Migration of Ionophores and Salts through a Water−Chloroform Liquid−Liquid Interface: Molecular Dynamics−Potential of Mean Force Investigations

Liquid-Liquid Extraction of the Eu(III) Cation by BTP Ligands into Ionic Liquids: Interfacial Features and Extraction Mechanisms Investigated by MD Simulations

What happens at the ionic-liquid (IL)/water interface when the Eu³⁺ cation is complexed and extracted by bis(dimethyltriazinyl) pyridine "BTP" ligands has been investigated by molecular dynamics and potential of mean force simulations on the interface crossing by key species: neutral BTP, its protonated BTPH⁺ form, Eu³⁺, and the Eu(BTP)₃³⁺ complex. At both the [BMI][Tf₂N]/water and [OMI][Tf₂N]/water interfaces, neither BTP nor Eu(BTP)₃³⁺ are found to adsorb. The distribution of Eu(BTP)₂³⁺ and Eu(BTP)³⁺ precursors of Eu(BTP)₃³⁺, and of their nitrate adducts, implies the occurrence of a stepwise complexation process in the interfacial domain, however. The analysis of the ionic content of the bulk phases and of their interface before and after extraction highlights the role of charge buffering by interfacial IL cations and anions, by different amounts depending on the IL. Comparison of ILs with octanol as the oil phase reveals striking differences regarding the extraction efficiency, the affinity of Eu(BTP)₃³⁺ for the interface, the effects of added nitric acid and of counterions (NO₃^- vs Tf₂N^-), charge neutralization mechanisms, and the extent of "oil" heterogeneity. Extraction into octanol is suggested to proceed via adsorption at the surface of water pools, nanoemulsions, or droplets, with marked counterion effects.

Understanding the Complex Surface Interplay for Extraction: A Molecular Dynamics Study

By means of classical molecular dynamics simulation the interfacial properties of methanol and n‐dodecane, which are two potential candidate solvents for use in non‐aqueous liquid–liquid extraction, were assessed. The question of how the interface changes depending on the concentration of extractant (tri‐n‐butyl phosphate) and salt (LiCl) is addressed. Two different models to represent systems were used to evaluate how LiCl and tri‐n‐butyl phosphate affect mutual miscibility, and how the last‐named behaves depending on the chemical environment. Tri‐n‐butyl phosphate increases the mutual solubility of the solvents, whereas LiCl counteracts it. The extractant was found to be mostly adsorbed on the interface between the solvents, and therefore the structural features of the adsorption were investigated. Adsorption of tri‐n‐butyl phosphate changes depending on its concentration and the presence of LiCl. It exhibits a preferential orientation in which the butyl chains point at the n‐dodecane phase and the phosphate group points at the methanol phase. For high concentrations of tri‐n‐butyl phosphate, its molecular orientation is preserved by diffusion of the excess molecules into both the methanol and n‐dodecane phases. However, LiCl hinders the diffusion into the methanol phase, and thus increases the concentration of tri‐n‐butyl phosphate at the interface and forces a rearrangement with subsequent loss of orientation.

The effect of solvent heterogeneity on the solvation and complexation of alkali cations by 18-crown-6: a simulation study in the 90 : 10 chloroform/methanol mixture

Although chloroform is in excess over methanol in the mixture, the predicted ion binding affinities and selectivities are more “methanol-like” than “chloroform-like”.

Liquid–liquid extraction of alkali cations by 18-crown-6: complexation and interface crossing studied by MD and PMF simulations

The 18C6/M⁺Pic⁻complexes form and adsorb “right at the nano-interface” where 18C6 prefers the K⁺guest.

