Communication: exploring the reorientation of benzene in an ionic liquid via molecular dynamics: effect of temperature and solvent effective charge on the slow dynamics

The rotational time correlation function (RTCF) of solute benzene molecules in the ionic liquid (1-butyl-3-methylimidazolium chloride) has been studied using classical molecular dynamics simulation. The effect of solvent charge on the functional form of RTCF was investigated by comparing four force fields for the solvent where the total charge on the anion and the cation was set to ±1e, ±0.7e, ±0.5e, and 0, respectively. For all three charged solvent models, the RTCF exhibits a long-time tail where the relaxation rate exhibits a significant slowdown. This feature is strengthened by higher solvent charges as well as lower temperatures, indicating the influence of the strong Coulombic fields arising from the solvent charges. The long-time tail is caused by the extraordinarily slow solvent structural relaxation of ionic liquids compared to the time scale of their local vibrational and librational dynamics.     

First principles molecular dynamics simulation of a task-specific ionic liquid based on silver-olefin complex: atomistic insights into a separation process

First principles molecular dynamics based on density functional theory is applied to a hypothetical ionic liquid whose cations and anions are silver-ethylene complex [Ag(C2H4)2+] and tetrafluoroborate [BF4-], respectively. This ionic liquid represents a group of task-specific silver complex-based ionic liquids synthesized recently. Molecular dynamics simulations at two temperatures are performed for five picoseconds. Events of association, dissociation, exchange, and recombination of ethylene with silver cation are found. A mechanism of ethylene transfer similar to the Grotthus type of proton transfer in water is identified, where a silver cation accepts one ethylene molecule and donates another to a neighboring silver cation. This mechanism may contribute to fast transport of olefins through ionic liquid membranes based on silver complexes for olefin/paraffin separation.     

Structure and dynamics of the interface between a Ag single crystal electrode and an aqueous electrolyte

The aim of this work is to elucidate the initial steps of the electrochemical oxidation of Ag(111) in alkaline electrolytes. We use electrochemical as well as ex situ (XPS) and in situ (SHG) spectroscopic techniques to reconstruct the Ag(111)/electrolyte interface as a complex dynamic entity. Moving in the direction from negative to positive potentials we first observe specific adsorption of hydroxide ions, which starts at ca. -1.1 V vs. Ag/Ag2O in 0.1 M NaOH. SHG data prove that hydroxide retains its negative charge. At -0.3 V oxidation of the surface sets in with the formation of negatively charged adsorbed oxygen species and Ag+ ions, which give rise to peaks at 528.2 +/- 0.2 eV and at 367.7 eV in the O 1s and the Ag 3d(5/2) XP spectra, respectively. Around -0.1 V the adlayer is transformed into an ordered surface oxide phase which grows via a nucleation and growth mechanism. Above the reversible Ag/Ag2O potential the 2D Ag(I) oxide transforms into a 3D Ag(I) oxide. The electrochemical oxidation is compared with the previously studied gas-phase process, demonstrating both remarkable similarities as well as some differences.     

Molecular insights into the potential and temperature dependences of the differential capacitance of a room-temperature ionic liquid at graphite electrodes

Molecular dynamics simulation studies of the structure and the differential capacitance (DC) for the ionic liquid (IL) N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonyl imide ([pyr(13)][TFSI]) near a graphite electrode have been performed as a function temperature and electrode potential. The IL exhibits a multilayer structure that extends 20-30 ? from the electrode surface. The composition and ion orientation in the innermost layer were found to be strongly dependent on the electrode potential. While at potentials near the potential of zero charge (PZC), both cations and anions adjacent to the surface are oriented primarily perpendicular to the surface, the counterions in first layer orient increasingly parallel to the surface with increasing electrode potential. A minimum in DC observed around -1 V(RPZC) (potential relative to the PZC) corresponds to the point of highest density of perpendicularly aligned TFSI near the electrode. Maxima in the DC observed around +1.5 and -2.5 V(RPZC) are associated with the onset of "saturation", or crowding, of the interfacial layer. The asymmetry of DC versus electrode polarity is the result of strong interactions between the fluorine of TFSI and the surface, the relatively large footprint of TFSI compared to pyr(13), and the tendency of the propyl tails of pyr(13) to remain adsorbed on the surface even at high positive potentials. Finally, an observed decreased DC and the disappearance of the minimum in DC near the PZC with increasing temperature are likely due to the increasing importance of entropic/excluded volume effects (interfacial crowding) with increasing temperature.     

