorides to uranyl in a room temperature ionic liquid.

By coupling EXAFS, UV-vis spectroscopy, and molecular dynamics and quantum mechanical calculations, we studied the competitive complexation of uranyl cations with nitrate and chloride ions in a water immiscible ionic liquid (IL), C(4)mimTf(2)N (C(4)mim(+): 1-butyl-3-methyl-imidazolium; Tf(2)N(-) = (CF(3)SO(2))(2)N)(-): bis(trifluoromethylsulfonyl)imide). Both nitrate and chloride are stronger ligands for uranyl than the IL Tf(2)N(-) or triflate anions and when those anions are simultaneously present, neither the limiting complex UO(2)(NO(3))(3)(-) nor UO(2)Cl(4)(2-) alone could be observed. At a U/NO(3)/Cl ratio of 1/2/2, the dominant species is likely UO(2)Cl(NO(3))(2)(-). When chloride is in excess over uranyl with different nitrate concentrations (U/NO(3)/Cl ratio of 1/2/6, 1/4/4, and 1/12/4) the solution contains a mixture of UO(2)Cl(4)(2-) and UO(2)Cl(3)(NO(3))(2-) species. Furthermore, it is shown that the experimental protocol for introducing these anions to the solution (either as uranyl counterion, as added salt, or as IL component) influences the UV-vis spectra, pointing to the formation of different kinetically equilibrated complexes in the IL.

Solvation of Ln((III)) lanthanide cations in the [BMI][SCN], [MeBu(3)N][SCN], and [BMI](5)[Ln(NCS)(8)] ionic liquids: a molecular dynamics study.

We report a molecular dynamics study on the solvation of trivalent lanthanide cations Ln((III)) (Ln = La, Eu and Yb) in the [BMI][SCN] and [MeBu(3)N][SCN] ionic liquids (ILs), based respectively on 1-methyl-3-butylimidazolium (BMI(+)) and tributylmethylammonium (MeBu(3)N(+)) cations, and on thiocyanate (SCN(-)) anions. For this purpose we first derive a force field representation of SCN(-) that simultaneously describes the [BMI][SCN] liquid and SCN(-) as a ligand for Ln((III)) ions and, in particular, for the energy difference between N- vs S-coordination, as compared to QM calculated values. In liquid simulations, we compare different initial states where the solute is either the "naked" Ln((III)) ion or its anionic Ln(NCS)(8)(5-) complex. In all cases, the first solvation shell of Ln((III)) is found to be purely anionic, with 6 to 8 N-coordinated ligands, depending on the nature of Ln((III)) and the immersed solute. This first shell is surrounded by 13 - 14 BMI(+) or 8 - 9 MeBu(3)N(+) cations, leading to an "onion type" solvation of Ln((III)). The comparison of gas phase optimized structures (that are all unstable from n = 5 NCS(-) ligands) to those observed in ILs points to the importance of solvation forces on the nature of the Ln((III)) complex, with a marked contribution of the IL cation. A given Ln(NCS)(n)(3-n) complex is found to be better stabilized by the imidazolium than by the ammonium based IL. Furthermore, according to free energy PMF (potential of mean force) calculations, the imidazolium based liquid favors somewhat higher coordination numbers (CNs), compared to the ammonium based IL. For instance, the coordination of an eight SCN(-) ligand to Eu(NCS)(7)(4-) is favored in the [BMI][SCN] liquid, but not in [MeBu(3)N][SCN]. For the La((III)) and Yb((III)) cations, the CNs are the same in both liquids (8 and 7, respectively), but the free energy differences between the two types of complexes differ markedly. The final part of the paper is devoted to the [BMI](5)[Ln(NCS)(8)] pure ILs, based on BMI(+) as cation and on the Ln(NCS)(8)(5-) complex (Ln = La((III)) and Yb((III))) as anionic component. In these liquids, Ln((III)) CNs are found to be similar to those found in the [BMI][SCN] solutions, and the dynamics is characterized by a fluid behavior of the BMI(+) ions diffusing around a quasi frozen network of anionic complexes.

Conformational and Cs+ complexation properties of norbadione-A: a molecular modeling study.