Calculation of the intrinsic solvation free energy profile of an ionic penetrant across a liquid-liquid interface with computer simulations

We introduce the novel concept of an intrinsic free energy profile, allowing one to remove the artificial smearing caused by thermal capillary waves, which renders difficulties for the calculation of free energy profiles across fluid interfaces in computer simulations. We apply this concept to the problem of a chloride ion crossing the interface between water and 1,2-dichloroethane and show that the present approach is able to reveal several important features of the free energy profile which are not detected with the usual, nonintrinsic calculations. Thus, in contrast to the nonintrinsic profile, a free energy barrier is found at the aqueous side of the (intrinsic) interface, which is attributed to the formation of a water "finger" the ion pulls with itself upon approaching the organic phase. Further, by the presence of a nonsampled region, the intrinsic free energy profile clearly indicates the coextraction of the first hydration shell water molecules of the ion when entering the organic phase.

Paper spray ionization of polar analytes using non-polar solvents

Non-polar solvents like hexane allow ionization of insoluble drugs, peptides, nucleotides and phospholipids as solids from paper. Ambient ionization is achieved simply by application of a high voltage to the wet paper. Transport and ionization mechanisms are discussed, including the possibility of field desorption from dendritic structures formed on the paper.

Molecular dynamics study of a heteroditopic-calix[4]diquinone-assisted transfer of KCl and dopamine through a water-chloroform liquid-liquid interface

The ability of two heteroditopic calix[4]diquinone receptors to transport a KCl ion-pair and a dopamine zwitterion through a water-chloroform interface was investigated via molecular dynamics (MD) simulations. Gas-phase conformational analysis has been carried on KCl and dopamine receptor binding associations and the lowest energy structures found in both cases show that the recognition of KCl and dopamine zwitterion occurs through multiple and cooperative N-H...anion and O...cation bonding interactions, with the receptor adopting equivalent folded conformations stabilized by pi-stacking interactions. The unconstrained MD simulations performed on KCl and dopamine complexes inserted in either the chloroform or water phase revealed that receptors are preferentially located at the interface with the hydrophobic tert-butyl groups of the calix[4]diquinone moiety immersed in the chloroform bulk while the polar anion binding cavity is directed toward the water phase. When the KCl complex is placed in chloroform, the release of the ion-pair occurs only after the first contact with the water interface, being a nonsimultaneous event, with the chloride anion leaving the receptor before the potassium cation. The dopamine, via the -NH(3)(+) binding entity, remains bound to the receptor during the entire time of the MD simulation (10 ns). In contrast, when both complexes were inserted in the water bulk, the full release of KCl and dopamine are fast events. The potentials of mean force (PMFs), associated with the migration of the complexes from chloroform to water through the interface, were calculated from steered molecular dynamics (SMD) simulations. The PMFs for the free KCl and zwitterionic dopamine migrations were also obtained for comparison purposes. The transport of KCl from water to chloroform (the reverse path) mediated by the receptor has a free energy barrier estimated in 6.50 kcal mol(-1), which is 3.0 kcal mol(-1) smaller than that found for the free KCl. The transport of dopamine complex along the reverse path is characterized by downhill energy profile, with a small free energy barrier of 6.56 kcal mol(-1).

Molecular transport across fluid interfaces: coupling between solute dynamics and interface fluctuations

We investigate the transport mechanism of a small hydrophobic solute molecule across two types of fluid interfaces, (i) an interface between two immiscible liquids and (ii) a surfactant-covered liquid-liquid interface. These systems are modeled by coarse-grained molecular dynamics simulations. It is demonstrated that the dynamics of the solute molecule near the interface significantly deviates from Markovian Brownian motion. Specifically, the correlation time of the random force acting on the solute strongly depends on the distance between the solute and the interface and increases by two orders of magnitude within a very narrow (less than 1 nm wide) region near the interface. The slow fluctuations of the random force in this narrow region are caused by capillary waves. The region location and width are determined by interface protrusions caused by attraction between the solute and the hydrophobic phase. We use results of molecular dynamics simulations to develop a stochastic model for the coupled solute-interface dynamics and estimate the rate of the solute transport across the interface. The observed phenomenon appears to be a general feature of mass transport across fluid or flexible membranes. The coupling between the solute transport and the interface fluctuations is the strongest in areas corresponding to a large free energy gradient or near a free energy barrier for the solute transport. This suggests a strong influence of the coupled solute-interface dynamics on the rate of mass transfer across interfaces.

