Collin D. Wick

The Effect of Polarizability for Understanding the Molecular Structure of Aqueous Interfaces.

A review is presented on recent progress of the application of molecular dynamics simulation methods with the inclusion of polarizability for the understanding of aqueous interfaces. Comparisons among a variety of models, including those based on density functional theory of the neat air-water interface, are given. These results are used to describe the effect of polarizability on modeling the microscopic structure of the neat air-water interface, including comparisons with recent spectroscopic studies. Also, the understanding of the contribution of polarization to the electrostatic potential across the air-water interface is elucidated. Finally, the importance of polarizability for understanding anion transfer across an organic-water interface is shown.

Computational Investigation of the First Solvation Shell Structure of Interfacial and Bulk Aqueous Chloride and Iodide Ions

Molecular dynamics simulations with polarizable interaction potentials were carried out to understand the solvation structure of chloride and iodide anions in bulk and interfacial water, showing qualitative similarities between the first solvation shell structures at the interface and bulk. For the more polarizable iodide, its solvation structure was found to be more anisotropic than chloride, and this trend persisted at both the interface and in the bulk. The anisotropy of the solvation structure correlated with polarizability, but it was also found to inversely correlate with anion size. When polarizability was reduced to near zero, a very small anisotropy in the water solvation structure around the ion still persisted. Polarizable anions were found to have on average an induced dipole in the bulk that was significantly larger than zero. This induced dipole resulted in the water hydrogen atoms having stronger interactions with the anions on one side of them, in which the dipole was pointing. In contrast, the other side of the anions, in which the induced dipole was pointing away from, had fewer water molecules present and, for the case of iodide, was rather devoid of water molecules all together at both the interface and in the bulk. This region formed a small cavity in the bulk, whereas at the air-water interface it was simply part of the air interface. In the bulk, this small cavity may be viewed as somewhat hydrophobic, and the need for the extinction of this cavity may be one of the major driving forces for the more polarizable anions to reside at the air-water interface.

Transferable potentials for phase equilibria-united atom description of five- and six-membered cyclic alkanes and ethers

While the transferable potentials for phase equilibria-united atom (TraPPE-UA) force field has generally been successful at providing parameters that are highly transferable between different molecules, the polarity and polarizability of a given functional group can be significantly perturbed in small cyclic structures, which limits the transferability of parameters obtained for linear molecules. This has motivated us to develop a version of the TraPPE-UA force field specifically for five- and six-membered cyclic alkanes and ethers. The Lennard-Jones parameters for the methylene group obtained from cyclic alkanes are transferred to the ethers for each ring size, and those for the oxygen atom are common to all compounds for a given ring size. However, the partial charges are molecule specific and parametrized using liquid-phase dielectric constants. This model yields accurate saturated liquid densities and vapor pressures, critical temperatures and densities, normal boiling points, heat capacities, and isothermal compressibilities for the following molecules: cyclopentane, tetrahydrofuran, 1,3-dioxolane, cyclohexane, oxane, 1,4-dioxane, 1,3-dioxane, and 1,3,5-trioxane. The azeotropic behavior and separation factor for the binary mixtures of 1,3-dioxolane/cyclohexane and ethanol/1,4-dioxane are qualitively reproduced.

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.

Simulated surface potentials at the vapor-water interface for the KCl aqueous electrolyte solution

Classical molecular dynamics simulations with polarizable potential models were carried out to quantitatively determine the effects of KCl salt concentrations on the electrostatic surface potentials of the vapor-liquid interface of water. To the best of our knowledge, the present work is the first calculation of the aqueous electrolyte surface potentials. Results showed that increased salt concentration enhanced the electrostatic surface potentials, in agreement with the corresponding experimental measurements. Furthermore, the decomposition of the potential drop into contributions due to static charges and induced dipoles showed a very strong effect (an increase of approximately 1 V per 1M) due to the double layers formed by KCl. However, this was mostly negated by the negative contribution from induced dipoles, resulting in a relatively small overall increase ( approximately 0.05 V per 1M) with increased salt concentration.

