EMSL: Liem X Dang's Publications

Chang TM, and LX Dang. 2009. "Computational Studies of Structures and Dynamics of 1, 3-Dimethylimidazolim Salt Liquid and their Interfaces Using Polarizable Potential Models ." Journal of Physical Chemistry A 113(10):2127-2135. doi:10.1021/jp809132w Abstract The structures, thermodynamics, dynamical properties of bulk and air/liquid interfaces of three ionic liquids, 1,3-dimethylimidazolium [dmim]+, Cl-, Br-, and I- are studied using molecular dynamics techniques. In bulk melts, the radial distribution functions reveal a significant long-range structural correlation in these ionic liquids. From the angular distribution analysis, the imidazolium rings are found to lie parallel to each other at short distances, consistent with the structures observed in the crystal state. The single-ion dynamics are studied via mean-square-displacements, velocity and orientational correlation functions. The diffusion coefficients and reorientational times are found to be much smaller than H2O. We also observe that anion size plays an important role in the dynamics of ionic liquids. The computed density profiles of the ionic liquid/vapor interface exhibit oscillatory behavior, indicative of surface layering at the interface. Further analysis reveals that the [dmim]+ ions show preferred orientation at the interface with the ring parallel to the surface and methyl group attached to the ring pointing into the vapor phase. The computed surface tensions indicated small differences between these ionic liquids and are inline with recent experimental measurements. The calculated potential drops of these ionic liquids are found to be small and negative. These results could imply that the cation dipoles are likely to orient in the plane that parallel to the surface normal axis. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Dang LX, and CD Wick. 2009. "Investigating hydroxide anion interfacial activity by classical and multi-state empirical valence bond molecular dynamics simulations." Journal of Physical Chemistry A 113(22):6356-6364. Abstract 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 multi-state empirical valence bond (MS-EVB) potential. The latter model was parameterized 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. The MS-EVB model gave a hydroxide anion that approached very close to the interface before it had a sharp increase in free energy at the Gibbs dividing surface. This showed a hydroxide anion that was present at the interface, but strongly repelled from its outer edge near the air. This work was supported by the US Department of Energy's Office of Basec Energy Sciences, Chemical Sciences program. Pacific northwest national Laboratory is operated by Battelle for DOE.

Daschbach JL, X Sun, TM Chang, PK Thallapally, BP McGrail, and LX Dang. 2009. "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 ." Journal of Physical Chemistry A 113(14):3369-3374. doi:10.1021/jp808490g Abstract The structure, dynamics, and free energies of absorption of CO2 by a low density structure of Calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO2:TBC4 have been studied using molecular dynamics simulations. From the computed radial distribution functions, we notice that the 1:1 loading peak shows a single broad peak from the caged CO2 separation. At higher temperature, the peak is only slightly broadened from the 300 K peak and the shoulder around 11 Å is reduced. The radial distribution function of the 2:1 loading shows a prominent sharp peak around 3 Å and a second peak around 4 Å, indicating a dimer center-of-mass separation of 3 Å that is smaller than that of optimized gas-phase dimer. This result suggests that dimer is sufficiently stabilized by the interaction with the TBC4. The relative angle distributions for paired CO2 molecules are flat and do not show a preference for the crossed geometry found in the vacuum dimer at 3 Å. Angular distributions relative to the TBC4 symmetry axis show a preference for alignment tilted relative to the TBC4 axis closer to the plane of the phenyl rings of the TBC4 cage. Translational velocity autocorrelation calculations show a single peak under all conditions studied with very little change with temperature. Rotational velocity autocorrelation calculations show relatively little structure with significant tailing to low frequencies indicating rotation is hindered in the conical TBC4 cavity in the low density structure. The free energy of inclusion for CO2 in this TBC4 structure at 300 and 450 K for various loadings show the inclusion of a single CO2 in the system is favorable at -4.2 kcal/mol at 300 K and -1.5 kcal/mol at 450 K. The fully loaded 1:1 CO2:TBC4 system is slightly less favorable at -3.6 and -1.0 kcal/mol for 300 and 450 K respectively. The first CO2 added beyond 1:1 loading shows a significant drop in absorption energy to -1.8 and +1.5 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 CO2 at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the energy of inclusion for addition of the CO2 above 1:1 is less favorable. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Glezakou VA, LX Dang, and BP McGrail. 2009. "Spontaneous activation of CO2 and possible corrosion pathways on the low-index Iron surface Fe(100)." Journal of Physical Chemistry C 113(9):3691-3696. Abstract This work examines fundamental reactions pertaining to the capture, sequestration and storage of CO2 or other contaminants such as H2O, Sox etc, as well as the corrosion mechanism of steels under SC-Co2 conditions. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Sun X, and LX Dang. 2009. "Computational studies of aqueous interfaces of RbBr salt solutions ." Journal of Chemical Physics 130(12):Article no. 124709. doi:10.1063/1.3096916 Abstract In this paper, we compute the structural factor and corresponding x-ray reflectivity of the aqueous interface of RbBr salt solutions using molecular dynamics techniques and polarizable and non-polarizable potential models. Our computed electron and number densities clearly demonstrate that the polarizable Br- anions are concentrated at the water surface, while the non-polarizable Br- anions are depleted from the surface. This observation contradicts a recently published conclusion that was based on x-ray reflectivity measurements. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Sun X, TM Chang, Y Cao, S Niwayama, WL Hase, and LX Dang. 2009. "Solvation of Dimethyl Succinate in a Sodium Hydroxide Aqueous Solution. A Computational Study." Journal of Physical Chemistry B 113(18):6473–6477. Abstract Molecular dynamics simulations were carried out to study dimethyl-succinate/water/NaOH solutions. The potential of mean force method is 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 van der Waals interactions. Thus, even at higher NaOH concentrations, the increasing electrostatic interaction between the diester and ions is not large enough to overcome the Lennard-Jones (LJ) interactions to favor the solvation of diester in bulk solutions. The calculated solvation free energies are found to be -2.6 ~ -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. This work was supported by the US Department of Energy (DOE) Office of Basic Energy Sciences, Chemical Sciences program. Pacific Northwest National Laboratory is operated by Battelle for DOE.

