Structure, Molecular Interactions, and Dynamics of Aqueous [BMIM][BF4] Mixtures: A Molecular Dynamics Study.

Molecular dynamics simulations with many-body polarizable force fields were carried out to investigate the thermodynamic, structural, and dynamic properties of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). The radial distribution functions exhibit well-defined features, revealing favored structural correlations between [bmim]+, [BF4]-, and H2O. The addition of water is shown to alter ionic liquid structural organizations by replacing counterions in the coordination shells and disrupt the cation-anion network. At low water concentration, the majority of water molecules are isolated from each other and have lower average dipole moment than that in pure water. With increasing hydration level, while [bmim][BF4] ionic network breaks up and becomes isolated ion pairs or free ions in the dilute limit, water begins to form clusters of increasing sizes and eventually forms a percolating network. As a result, the average water dipole moment increases and approaches its bulk value. Water is also observed to have a substantial influence on the dynamics of ionic liquids. At low water content, the cation and anion have similar diffusion coefficients due to the correlated ionic motion of long-lived ion pairs. As the water concentration increases, both ions exhibit greater mobility and faster rotations from the breakup of ionic network. Consequently, the ionic conductivity of [bmim][BF4] aqueous solutions rises with increasing water composition.

Li+ solvation and kinetics of Li+-BF4-/PF6- ion pairs in ethylene carbonate. A molecular dynamics study with classical rate theories.

Using our polarizable force-field models and employing classical rate theories of chemical reactions, we examine the ethylene carbonate (EC) exchange process between the first and second solvation shells around Li+ and the dissociation kinetics of ion pairs Li+-[BF4] and Li+-[PF6] in this solvent. We calculate the exchange rates using transition state theory and correct them with transmission coefficients computed by the reactive flux, Impey, Madden, and McDonald approaches, and Grote-Hynes theory. We found that the residence times of EC around Li+ ions varied from 60 to 450 ps, depending on the correction method used. We found that the relaxation times changed significantly from Li+-[BF4] to Li+-[PF6] ion pairs in EC. Our results also show that, in addition to affecting the free energy of dissociation in EC, the anion type also significantly influences the dissociation kinetics of ion pairing.

Mesoscale Polymer Dissolution Probed by Raman Spectroscopy and Molecular Simulations.

The diffusion of various solvents into a polystyrene (PS) matrix was probed experimentally by monitoring the temporal profiles of the Raman spectra and theoretically from molecular dynamics simulations. The simulation results assist in providing a fundamental, molecular-level connection between the mixing/dissolution processes and the difference, Δδ = δsolvent - δPS, in the values of the Hildebrand parameter (δ) between the two components of the binary systems: solvents having values of δ similar to those for PS (small Δδ) exhibit fast diffusion into the polymer matrix, whereas the diffusion slows down considerably when the δ's are different (large Δδ). To this end, the Hildebrand parameter was identified as a useful descriptor that governs the process of mixing in polymer-solvent binary systems. The experiments also provide insight into further refinements of the models specific to non-Fickian diffusion phenomena that need to be used in the simulations.

Rate theory of solvent exchange and kinetics of Li(+) - BF4 (-)/PF6 (-) ion pairs in acetonitrile.

In this paper, we describe our efforts to apply rate theories in studies of solvent exchange around Li(+) and the kinetics of ion pairings in lithium-ion batteries (LIBs). We report one of the first computer simulations of the exchange dynamics around solvated Li(+) in acetonitrile (ACN), which is a common solvent used in LIBs. We also provide details of the ion-pairing kinetics of Li(+)-[BF4] and Li(+)-[PF6] in ACN. Using our polarizable force-field models and employing classical rate theories of chemical reactions, we examine the ACN exchange process between the first and second solvation shells around Li(+). We calculate exchange rates using transition state theory and weighted them with the transmission coefficients determined by the reactive flux, Impey, Madden, and McDonald approaches, and Grote-Hynes theory. We found the relaxation times changed from 180 ps to 4600 ps and from 30 ps to 280 ps for Li(+)-[BF4] and Li(+)-[PF6] ion pairs, respectively. These results confirm that the solvent response to the kinetics of ion pairing is significant. Our results also show that, in addition to affecting the free energy of solvation into ACN, the anion type also should significantly influence the kinetics of ion pairing. These results will increase our understanding of the thermodynamic and kinetic properties of LIB systems.