Relationship between interfacial transfer and adsorption-desorption of surface-active bis-ammonium ions at a liquid/liquid interface

Cyclic voltammograms and interfacial tension-applied potential curves were recorded at the interface between water containing surface-active bis-quaternary ammonium ions, bis-A(2+), and an organic solvent such as 1,2-dichloroethane or nitrobenzene. An ordinary diffusion-controlled voltammetric wave for the transfer of bis-A(2+) from aqueous phase to organic phase, the first wave, was followed by a typical adsorption-related wave, the second wave. It was found from the potential dependence of the interfacial tension of bis-A(2+) that the second wave was due to the desorption of bis-A(2+) toward the organic phase. The influence of the structure of bis-A(2+) on voltammograms was investigated, and the potential for the first wave was found to depend on both the length of the side chain and that of the spacer chain, whereas the potential for the second wave depended on the latter only. The thermodynamic relations among three processes of the ion transfer, adsorption, and desorption were discussed based on the experimental results.     

Free energy and dynamics of electron-transfer reactions in a room temperature ionic liquid

Reaction free energetics and dynamics of unimolecular electron-transfer processes in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF6-) are investigated via molecular dynamics computer simulations employing a model diatomic solute and compared with those in aprotic acetonitrile. Using the free energy perturbation method, diabatic free energy curves relevant to charge separation and recombination processes are studied over a wide range of the reaction coordinate. The diabatic curves are found to vary with the solute charge distribution, especially in EMI+PF6-. Nevertheless, if the free energy of reaction is not that substantial, the Marcus free energy relationship holds reasonably well, provided that the reorganization free energy averaged between the reactant and product states is employed. The effective polarity, measured as solvation-induced stabilization of dipolar solutes, is higher for EMI+PF6- than for acetonitrile, consonant with many solvatochromic measurements. Thus, in the normal regime, activation barriers for charge separation and recombination reactions are, respectively, lower and higher in EMI+PF6- than in acetonitrile. The influence of solvent dynamics on reaction kinetics through modulations of activation, deactivation, and barrier crossing is analyzed. Even though overall solvent relaxation dynamics in EMI+PF6- are considerably slower than those in acetonitrile, the deviation of the rate constant from the transition state theory predictions is found to be small for both solvents. Implications of this finding for other reactions in ionic liquids are briefly discussed.     

Electron transfer across the polarized interface between water and a hydrophobic redox-active ionic liquid

Here we report the synthesis of a new redox-active ionic liquid (IL), (ferrocenylmethyl)dodecyldimethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate that can be used to form the polarizable water│IL interface at an elevated temperature (43 °C). Experimental approach is based on the cyclic voltammetry of the charge transfer processes occurring at the IL membrane supported on a thin microporous filter. Evidence is provided of the interfacial electron transfer between the ferrocenated cation of IL and an electron acceptor, IrCl₆^2?, in the adjacent aqueous phase.     

Silver based mercury film electrode: Part II. The influence of aging of electrode on the electrode processes of various metal ions

The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10^?3M range show a prepeak-postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer 1 μm thick.     

