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Title: Molecular Mechanism of Transporting a Polarizable Iodide Anion Across the Water-CCl4 Liquid/Liquid Interface

Abstract

The result of transferring a polarizable iodide anion across the H2O-CCl4 liquid/liquid interface was investigated. The computed transfer free energy profile or potential of mean force exhibits a minimum near the Gibbs dividing surface, and its characteristics are similar to those of found in a corresponding water vapor/liquid interface study involving a smaller minimum free energy. Molecular dynamics simulations also were carried out to compare the concentrations of NaCl, NaBr, and NaI at H2O-vapor and H2O-CCl4 interfaces. While the concentration of bromide and iodide ions were lower at the H2O-CCl4 interface when compared to the H2O-vapor interface, the chloride ion concentrations were similar at both interfaces. Analysis of the solvation structures of iodide and chloride ions revealed that the more polarizable iodide ion was less solvated than the chloride ion at the interface. This characteristic brought the iodide ion in greater contact with CCl4 than the chloride ion, resulting in repulsive interactions with CCl4, which reduced its propensity for the interface. 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. Themore » DOE Division of Chemical Sciences and the Scientific Computing Staff, Office of Science provided computer resources at the National Energy Research Supercomputer Center (Berkeley, California) that supported this research.« less

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
908734
Report Number(s):
PNNL-SA-53038
Journal ID: ISSN 0021-9606; JCPSA6; KC0301020; TRN: US200722%%766
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics, 126(13); Journal Volume: 126; Journal Issue: 13
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; MASS TRANSFER; FREE ENERGY; IODIDES; SOLVATION; WATER; CARBON TETRACHLORIDE; INTERFACES; MOLECULAR DYNAMICS METHOD; liquid interface; iodide anion; molecular dynamics simulation