The effect of a solvent modifier in the cesium extraction by a calix[4]arene: a molecular dynamics study of the oil phase and the oil-water interface

We report a molecular dynamics (MD) study of the effect of a fluorinated lipophilic alcohol (referred to as "cs3" by Delmau et al, Solvent Extr. Ion Exch., 2005, 23, 23) used as a phase modifier in the solvent extraction of Cs(+) NO(3)(-) by a calix[4]arene. It is shown that adding cs3 to a chloroform phase improves the solvation of all partners of the extraction system, i.e. the free calixarene ligand L, its LCs(+) complex and its counterion. This effect is most pronounced for the NO(3)(-) anion that is H-bonded to the -OH and terminal -CF(2)H protons of cs3. On the average, the dissociated nitrate interacts with 2 to 4 cs3 molecules, whereas the associated nitrate (LCs(+)NO(3)(-) complex) interacts with one cs3 dimer. The remaining modifier molecules are mainly dissolved in the solution as dimers or trimers and, to a lesser extent, as monomers and tetramers. Insights into the question of ion pairing in the organic phase are obtained via free energy perturbation (FEP) calculations, showing that the LCs(+)NO(3)(-) paired complex is more stable than its analogue with the dissociated anion. Furthermore, the nitrate dissociation energy is ca. twice as small in the cs3-modified solution than in a pure chloroform phase. We also simulated chloroform/cs3/water ternary systems, showing that the modifiers are surface active and stabilize the formation of water nanodroplets, while other modifier molecules drag some water to the organic phase. Calixarene complexes also adsorb at the aqueous interface in the presence of modifiers, confirming the importance of interfacial phenomena in the assisted cation extraction process. The microscopic insights obtained by the simulations are consistent with experimental results and allow us to better understand why the extraction is enhanced after addition of modifiers to the organic phase.

Rhodium-Catalyzed Hydroformylation of 1-Hexene in an Ionic Liquid: A Molecular Dynamics Study of the Hexene/[BMI][PF6] Interface

We report a molecular dynamics study of biphasic systems involved in the rhodium-catalyzed hydroformylation of 1-hexene in the 1-butyl-3-methyl-imidazolium hexafluorophosphate ionic liquid ([BMI][PF(6)] IL). We first describe the neat [BMI][PF(6)] interfaces with hexene (the substrate) and heptanal (the linear reaction product) as organic phases. The former interface is molecularly sharp with BMI+ cations preferentially oriented "perpendicular" (i.e., pointing their butyl chains toward the organic phase), whereas hexene molecules tend to be somewhat parallel to the interface. The interface with heptanal is approximately twice as broad, due to BMI+...O(heptanal) attractions, and the solvent molecules are disordered at the interface. No IL ions solubilize in the organic phase(s) whereas ca. 2-3 hexene or heptanal molecules diffused into the IL phase. The presence of the CO and H2 gases does not modify the nature of the hexene/IL interface, as these gases are mainly solubilized in the organic phase, respectively, as diluted species and in the form of a "gaseous" droplet. In the IL phase, one finds a few CO monomers, whereas the less soluble H2 molecules spend only transient excursions. We next simulate the phase separation of "randomly mixed" IL/hexene liquids with the [RhH(CO)L(3)] precatalyst as a solute, comparing the PPh(3) to the TPPTS(3-) ligands (L). The phases separate much more slowly than in the case of classical liquids, and the neutral complex with PPh(3) ligands solubilizes in the hexene phase, displaying loose dynamical contacts with the IL interface. This contrasts with the -9 charged [RhH(CO)(TPPTS)(3)](9-) complex that sits "immobilized" on the IL side of the interface and is mainly solvated by BMI+ cations. Finally, we characterize the solvation of -6 charged [RhH(CO)(TPPTS)(2)](6-), [RhH(CO)(2)(TPPTS)(2)](6-), and [RhH(CO)(TPPTS)(2)(hexene)](6-) complexes involved as reaction intermediates in the hydroformylation reaction and of the free TPPTS(3-) ligand itself in the bulk IL.