We report a quantum mechanical (QM) and classical molecular dynamics (MD) study of the conformational and complexation properties of norbadione-A (NBA), a key pigment involved in the Cs+ complexation by mushrooms. The Z versus E isomers of its pulvinic moieties are compared in their neutral (Pulv0), mono- (Pulv(-1)) and di-deprotonated (Pulv(-2)) states, and the 1H chemical shifts are calculated ab initio. Pulv(-1) is found to be stabilized in the E form by an internal COOH(-)O(enolate) hydrogen-bond. No energy minimum is found for the corresponding COO(-)HO(enol) state, indicating that the conjugated enol function of Pulv0 is more acidic than the COOH function. Further deprotonation leads to the Z and E forms of Pulv(-2) that are close in energy and both account for a marked downfield shift delta of ortho-H8 protons. A similar shift is found upon deprotonation of the enol function of an ester analogue of Pulv0. Therefore, contrary to previous assumptions (ref. 7: P. Kuad, et al., J. Am. Chem. Soc., 2005, 127, 1323), the large shift of delta(H8) around pH 9.5 upon deprotonation of NBA or of pulvinic acid cannot be taken as an indicator of an E-to-Z conformational switch, but merely reflects the pH-induced conformational change of the carboxylate group adjacent to the (H8)-ring. The QM and MD studies on NBA(2-) and NBA(4-) support the view that both species prefer the E/E form with two intramolecular COOH(-)O(enolate) hydrogen-bonds in the gas phase and in solution. Finally, we simulated mono- and di-nuclear complexes of Cs+ with NBA(2-) and NBA(4-) by MD, showing that only the NBA(4-) state populated at high pH values can bind two Cs+ cations, with both E and Z conformations of the pulvinic arms.

Molecular dynamics study of dicarbollide anions in nitrobenzene solution and at its aqueous interface. Synergistic effect in the Eu(III) assisted extraction.

We report a molecular dynamics study of chlorinated cobalt bis(dicarbollide) anions [(B(9)C(2)H(8)Cl(3))(2)Co](-)"CCD(-)" in nitrobenzene and at the nitrobenzene-water interface, with the main aim to understand the solution state of these hydrophobic species and why they act as strong synergists in assisted liquid-liquid extraction of metallic cations. Neat nitrobenzene is found to well solubilize CCD(-), Cs(+) salts in the form of diluted pairs or oligomers, without displaying aggregation. In biphasic nitrobenzene-water systems, CCD(-) anions mainly partition to the organic phase, thus attracting Cs(+) or even more hydrophilic counterions like Eu(3+) into that phase. The remaining CCD(-) anions adsorb at the interface, but are less surface active than at the chloroform interface. Finally, we compare the interfacial behavior of the Eu(BTP)(3)(3+) complex in the absence and in the presence of CCD(-) anions and extractant molecules. It is found that in the absence of CCDs, the complex is trapped at the interface, while when the CCDs are concentrated enough, the complex is extracted to the nitrobenzene phase. These results are compared to those obtained with chloroform or octanol as organic phase and discussed in the context of synergistic effect of CCDs in liquid-liquid extraction, pointing to the importance of dual solvation properties of nitrobenzene or octanol to solubilize the CCD(-) salts as well as the extracted complex.

Uranyl coordination in ionic liquids: the competition between ionic liquid anions, uranyl counterions, and Cl- anions investigated by extended X-ray absorption fine structure and UV-visible spectroscopies and molecular dynamics simulations.

The first coordination sphere of the uranyl cation in room-temperature ionic liquids (ILs) results from the competition between its initially bound counterions, the IL anions, and other anions (e.g., present as impurities or added to the solution). We present a joined spectroscopic (UV-visible and extended X-ray absorption fine structure)-simulation study of the coordination of uranyl initially introduced either as UO2X2 salts (X-=nitrate NO3-, triflate TfO-, perchlorate ClO4-) or as UO2(SO4) in a series of imidazolium-based ILs (C4mimA, A-=PF6-, Tf2N-, BF4- and C4mim=1-methyl-3-butyl-imidazolium) as well as in the Me3NBuTf2N IL. The solubility and dissociation of the uranyl salts are found to depend on the nature of X- and A-. The addition of Cl- anions promotes the solubilization of the nitrate and triflate salts in the C4mimPF6 and the C4mimBF4 ILs via the formation of chloro complexes, also formed with other salts. The first coordination sphere of uranyl is further investigated by molecular dynamics (MD) simulations on associated versus dissociated forms of UO2X2 salts in C4mimA ILs as a function of A- and X- anions. Furthermore, the comparison of UO2Cl(4)2-, 2 X- complexes with dissociated X- anions, to the UO2X2, 4 Cl- complexes with dissociated chlorides, shows that the former is more stable. The case of fluoro complexes is also considered, as a possible result of fluorinated IL anion's degradation, showing that UO2F42- should be most stable in solution. In all cases, uranyl is found to be solvated as formally anionic UO2XnAmClp2-n-m-p complexes, embedded in a cage of stabilizing IL imidazolium or ammonium cations.

Synergistic effect of dicarbollide anions in liquid-liquid extraction: a molecular dynamics study at the octanol-water interface.