New insights into flavivirus biology: the influence of pH over interactions between prM and E proteins

Diseases caused by flaviviruses, such as dengue and zika, are globally recognized as major threats. During infection, a critical point in their replicative cycle is the maturation step, which occurs throughout the cellular exocytic pathway. This step is a pH-dependent process that involves the modification of the viral envelope by converting prM (pre-membrane) into M (membrane) proteins with the release of a “pr peptide”. After this reaction, the pr peptides remain bound to the viral envelope while the virions cross the acidic trans-Golgi network, and are released only at neutral pH after secretion of the virus particles. Despite this current knowledge, the molecular basis of the flavivirus maturation step is largely unknown. Here, based on the crystal structure of the dengue pr–E complex (“pr peptide” bound to virus envelope protein) and using molecular dynamics simulations, we found that the pH shift from acidic to neutral yields considerable structural changes in the system. Dynamic cross correlation maps and root mean square deviation analyses revealed that the pr–E junction is clearly unstable under neutral pH. Secondary structure analysis also revealed that the fusion loop region, present in the E protein, is sensitive to pH and tends to unstructure at a neutral environment. Moreover, we found that five residues present in the E protein, Gly¹⁰², His²⁴⁴, Thr⁷⁰, Thr⁶⁸ and Asn⁶⁷ are critical to confer stability to the pr–E complex while inside the Golgi apparatus. This work brings details about the dynamical behavior of the pr–E system, helps to better understand the flavivirus biology and may also be of use in the development of novel antiviral strategies.     

Interfacial tension and electrocapillary measurements of the room temperature ionic liquid/aqueous interface

The surface and aqueous interfacial tensions for a series of water-immiscible room-temperature ionic liquids (RTILs) have been measured. The RTILs used in this study were based on 1-alkyl-3-methylimidazolium cations (Cnmim, n=6, 8, 10, and 12) and bis(perfluoromethylsulfonyl)imide (BMSI) and bis(perfluoroethylsulfonyl)imide (BETI) anions. It was found that the surface tensions of the RTILs increased with an increasing cation chain length similar to the behavior of n-alkanes. Interfacial tensions of the RTILs with aqueous solutions, however, were found to decrease with the cation chain length, which has been attributed to the increased surface activity of the longer chain cations. We have also demonstrated the first use of electrocapillary measurements to study the polarizable RTIL/aqueous interfaces. From the electrocapillary data, the potential of zero charge (PZC) for these RTIL/aqueous interfaces was determined, as well as the relative surface excess charge and capacitance. The PZC was found to be dependent upon the structure of the anions and cations with PZC values ranging from -357 mV for C6mimBETI and -161 mV for C10mimBMSI. The electrocapillary results also show that the cations of the RTIL are becoming increasingly surface-active as the alkyl chain on the cation is lengthened, thereby modulating the interfacial potential.     

Direct gas-liquid interfacial dynamics: the reaction between O(3P) and a liquid hydrocarbon

We report the first measurements of internal energy distributions of the OH produced via a direct mechanism, isolated from other components on the basis of time-of-flight, in the interfacial reaction between gas-phase O((3)P) atoms and the liquid hydrocarbon squalane, C(30)H(62). O((3)P) atoms were generated by laser photolysis of NO(2) above the liquid. Resulting hydroxyl radicals that escape from the surface were detected by laser-induced fluorescence. Time-of-flight profiles demonstrate that the kinetic energy of the fastest OH (nu' = 1) is lower than that of (nu' = 0). Rotational distributions were measured at the rising edge of their appearance for both OH (nu' = 0) and (nu' = 1). They were found to differ substantially more than at the peak of their profiles. They were also less dependent on the bulk liquid temperature. We conclude that the new data confirm strongly that at least two mechanisms contribute to the production of OH. The higher-velocity component has translational and rotational energy distributions, observed cleanly for the first time, consistent with a direct mechanism. The close correspondence of these rotational distributions to those from the corresponding homogeneous gas-phase reaction of O((3)P) with smaller hydrocarbons suggests a very similar, near collinear direct abstraction. This is accompanied by a slower component with kinetic energy and rotational (but not vibrational) distributions reflecting the temperature of the liquid, consistent with a distinct trapping-desorption mechanism.     