Importance of Interfacial Adsorption in the Biphasic Hydroformylation of Higher Olefins Promoted by Cyclodextrins: A Molecular Dynamics Study at the Decene/Water Interface

We report herein a molecular dynamics study of the main species involved in the hydroformylation of higher olefins promoted by cyclodextrins in 1-decene/water biphasic systems at a temperature of 350 K. The two liquids form a well-defined sharp interface of approximately 7 A width in the absence of solute; the decene molecules are generally oriented "parallel" to the interface where they display transient contacts with water. We first focused on rhodium complexes bearing water-soluble TPPTS(3-) ligands (where TPPTS(3-) represents tris(m-sulfonatophenyl)phosphine) involved in the early steps of the reaction. The most important finding concerned the surface activity of the "active" form of the catalyst [RhH(CO)(TPPTS)(2)](6-), the [RhH(CO)(2)(TPPTS)(2)](6-) complex, and the key reaction intermediate [RhH(CO)(TPPTS)(2)(decene)](6-) (with the olefin pi-coordinated to the metal center) which are adsorbed at the water side of the interface in spite of their -6 charge. The free TPPTS(3-) ligands themselves are also surface-active, whereas the -9 charged catalyst precursor [RhH(CO)(TPPTS)(3)](9-) prefers to be solubilized in water. The role of cyclodextrins was then investigated by performing simulations on 2,6-dimethyl-beta-cyclodextrin ("CD") and its inclusion complexes with the reactant (1-decene), a reaction product (undecanal), and the corresponding key reaction intermediate [RhH(CO)(TPPTS)(2)(decene)](6-) as guests; they were all shown to be surface-active and prefer the interface over the bulk aqueous phase. These results suggest that the biphasic hydroformylation of higher olefins takes place "right" at the interface and that the CDs promote the "meeting" of the olefin and the catalyst in this peculiar region of the solution by forming inclusion complexes "preorganized" for the reaction. Our results thus point to the importance of adsorption at the liquid/liquid interface in this important phase-transfer-catalyzed reaction.

Comparing an Ionic Liquid to a Molecular Solvent in the Cesium Cation Extraction by a Calixarene: A Molecular Dynamics Study of the Aqueous Interfaces

We report a molecular dynamics (MD) study of the interfacial behavior of key partners involved in the Cs(+) cation extraction by a calix[4]arene-crown-6 host (L), comparing an ionic liquid (IL) to a classical molecular solvent (chloroform) as receiving "oil" phase. The IL is composed of hydrophobic 1-butyl-3-methylimidazolium cations (BMI(+)) and bis(trifluoromethylsulfonyl)imide anions (Tf(2)N(-)) and forms a biphasic system with water. The simulations reveal similarities but also interesting differences between the two types of interfaces. Much longer times are needed to "equilibrate" IL systems, compared to classical liquid mixtures, and there is more intersolvent mixing with the IL than with chloroform, especially concerning the water-in-oil content. There is also some excess of the BMI(+) cations over the Tf(2)N(-) anions in the aqueous phase. Simulations on the Na(+)NO(3)(-) and Cs(+)NO(3)(-) ions show that they sometimes interact at the interface with the IL ions, forming hydrated intimate ion pairs, whereas they are "repelled" by the classical interface. The LCs(+) complex and L ligand also behave differently, depending on the "oil phase". They are better solvated by the IL than by chloroform and thus poorly attracted at the IL interface, whereas they adsorb at the chloroform interface, adopting well-defined amphiphilic orientations. The results are discussed in the context of assisted ion transfer and provide a number of arguments explaining the specificity and efficiency of IL based, compared to classical extraction systems.