We report a molecular dynamics study of chlorinated cobalt bis(dicarbollide) anions [(B(9)C(2)H(8)Cl(3))(2)Co](-)"CCD(-)" in octanol and at the octanol-water interface, with the main aim to understand why these hydrophobic species act as strong synergists in assisted liquid-liquid cation extraction. Neat octanol is quite heterogeneous and is found to display dual solvation properties, allowing to well solubilize CCD(-), Cs(+) salts in the form of diluted pairs or oligomers, without displaying aggregation. At the aqueous interface, octanol behaves as an amphiphile, forming either monolayers or bilayers, depending on the initial state and confinement conditions. In biphasic octanol-water systems, CCD(-) anions are found to mainly partition to the organic phase, thus attracting Cs(+) or even more hydrophilic counterions like Eu(3+) into that phase. The remaining CCD(-) anions adsorb at the interface, but are less surface active than at the chloroform interface. Finally, we compare the interfacial behavior of the Eu(BTP)(3)(3+) complex in the absence and in the presence of CCD(-) anions and extractant molecules. It is found that when the CCD(-)'s are concentrated enough, the complex is extracted to the octanol phase. Otherwise, it is trapped at the interface, attracted by water. These results are compared to those obtained with chloroform as organic phase and discussed in the context of synergistic effect of CCD(-) in liquid-liquid extraction, pointing to the importance of dual solvation properties of octanol and of the hydrophobic character of CCD(-) for synergistic extraction of cations.

Molecular dynamics simulations of the aqueous interface with the [BMI][PF6] ionic liquid: Comparison of different solvent models.

We report a Molecular Dynamics (MD) study of the interface between water and the hygroscopic room temperature Ionic Liquid "IL" [BMI][PF6] (1-butyl-3-methyl-imidazolium hexafluorophosphate), comparing the TIP3P, SPC/E and TIP5P models for water and two IL models where the ions are +/-1 or +/-0.9 charged. A recent MD study (A. Chaumont, R. Schurhammer and G. Wipff, J. Phys. Chem. B, 2005, 109, 18964) showed that using TIP3P water in conjunction with the IL(+/-1) model led to water-IL mixing without forming an interface, whereas a biphasic system could be obtained with the IL(+/-0.9) model. With the TIP5P and SPC/E models, the juxtaposed aqueous and IL phases are found to remain distinct for at least 20 ns. The resulting IL humidity, exaggerated with the IL(+/-1) model, is in better agreement with experiment using the IL(+/-0.9) model. We also report demixing simulations on the "randomly mixed" liquids, using the IL(+/-0.9) model for the ionic liquid. With the three tested water models, the phases separate very slowly ( approximately 20 ns or more) compared to "classical" chloroform-water mixtures (less than 1 ns), leading to biphasic systems similar to those obtained after equilibration of the juxtaposed liquids. The characteristics of the interface (size, polarity, ion orientation, electrostatic potential) are compared with the different models. Possible reasons why, among the three tested water models, the widely-used TIP3P model exaggerates the inter-solvent mixing, are analyzed. The difficulty in computationally and experimentally equilibrating water-IL mixtures is attributed to the slow dynamics and micro-heterogeneity of the IL and to the different states of water in the IL phase.

The [BMI][Tf2N] ionic liquid/water binary system: a molecular dynamics study of phase separation and of the liquid-liquid interface.

We report molecular dynamics (MD) simulations of the aqueous interface of the hydrophobic [BMI][Tf2N] ionic liquid (IL), composed of 1-butyl-3-methylimidazolium cations (BMI+) and bis(trifluoromethylsulfonyl)imide anions (Tf2N-). The questions of water/IL phase separation and properties of the neat interface are addressed, comparing different liquid models (TIP3P vs TIP5P water and +1.0/-1.0 vs +0.9/-0.9 charged IL ions), the Ewald vs the reaction field treatments of the long range electrostatics, and different starting conditions. With the different models, the "randomly" mixed liquids separate much more slowly (in 20 to 40 ns) than classical water-oil mixtures do (typically, in less than 1 ns), finally leading to distinct nanoscopic phases separated by an interface, as in simulations which started with a preformed interface, but the IL phase is more humid. The final state of water in the IL thus depends on the protocol and relates to IL heterogeneities and viscosity. Water mainly fluctuates in hydrophilic basins (rich in O(Tf2N) and aromatic CH(BMI) groups), separated by more hydrophobic domains (rich in CF3(Tf2N) and alkyl(BMI) groups), in the form of monomers and dimers in the weakly humid IL phase, and as higher aggregates when the IL phase is more humid. There is more water in the IL than IL in water, to different extents, depending on the model. The interface is sharper and narrower (approximately 10 A) than with the less hydrophobic [BMI][PF6] IL and is overall neutral, with isotropically oriented molecules, as in the bulk phases. The results allow us to better understand the analogies and differences of aqueous interfaces with hydrophobic (but hygroscopic) ILs, compared to classical organic liquids.

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).

Adsorption at the liquid-liquid interface in the biphasic rhodium catalyzed hydroformylation of olefins promoted by cyclodextrins: a molecular dynamics study.