A polymer in an ionic liquid: Structural properties of a system during attraction between the polymer and cations of the ionic liquid

In terms of the polymer integral equation theory, structuring in ionic liquids containing flexiblechain polymers under the conditions of good solubility is studied for the first time. The influence of the polymer concentration on the structural properties of system components is studied at different lengths of polymer molecules. The structural organization of the ionic liquids is shown to be fairly stable after addition of the polymer. When there is attraction between the polar groups of cations of the ionic liquid and polymer molecules, the calculated structural factors suggest intermediate-range scales of polymer-component ordering. Given this, for any considered length of polymer molecules, there is a range of polymer concentrations in which the characteristic scale of ordering decreases with an increase in polymer density according to a power law.     

The influence of temperature on the phase behavior of ionic liquid aqueous two-phase systems

In this article, the equilibrium behavior of solutions of 1-ethyl-3-methylimidazolium dimethyl phosphate ([Emim]DMP) and ethyl acetate or acetone in aqueous two-phase system (ATPS) was discussed to understand the liquid–liquid equilibrium (LLE) behavior of these organic solvents. Thus, we determined phase diagrams and LLE data at 303.15, 308.15, 313.15, and 323.15 K for the investigated biphasic systems. Four empirical equations were used to study the tie lines. The results showed that for the [Emim]DMP?+?acetone?+?water biphasic systems within the investigated temperature range, temperature influences the phase behavior, but for the [Emim]DMP?+?ethyl acetate?+?water biphasic systems within the investigated temperature range, there are no notable changes on the phase behavior with rising temperature. The results may have important applications for the separation of antibiotics and for the recovery of ionic liquids (ILs).     

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.     

Solvation dynamics and electric field relaxation in an imidazolium-PF6 ionic liquid: from room temperature to the glass transition

Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of eta/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature T(g). The dielectric modulus relaxation follows this trend only for temperatures T > 1.2T(g) and departs significantly from eta/T in the 1.1T(g) > T > T(g) range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2T(g).     

Revealing the influence of the strength of coulomb interactions on the viscosity and interfacial tension of ionic liquid cosolvent mixtures

Within this work, viscosity and interfacial tension of selected ionic liquid cosolvent mixtures, [EMIM][EtOSO3] (1-ethyl-3-methyl-immidazolium ethyl sulfate) with water and ethanol, were studied as a function of composition by surface light scattering (SLS) and the pendant drop method in a consistent manner, allowing a close insight into the nature of interactions. Here, we show that the viscosity behavior clearly reflects the bulk structure of the ionic liquid cosolvent mixtures and correlates to the fluid structure at the phase boundary. In contrast to former work, we found the viscosity of ionic liquid [EMIM][EtOSO3] to be decreasing the stronger by small amounts of the cosolvents and the lower their dielectric constant. Furthermore, two distinct trends for the dependence of the viscosity on the cosolvent concentration were resolved. These were assigned to ion-dipole interactions dominating in the salt-rich region and to dipole-dipole interactions in the diluted one. A crossover between both regions is reflected by the interfacial tension data, where it seems that up to a "critical" concentration almost no cosolvent is present at the phase boundary.     

Theory of a reversible redox reaction in an ionic liquid that is coupled to an ion transfer across the aqueous electrolyte/ionic liquid interface

A theory is provided for a reversible electro-oxidation of a neutral redox probe dissolved in room-temperature ionic liquid, which is sandwiched between an electrode surface and an aqueous solution as a thin film. If the peak potentials in cyclic voltammetry depend on the bulk concentration of electrolyte in water, the oxidation is most probably coupled to the transfer of anions from water into ionic liquid; but if the peak potentials are independent of the electrolyte concentration, the transfers of anions from water into ionic liquid and cations from ionic liquid into water are equally probable. Dedicated to Professor Dr. Yakov I. Tur’yan on the occasion of his 85th birthday.