Experimental and theoretical study of molecular response of amine bases in organic solvents.

Reorientational correlation times of various amine bases (namely, pyridine, 2,6-lutidine, 2,2,6,6-tetramethylpiperidine) and organic solvents (dichloromethane, toluene) were determined by solution-state NMR relaxation time measurements and compared with predictions from molecular dynamics (MD) simulations. The amine bases are reagents in complex reactions catalyzed by frustrated Lewis pairs (FLP), which display remarkable activity in metal-free H2 scission. The comparison of measured and simulated correlation times is a key test of the ability of recent MD and quantum electronic structure calculations to elucidate the mechanism of FLP activity. Correlation times were found to be in the range of 1.4-3.4 (NMR) and 1.23-5.28 ps (MD) for the amines and 0.9-2.3 (NMR) and 0.2-1.7 ps (MD) for the solvent molecules.

Computational observation of pockets of enhanced water concentration at the 1-octanol/water interface.

Molecular dynamics simulations with polarizable potentials were carried out to investigate the 1-octanol-water interface in which a significant amount of water migrated into the 1-octanol phase. A region of enhanced water concentration, around three times the average concentration in water saturated 1-octanol, was present 18 Å from the Gibbs dividing surface into the 1-octanol phase. This coincided with two layers of 1-octanol molecules, forming a somewhat ordered bilayer with the first layer having its hydroxyl group pointed toward the water phase. The second layer of 1-octanol had hydroxy groups pointed in the opposite direction on average. A consequence of this was a region of high alkyl concentration and reduced polarity, as has been previously observed. Water structure in the octanol phase contracted as it approached the 1-octanol phase, opposite what was observed at the n-octane-water interface with polarizable potentials. In contrast, 1-octanol hydroxy structure expanded as it came in contact with water.

Computational studies of [bmim][PF6]/n-alcohol interfaces with many-body potentials.

In this paper, we present the results from molecular dynamics simulations of the equilibrium properties of liquid/liquid interfaces of room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and simple alcohols (i.e., methanol, 1-butanol, and 1-hexanol) at room temperature. Polarizable potential models are employed to describe the interactions among species. Results from our simulations show stable interfaces between the ionic liquid and n-alcohols, and we found that the interfacial widths decrease from methanol to 1-butanol systems and then increase for 1-hexanol interfaces. Angular distribution analysis reveals that the interface induces a strong orientational order of [bmim] and n-alcohol molecules near the interface, with [bmim] extending its butyl group into the alcohol phase, whereas the alcohol has the OH group pointing into the ionic liquid region, which is consistent with the recent sum-frequency-generation experiments. We found the interface to have a significant influence on the dynamics of ionic liquids and n-alcohols. The orientational autocorrelation functions illustrate that [bmim] rotates more freely near the interface than in the bulk, whereas the rotation of n-alcohol is hindered at the interface. Additionally, the time scale associated with the diffusion along the interfacial direction is found to be faster for [bmim] but slowed down for n-alcohols approaching the interface. We also calculate the dipole moment of n-alcohols as a function of the distance normal to the interface. We found that, even though methanol and 1-butanol have different dipole moments in bulk phase, they reach a similar value at the interface.

Molecular mechanism of CO2 and SO2 molecules binding to the air/liquid interface of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid: a molecular dynamics study with polarizable potential models.

Molecular dynamics simulations with many-body interactions were carried out to understand the bulk and interfacial absorption of gases in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). A new polarizable molecular model was developed for BMIMBF4, which was found to give the correct liquid density but which also had good agreement with experiment for its surface tension and X-ray reflectivity. The potential of mean force of CO(2) and SO(2) was calculated across the air-BMIMBF4 interface, and the bulk free energies were calculated with the free-energy perturbation method. A new polarizable model was also developed for CO(2). The air-BMIMBF4 interface had enhanced BMIM density, which was mostly related to its butyl group, followed by enhanced BF4 density a few angstroms toward the liquid bulk. The density profiles were observed to exhibit oscillations between high BMIM and BF4 density indicating the presence of surface layering induced by the interface. The potential of mean force for CO(2) and SO(2) showed more negative free energies in regions of enhanced BF4 density, while more positive free energies were found in regions of high BMIM density. Moreover, these gases showed free-energy minimums at the interface, where the BMIM alkyl groups were found to be most prevalent. Our results show the importance of ionic liquid interfacial ordering for understanding gas solvation in them.