Chang TM, and LX Dang. 2008. "Computational studies of liquid water and diluted water in carbon tetrachloride." Journal of Physical Chemistry A 112(8):1694-1700. doi:10.1021/jp711092v Abstract 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 Å 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 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 were non-isotropic, with the out-of plane vector correlation times in water/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 and Skinner et al. JACS 2001, 123, 8047). For diluted water in CCl4, we found the computed rotational correlation times also were non-isotropic 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 concentrations, 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. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Daschbach JL, PK Thallapally, BP McGrail, and LX Dang. 2008. "Dynamics and free energies of CH4 and CO2 in the molecular solid of the p-tert-butylcalix[4]arene." Chemical Physics Letters 453(4-6):123-128. doi:10.1016/j.cplett.2008.01.031 Abstract The dynamics of the guest molecules CO2 and CH4 in p-tert-butylalix[4]arene (TBC4) motion was studied from 100 K to 400 K using the velocity autocorrelation approach. The rattling motion of the CH4 guest molecules exhibit an increase in the Einstein frequency with increasing temperature as has been observed experimentally and in simulations for some rare gas clathrate systems. The CH4 molecule shows an increase from 75 cm-1 at 100 K to 91 cm-1 at 400 K. The CO2 rattling motion exhibits a single peak and less temperature dependence going from 76 cm-1 at 100 K to 72 cm-1 at 400 K. The rotational spectra for CO2 exhibit two peaks at 100 K with the higher frequency peak shifting to lower frequency with increasing temperature. The computed rotational Einstein frequencies go from 86 cm-1 at 100 K to 71 cm-1 at 400 K. The temperature dependent Gibbs absorption free energies of the guest molecules CO2 and CH4 in TBC4 have also been studied at 1 bar using thermodynamic integration from 10 K to 400 K. As expected, the simulated absorption free energy for CO2 is greater at all temperatures than for CH4. At 10 K, the simulated values of -11.4 kcal/mol and -8.4 kcal/mol for CO2 and CH4, respectively, while at 300 K the values are -5.4 kcal/mol and -4.4 kcal/mol. This work was performed at the Pacific Northwest National Laboratory (PNNL) and was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). PNNL is operated by Battelle for the DOE.

Wick CD, and LX Dang. 2008. "Molecular Dynamics Study of Ion Transfer and Distribution at the Interface of Water and 1,2-Dichloroethane (Letter)." Journal of Physical Chemistry C 112(3):647-649. doi:10.1021/jp076608c Abstract Molecular dynamics simulations were carried out to study Cl-’s propensity for and its transfer across the H2O-1,2-dichloroethane (DCE) interface, comparing it with the H2O-CCl4 and H2O-vapor interfaces. It was found that, primarily because the DCE molecules had a preferred orientation at the H2O-DCE interface that resulted in unfavorable interactions with Cl-, Cl- was repelled from the H2O-DCE interface. For CCl4, which has a larger Cl- free energy of transfer from H2O than DCE, Cl- had a propensity for the interface, as well as for the H2O-vapor interface. Calculated thermodynamic properties for pure DCE, the H2O-DCE surface tension, and the free energy of Cl- transfer across the H2O-DCE interface agreed very well with experiment. This study shows that a coexisting solvent’s preferred orientation at the interface can be used to control the propensity of a solute for the aqueous interface. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

Wick CD, and LX Dang. 2008. "Recent advances in understanding transfer ions across aqueous interfaces." Chemical Physics Letters 458(1-3):1-5. doi:10.1016/j.cplett.2008.03.097 Abstract Understanding the composition of aqueous interfaces, and the mechanism for ion transport across them is of fundamental importance for biological, environmental, and industrial processes. Molecular dynamics simulations, using the potential of mean force technique serves as a technique to map out the free energy profile across interfaces. In some cases, where the free energy of ion transfer is known experimentally between two phases, the potential of mean force technique can allow validation of the simulation results against experiment for this property. In addition, the inclusion of polarizability in the interaction potential can be of paramount importance for understanding interfacial properties and the ion transfer mechanism in interfacial environments. This review discusses some of the recent studies of ion transport across aqueous interfaces, and gives insights on the ion transport mechanism and why certain interfacial behavior is observed. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.