Citation Formats

Wick, Collin D., and Dang, Liem X. Molecular Mechanism of Transporting a Polarizable Iodide Anion Across the Water-CCl4 Liquid/Liquid Interface. United States: N. p., 2007. Web. doi:10.1063/1.2717164.
Wick, Collin D., & Dang, Liem X. Molecular Mechanism of Transporting a Polarizable Iodide Anion Across the Water-CCl4 Liquid/Liquid Interface. United States. doi:10.1063/1.2717164.
Wick, Collin D., and Dang, Liem X. Sat . "Molecular Mechanism of Transporting a Polarizable Iodide Anion Across the Water-CCl4 Liquid/Liquid Interface". United States. doi:10.1063/1.2717164.
@article{osti_908734,
title = {Molecular Mechanism of Transporting a Polarizable Iodide Anion Across the Water-CCl4 Liquid/Liquid Interface},
author = {Wick, Collin D. and Dang, Liem X.},
abstractNote = {The result of transferring a polarizable iodide anion across the H2O-CCl4 liquid/liquid interface was investigated. The computed transfer free energy profile or potential of mean force exhibits a minimum near the Gibbs dividing surface, and its characteristics are similar to those of found in a corresponding water vapor/liquid interface study involving a smaller minimum free energy. Molecular dynamics simulations also were carried out to compare the concentrations of NaCl, NaBr, and NaI at H2O-vapor and H2O-CCl4 interfaces. While the concentration of bromide and iodide ions were lower at the H2O-CCl4 interface when compared to the H2O-vapor interface, the chloride ion concentrations were similar at both interfaces. Analysis of the solvation structures of iodide and chloride ions revealed that the more polarizable iodide ion was less solvated than the chloride ion at the interface. This characteristic brought the iodide ion in greater contact with CCl4 than the chloride ion, resulting in repulsive interactions with CCl4, which reduced its propensity for the interface. 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. The DOE Division of Chemical Sciences and the Scientific Computing Staff, Office of Science provided computer resources at the National Energy Research Supercomputer Center (Berkeley, California) that supported this research.},
doi = {10.1063/1.2717164},
journal = {Journal of Chemical Physics, 126(13)},
number = 13,
volume = 126,
place = {United States},
year = {Sat Apr 07 00:00:00 EDT 2007},
month = {Sat Apr 07 00:00:00 EDT 2007}
}
  • In this work, we used molecular dynamics techniques and mean force approaches to compute the ion transfer free energy for the water/dichloromethane liquid-liquid interface. We used polarizable potential models to describe the interactions among the species, and both forward and reverse directions were carried out to estimate the error bar of the computed free energy results. Based on the results of our calculations, we have proposed a mechanism that describes the transport of a chlorine ion across the interface. The computed ion transfer free energy is 14 & No.177; 2 kcal/mol, which is in reasonable agreement with the experimentally reportedmore » value of 10 kcal/mol. A smooth transition from the aqueous phase to the non-aqueous phase on the free energy profile clearly indicates that the ion transfer mechanism is a nonactivated process. The computed hydration number for the chlorine ion indicates that some water molecules are associated with the ion inside the non-aqueous phase. This result is in excellent agreement with the experimental interpretation of the ion transfer mechanism reported recently by Osakai et al. (J. Phys. Chem. 1997, 101, 8341).« less
  • To enhance our understanding of the molecular mechanism of ion adsorption to the interface of mixtures, we systematically carried out a free energy calculations study involving the transport of an iodide anion across the interface of a water-methanol mixture. Many body affects are taken into account to describe the interactions among the species. The surface propensities of I- at interfaces of pure water and methanol are well understood. In contrast, detailed knowledge of the molecular level adsorption process of I- at aqueous mixture interfaces has not been reported. In this paper, we explore how this phenomenon will be affected formore » mixed solvents with varying compositions of water and methanol. Our potential of mean force study as function of varying compositions indicated that I- adsorption free energies decrease from pure water to pure methanol but not linearly with the concentration of methanol. We analyze the computed density profiles and hydration numbers as a function of concentrations and ion positions with respect to the interface to further explain the observed phenomenon. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. The calculations were carried out using computer resources provided by BES.« less
  • 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 also had good agreement with experiment for its surface tension and X-ray reflectivity. The potential of mean force of CO2 and SO2 were 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 CO2. The air-BMIMBF4 interface had enhanced BMIM density, which wasmore » mostly related to its butyl group, followed by enhanced BF4 density a few angstroms towards 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 CO2 and SO2 showed more negative free energies in regions of enhanced BF4 density, while more positive free energies 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. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less
  • No abstract prepared.
  • A comprehensive analysis of the H2O structure about aqueous iodide (I-) is reported from molecular dynamics (MD) simulation and x-ray absorption fine structure (XAFS) measurements. XAFS spectra from the iodide K-, L1-, and L3- edges were co-refined to establish the complete structure of the first hydration shell about aqueous I-. The results show approximately 6.3 water molecules located at I-H and I-O distances of 2.65 Å and 3.50 Å, respectively. Whereas the I-O bond is moderately disordered (Debye Waller factor, σ2 = 0.017 Å2) due to the relatively low charge-to-ion radius ratio, the I-H interaction shows even higher disorder (σ2more » = 0.036 Å2) due to the variable angular orientation of water at the ion surface. Molecular dynamics simulations employing both DFT (+dispersion) and classical potentials generate quite similar structures and they both agree to a large extent with the structure from the experimental XAFS. However the DFT-MD simulations provide a description of molecular structure that is more consistent with the XAFS experiment data. We employ a molecular anaylsis in which we incrementally evaluate the structural contributions from each of the nearest-neighbor water molecules about the iodide to provide a clear picture of the hydrated structure. For the DFT description of molecular interaction, a water molecule in the first shell has more freedom to rotate about the O atom when compared to motions resulting from a classical potential. Further, the hydrogen bonding of first-shell water with the second shell water establishes an strong ordering of the water about I- surface leading to characteristic O-I-O angles of 79 and 142°. This ordering, in addition to the higher coordination number leads to a more symmetric solvation from the DFT-MD configurations relative to the classical potential simulation.« less