Interpreting vibrational sum-frequency spectra of sulfur dioxide at the air/water interface: a comprehensive molecular dynamics study.

We investigated the solvation and spectroscopic properties of SO(2) at the air/water interface using molecular simulation. Molecular interactions from both Kohn-Sham (KS) density functional theory (DFT) and classical polarizable models were used to understand the properties of SO(2):(H(2)O)(x) complexes in the vicinity of the air/water interface. The KS-DFT was included to allow comparisons with vibrational sum-frequency spectroscopy through the identification of surface SO(2):(H(2)O)(x) complexes. Using our simulation results, we were able to develop a much more detailed picture of the surface structure of SO(2) consistent with spectroscopic data obtained by Richmond and co-workers (J. Am. Chem. Soc. 2005, 127, 16806). We also found many similarities and differences between the two interaction potentials, including a noticeable weakness of the classical potential model in reproducing the asymmetric hydrogen bonding of water with SO(2) due to its inability to account for SO(2) resonance structures.

Solvation of dimethyl succinate in a sodium hydroxide aqueous solution. A computational study.

Molecular dynamics simulations were carried out to study dimethyl succinate/water/NaOH solutions. The potential of mean force method was used to determine the transport mechanism of a dimethyl succinate (a diester) molecule across the aqueous/vapor interface. The computed number density profiles show a strong propensity for the diester molecules to congregate at the interface, with the solubility of the diester increasing with increasing NaOH concentration. It is observed that the major contribution to the interfacial solvation free-energy minimum is from electrostatic interactions. Even at higher NaOH concentrations, the increasing electrostatic interaction between the diester and ions is not large enough to favor the solvation of diester in bulk solutions. The calculated solvation free energies are found to be -2.6 to -3.5 kcal/mol in variant concentrations of NaOH aqueous solutions. These values are in qualitative agreement with the corresponding experimental measurements. The computed surface potential indicates that the contribution of diester molecules to the total surface potential is about 25%, with the major contribution from interfacial water molecules.

Computational studies of structures and dynamics of 1,3-dimethylimidazolim salt liquids and their interfaces using polarizable potential models.

The structures, thermodynamics, and dynamical properties of bulk and air/liquid interfaces of three ionic liquids, 1,3-dimethylimidazolium [dmim](+) with Cl(-), Br(-), and I(-) were studied using molecular dynamics techniques and polarizable potential models. In bulk melts, the radial distribution functions reveal a significant long-range structural correlation in these ionic liquids. The single-ion dynamics are studied via mean-square-displacements, velocity and orientational correlation functions. We observe that anion size plays an important role in the dynamics of ionic liquids, with larger anions inducing faster cation and anion motion. The computed density profiles of the ionic liquid/vapor interface exhibit oscillatory behavior, indicative of surface layering at the interface. The computed surface tensions indicate small differences between these ionic liquids and decrease with the increasing anion size. The magnitudes of the computed potential drops of these ionic liquids are found to be small and negative and increase with the decreasing anion size. These results could imply that the cation dipoles on average orient more in the interfacial plane than perpendicular to it. Our results showed that anion type plays a major role in determining IL interfacial behavior.

Computational studies of load-dependent guest dynamics and free energies of inclusion for CO2 in low-density p-tert-butylcalix[4]arene at loadings up to 2:1.

The structure, dynamics, and free energies of absorption of CO(2) by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO(2):TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO(2)/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions of CO(2) molecules, a CO(2) dimer is observed for TBC4-T and a cage-interstitial CO(2) structure is suggested for TBC4-U. The dynamics of the CO(2) molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO(2) in this TBC4 structure for various loadings is -4.0 kcal/mol at 300 K and -1.8 kcal/mol at 450 K, showing that CO(2) inclusion is favored. The fully loaded 1:1 CO(2):TBC4 system is slightly less favorable at -3.9 and -1.2 kcal/mol at 300 and 450 K, respectively. The first CO(2) added beyond 1:1 loading shows a significant drop in absorption energy to -1.9 and +1.9 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO(2) at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the free energy of inclusion for addition of the CO(2) above 1:1 is less favorable.