Using molecular dynamics (MD) simulations, we investigate the interfacial distribution of partners involved in the phase transfer rhodium catalyzed hydroformylation of olefins promoted by beta-cyclodextrins (beta-CDs). The beta-CDs, the reactant (alkene), product (aldehyde), several rhodium complexes (the catalyst, its precursor, and its alkene adduct) are simulated at the water-"oil" interface, where oil is represented by chloroform or hexane. It is shown that unsubstituted beta-CD and its 6-methylated and 2,6-dimethylated analogues adsorb at the interface, whereas the liposoluble permethylated CD does not. The precursor of the catalyst [RhH(CO)(TPPTS)3]9- (with triphenylphosphine trisulfonated TPPTS3- ligands) sits in water, but the less charged [RhH(CO)(TPPTS)2]6- catalyst and the [RhH(CO)(TPPTS)2(alkene)]6- reaction intermediate are clearly surface active. The TPPTS3- anions also concentrate at the interface, where they adopt an amphiphilic conformation, forming an electrical double layer with their Na+ counterions. Thus, the most important key partners involved in the hydroformylation reaction concentrate at the interface, thereby facilitating the reaction, a process which may be further facilitated upon complexation by CDs. These results point to the importance of adsorption at the liquid-liquid interface in the two-phase hydroformylation reaction of olefins promoted by beta-CDs and provide microscopic pictures of this peculiar region of the solution.

Alkali cation extraction by calix[4]crown-6 to room-temperature ionic liquids. The effect of solvent anion and humidity investigated by molecular dynamics simulations.

We report a molecular dynamics study on the solvation of M+ (Na+ to Cs+) alkali cations and of their LM+ complexes with a calix[4]arene host (L = 1,3-dimethoxy-calix[4]arene-crown-6 in the 1,3-alternate conformation) in the [BMI][PF6] and [BMI][Tf2N] room-temperature ionic liquids "ILs" based on the BMI+ (1-butyl-3-methylimidazolium) cation. The comparison of the two liquids and the dry versus humid form of the former one (with a 1:1 ratio of H2O and BMI+PF6- species) reveals the importance of humidity: in [BMI][PF6]-dry as in the [BMI][Tf2N] liquid, the first solvation shell of the "naked" M+ ions is composed of solvent anions only (four PF6- anions, and from four to five Tf2N- anions, respectively, quasi-neutralized by a surrounding cage of BMI+ cations), while in the [BMI][PF6]-humid IL, it comprises from one to three solvent anions and about four H2O molecules. In the LM+ complexes, the cation is shielded from solvent, but still somewhat interacts with a solvent anion in the dry ILs and with water in the humid IL. We also report tests on M+ interactions with solvent anions PF6- and Tf2N- in the gas phase, showing that the AMBER results are in satisfactory agreement with QM results obtained at different levels of theory. The question of ion recognition by L is then examined by free energy perturbation studies in the three liquids, predicting a high Cs+/Na+ selectivity upon liquid extraction from an aqueous phase, in agreement with experimental results on a parent calixarene host. A similar Cs+/Na+ selectivity is predicted upon complexation in a homogeneous IL phase, mainly due to the desolvation energy of the free cations. Thus, despite their polar character, ionic liquids qualitatively behave as classical weakly polar organic liquids (e.g., choroform) as far as liquid-liquid extraction is concerned but more like polar liquids (water, alcohols) as far as complexation in a single phase is concerned.

Effect of the TBP and water on the complexation of uranyl nitrate and the dissolution of nitric acid into supercritical CO2. A Theoretical Study.

We report theoretical studies on the complexation of uranyl nitrate and the dissolution of nitric acid in supercritical CO2 by TBP. According to quantum mechanical calculations, TBP (modeled by trimethyl phosphate TMP) displays stronger hydrogen-bonding interactions with HNO3 than with H2O, and this has been modeled in force-field calculations. Different combinations of water, TBP, and acid are compared in SC-CO2 and simulated by molecular dynamics (MD), demonstrating the importance of TBP and water concentrations. In MD simulations, which started from "random" mixtures of water, TBP, nitric acid, and uranyl nitrate, complexation of uranyl by TBP is observed and the yield increases with the TBP concentration. TBP molecules are also necessary to dissolve nitric acid in the supercritical phase. Indeed, without TBP, nitric acid alone self aggregates via hydrogen-bonding interactions. Adding water to this solution leads to the formation of water microdomains containing the acid and uranyl salts. The simulations show that a high TBP/nitric acid ratio is needed to fully dissolve the acid in the supercritical phase and to form CO2-philic UO2(NO3)2(TBP)2 complexes. The resulting hydrogen-bonding and solvation patterns are analyzed. The results are consistent with experimental observations and provide microscopic views of this important extraction system.