Investigating Hydroxide Anion Interfacial Activity by Classical and Multistate Empirical Valence Bond Molecular Dynamics Simulations

Molecular dynamics simulations were carried out to understand the propensity of the hydroxide anion for the air-water interface. Two classes of molecular models were used, a classical polarizable model and a polarizable multistate empirical valence bond (MS-EVB) potential. The latter model was parametrized to reproduce the structures of small hydroxide-water clusters based on proton reaction coordinates. Furthermore, nuclear quantum effects were introduced into the MS-EVB model implicitly by refitting its potential energy function to account for them. The final MS-EVB model showed reasonable agreement with experiment and ab initio molecular dynamics simulations for dynamical and structural properties. The free-energy profiles for both the classical and MS-EVB models were mapped out across the air-water interface, and the classical model gave a higher free energy at the interface with respect to bulk. However, the MS-EVB model gave little free-energy difference between when the hydroxide anion was in the bulk and when it was present at the air-water interface with its oxygen fully solvated and its hydrogen pointing toward the vapor. When the hydroxide oxygen started to desolvate, the free energy increased dramatically, suggesting that the hydroxide anion can be found in the interfacial region.

Electrostatic dampening dampens the anion propensity for the air-water interface

Molecular dynamics simulations with polarizable potentials and electrostatic dampening were carried out to understand the influence of electrostatic dampening on the propensity of anions for the air-water interface. New anion molecular models incorporating these features were developed for this work. The results showed that electrostatic dampening reduced the average anion induced dipole in bulk water, in agreement with previous investigations [M. Masia, J. Chem. Phys. 128, 18 (2008)]. As a consequence, electrostatic dampening was found to significantly reduce, but not eliminate, the influence of polarizability on the anion propensity for the air-water interface. The Br(-) and I(-) models showed reduced propensity for the air-water interface with respect to previous models parametrized in a similar manner, but with no electrostatic dampening.

Temperature effects on the retention of n-alkanes and arenes in helium–squalane gas–liquid chromatography: Experiment and molecular simulation

Experiments and molecular simulations were carried out to study temperature effects (in the range of 323 to 383 K) on the absolute and relative retention of n-hexane, n-heptane, n-octane, benzene, toluene and the three xylene isomers in gas-liquid chromatography. Helium and squalane were used as the carrier gas and retentive phase, respectively. Both the experiments and the simulations show a markedly different temperature dependence of the retention for the n-alkanes compared to the arenes. For example, over the 60 K temperature range studied, the Kovats retention index of benzene is found to increase by about 16 or 18+/-10 retention index units determined from the experiments or simulations, respectively. For toluene and the xylenes, the experimentally measured increases are similar in magnitude and range from 14 to 17 retention index units for m-xylene to o-xylene. The molecular simulation data provide an independent method of obtaining the transfer enthalpies and entropies. The change in retention indices is shown to be the result of the larger entropic penalty and the larger heat capacity for the transfer of the alkane molecules.

Anion Effects on Interfacial Absorption of Gases in Ionic Liquids. A Molecular Dynamics Study

Molecular dynamics simulations with many-body interactions were carried out to systematically study the effect of anion type, tetrafluoroborate [BF(4)] or hexafluorophosphate [PF(6)], paired with the cation 1-butyl-3-methylimidazolium [bmim], on the interfacial absorption of gases in room temperature ionic liquids (RTILs). The potentials of mean force (PMF) of CO(2) and H(2)O at 350 K were calculated across the air-liquid interfaces of [bmim][BF(4)] and [bmim][PF(6)]. We found that the PMFs for H(2)O exhibited no interfacial minima at both interfaces, while the corresponding PMFs for CO(2) had significant free energy minima there. However, the PMFs for H(2)O showed a much higher interfacial free energy than in the bulk for [bmim][BF(4)], but only a slightly higher interfacial free energy for [bmim][PF(6)] than in bulk. The reason for this was due to the more hydrophilic nature of the [BF(4)] anion, and the fact that [BF(4)] was found to have little propensity for the interface. Our results show that H(2)O is much more likely to be found at the air-[bmim][PF(6)] interface than at the air-[bmim][BF(4)] interface. The free energies of solvation were found to be more negative for [bmim][BF(4)] than [bmim][PF(6)] for water and similar for CO(2). This observation is consistent with experimental Henrys law coefficients. Our results show that anion type, in addition to affecting the free energy of solvation into RTILs, should also significantly influence the uptake mechanism.

Simulating the vapour–liquid equilibria of large cyclic alkanes

Self-adapting fixed endpoint configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapour–liquid coexistence curves of cyclic alkanes from c-pentane to c-octadecane. In general, the critical temperatures and densities of the cyclic alkanes are substantially higher than those of their linear counterparts. Furthermore, the simulation data point to a maximum in the critical density for cyclic alkanes with about eight carbon atoms as also observed for the linear alkanes.