Computational studies of liquid water and diluted water in carbon tetrachloride.

Molecular dynamics simulations were carried out to study solvent effects on the energetic and dynamical properties of water molecules in liquid water and in carbon tetrachloride (CCl4). In these studies, the free-energy profiles or potentials of mean force (PMF) for water dimers in both solvents were computed. The computed PMF results showed a stable minimum near 3 A for the O-O separation, with a minimum free energy of about -2.8 kcal/mol in CCl4, as compared to a value of -0.5 kcal/mol in liquid water. The difference in free energy in water as compared to that in CCl4 was expected and is the result of competition from surrounding water molecules that are capable of forming hydrogen bonds in the liquid water. This capability is absent in the diluted water found in CCl4. We found that the rotational motions of H2O/D2O were nonisotropic, with the out-of-plane vector correlation times in H2O/D2O varying from 5.6/5.8 ps at 250 K to 0.57/0.56 ps at 350 K and the corresponding OH/OD bond vectors varying from 6.5/7.7 ps to 0.75/0.75 ps. The results compare reasonably well to the available NMR experimental and computer simulation data on the same system (Farrar; Skinner; et al. J. Am. Chem. Soc. 2001, 123, 8047). For diluted water in CCl4, we found the computed rotational correlation times also were nonisotropic and much longer than the corresponding NMR experimental values at the same concentration (Farrar; et al. J. Phys. Chem. A 2007, 111, 6146). Upon analyzing the water hydrogen-bonding patterns as a function of water concentration, we conclude that the differences in the rotational correlation times mainly result from the formation of water hydrogen-bonding networks as the water concentration is increased in liquid CCl4. In addition, we found the rotational correlation times to be substantially faster in liquid CCl4 than in liquid water.

Molecular mechanisms of hydrogen-loaded beta-hydroquinone clathrate.

Molecular dynamics simulations are used to investigate the molecular interactions of hydrogen-loaded beta-hydroquinone clathrate. It is found that, at lower temperatures, higher loadings are more stable, whereas at higher temperatures, lower loadings are more stable. Attractive forces between the guest and host molecules lead to a stabilized minimum-energy configuration at low temperatures. At higher temperatures, greater displacements take the system away from the shallow energy minimum, and the trend reverses. The nature of the cavity structure is nearly spherical for a loading of one, leads to preferential occupation near the hydroxyl ring crowns of the cavity with a loading of two, and at higher loadings, leads to occupation of the interstitial sites (the hydroxyl rings) between cages by a single H(2) molecule with the remaining molecules occupying the equatorial plane of the cavity. Occupation of the interstitial positions of the cavities leads to facile diffusion.

On NO3--H2O interactions in aqueous solutions and at interfaces.

The constrained molecular-dynamics technique was employed to investigate the transport of a nitrate ion across the water liquid/vapor interface. We developed a nitrate-ion-water polarizable potential that accurately reproduces the solvation properties of the hydrated nitrate ion. The computed free-energy profile for the transfer of the nitrate ion across the air/water interface increases monotonically as the nitrate ion approaches the Gibbs dividing surface from the bulk liquid side. The computed density profiles of 1M KNO(3) salt solution indicate that the nitrate and potassium ions are both found below the aqueous interface. Upon analyzing the results, we conclude that the probability of finding the nitrate anion at the aqueous interface is quite small.

Liquid-vapor interface of methanol-water mixtures: a molecular dynamics study.

Molecular dynamics simulations were carried out to investigate the structural and thermodynamic properties and variations in the dipole moments of the liquid-vapor interfaces of methanol-water mixtures. Various methanol-water compositions were simulated at room temperature. We found that methanol tends to concentrate at the interface, and the computed surface tension shows a composition dependence that is consistent with experimental measurements. The methanol molecule shows preferred orientation near the interface with the methyl group pointing into the vapor phase. The methanol in the mixture is found to have larger dipole moments than that of pure liquid methanol. The strong local field induced by the surrounding water molecules is partly the reason for this difference. The dependence of hydrogen-bonding patterns between methanol and water on the interface and the composition of the mixture is also discussed in the paper.