Free energy of solvation of simple ions: Molecular-dynamics study of solvation of Cl− and Na+ in the ice/water interface

Molecular-dynamics simulations of Cl(-) and Na(+) ions are performed to calculate ionic solvation free energies in both bulk simple point-charge/extended water and ice 1 h at several different temperatures, and at the basal ice 1 h/water interface. For the interface we calculate the free energy of "transfer" of the ions across the ice/water interface. For the ions in bulk water in the NPT ensemble at 298 K and 1 atm, results are found to be in good agreement with experiments, and with other simulation results. Simulations performed in the NVT ensemble are shown to give equivalent solvation free energies, and this ensemble is used for the interfacial simulations. Solvation free energies of Cl(-) and Na(+) ions in ice at 150 K are found to be approximately 30 and approximately 20 kcal mol(-1), respectively, less favorable than for water at room temperature. Near the melting point of the model the solvation of the ions in water is the same (within statistical error) as that measured at room temperature, and in the ice is equivalent and approximately 10 kcal mol(-1) less favorable than the liquid. The free energy of transfer for each ion across ice/water interface is calculated and is in good agreement with the bulk observations for the Cl(-) ion. However, for the model of Na(+) the long-range electrostatic contribution to the free energy was more negative in the ice than the liquid, in contrast with the results observed in the bulk calculations.

Molecular-dynamics simulation of the effect of ions on a liquid–liquid interface for a partially miscible mixture

Molecular-dynamics simulations were performed to model the effect of added salt ions on the liquid-liquid interface in a partially miscible system. Simulations of the interface between saturated phases of a model 1-hexanol+water system show a bilayer structure of 1-hexanol molecules at the interface with -OH heads of the first layer directed into the water phase and the opposite orientation for the second layer. The alignment of the polar -OH groups at the interface stabilizes a charge separation of sodium and chloride ions when salt is introduced into the aqueous phase, producing an electrical double layer. Chloride ions aggregate nearer the interface and sodium ions move toward the bulk water phase, consistent with the explanation that the -OH alignment presents a region of partial positive charges to which the hydrated chloride atoms are attracted. Ions near the interface were found to be less solvated than those in the bulk phase. An electric field was also applied to drive ions through the interface. Ions crossing the interface tended to shed water molecules as they entered the hexanol bilayer, leaving a trail of water molecules. Stabilization and facilitated transport of the ion by interactions with the second layer of hexanol molecules appeared to be an important step in the mechanism of sodium ion transport.

Macrotricyclic quaternary tetraammonium receptors: halide anion recognition and interfacial activity at an aqueous interface. A molecular dynamics investigation

We report a molecular dynamics study of the halide inclusion complexes X(-) subset L(4+) of a macrotricyclic tetrahedral receptor L(4+) built from four quaternary ammonium sites connected by six (CH(2))(n) chains. The hydrophilic/hydrophobic character of the complexes is investigated at a water/chloroform interface, represented explicitly and, despite their +3 charge and "spherical" shape, they are found to display amphiphilic behavior and to concentrate at the interface. The more lipophilic N-substituted CH(2)phi derivative, as well as less charged models are more surface active than the N-Me substituted host. In relation with the Hofmeister series, I(-) exo neutralizing counterions are compared with Cl(-) anions and are found to sit closer to the interface, which becomes more neutral. The "macrocyclic interfacial effect" is investigated by a comparison of L(4+) complexes with their acyclic counterparts. Finally, we address the question of anion binding selectivity by L(4+) and its L(1) (4+) and L(2) (4+) topological isomers. F(-) is too small for these three hosts, while I(-) is too big. According to free energy perturbation calculations, Cl(-) is preferred to Br(-), but somewhat more by L(1) (4+) than by L(4+).