• Bulletin of the Korean Chemical Society
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• v.19 no.4
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• pp.417-422
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• 1998

The electrocatalytic reduction of molecular oxygen was investigated with a Co(II)-glyoxal bis(2-hydroxyanil) complex coated-glassy carbon (GC) electrode in aqueous media. The reduction of $O_2$ at the modified electrode was an irreversible and diffusion-controlled reaction. The complex coated-GC electrode demonstrated an excellent electrocatalytic effect for $O_2$ reduction in an acetate buffer solution of pH 3.2. The coated electrode made the $O_2$ reduction potential shift of 60-510 mV in a positive direction compared to the bare GC electrode depending on pH. The Co(II)-glyoxal bis(2-hydroxyanil) coated electrode converted about 51% of the $O_2$ to $H_2O_2$ via a two-electron reduction pathway, with the balance converted to H_2O$.

• Bulletin of the Korean Chemical Society
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• v.18 no.5
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• pp.478-484
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• 1997

In a recent paper[Bull. Kor. Chem. Soc. 17, 735 (1996)] we reported results of molecular dynamic (MD) simulations for the thermodynamic and structural properties of liquid n-alkanes, from n-butane to n-heptadecane, using three different models. Two of the three classes of models are collapsed atomic models while the third class is an atomistically detailed model. In the present paper we present results of MD simulations for the dynamic properties of liquid n-alkanes using the same models. The agreement of two self-diffusion coefficients of liquid n-alkanes calculated from the mean square displacements (MSD) via the Einstein equation and the velocity auto-correlation (VAC) functions via the Green-Kubo relation is excellent. The viscosities of n-butane to n-nonane calculated from the stress auto-correlation (SAC) functions and the thermal conductivities of n-pentane to n-decane calculated from the heat-flux auto-correlation (HFAC) functions via the Green-Kubo relations are smaller than the experimental values by approximately a factor of 2 and 4, respectively.

• Bulletin of the Korean Chemical Society
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• v.18 no.5
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• pp.501-509
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• 1997

In recent papers[Bull. Kor. Chem. Soc. 1996, 17, 735; ibid 1997, 18, 478] we reported results of molecular dynamics (MD) simulations for the thermodynamic, structural, and dynamic properties of liquid normal alkanes, from n-butane to n-heptadecane, using three different models. Two of the three classes of models are collapsed atomic models while the third class is an atomistically detailed model. In the present paper we present results of MD simulations for the corresponding properties of liquid branched-chain alkanes using the same models. The thermodynamic property reflects that the intermolecular interactions become weaker as the shape of the molecule tends to approach that of a sphere and the surface area decreases with branching. Not like observed in the straight-chain alkanes, the structural properties of model Ⅲ from the site-site radial distribution function, the distribution functions of the average end-to-end distance and the root-mean-squared radii of gyration are not much different from those of models Ⅰ and Ⅱ. The branching effect on the self diffusion of liquid alkanes is well predicted from our MD simulation results but not on the viscosity and thermal conductivity.

• Journal of Electrochemical Science and Technology
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• v.11 no.2
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• pp.148-154
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• 2020

Highly ordered mesoporous manganese oxide films were electrodeposited onto indium tin oxide coated (ITO) glass using sodium dodecyl sulfate (SDS) and ethylene glycol (EG) which were used as a templating agent and stabilizer for the formation of micelle, respectively. The manganese oxide films synthesized with surfactant templating exhibited a highly mesoporous structure with a long-range order, which was confirmed by SAXRD and TEM analysis. The unique porous structure offers a more favorable diffusion pathway for electrolyte transportation and excellent ionic conductivity. Among the synthesized samples, Mn₂O₃-SDS+EG exhibited the best electrochemical performance for a supercapacitor in the wide range of scan rate, which was attributed to the well-developed mesoporous structure. The Mn₂O₃ prepared with SDS and EG displayed an outstanding capacitance of 72.04 F g^-1, which outperform non-porous Mn₂O₃ (32.13 F g^-1) at a scan rate of 10 mV s^-1.

• Bulletin of the Korean Chemical Society
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• v.35 no.9
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• pp.2679-2683
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• 2014

Mechanisms underlying the impacts of interactions between carbon nanoparticles (CNPs) and ionic liquids (ILs) on the physicochemical behavior of CNPs need to be more full worked out. This manuscript describes a theoretical investigation at multiple levels on the interactions of fullerene $C_{60}$ with 21 imidazolium-based ILs of varying alkyl side chain lengths and anionic types and their impacts on $C_{60}$ dispersion behavior. Results show that ${\pi}$-cation interaction contributed to mechanism of the $C_{60}$-IL interaction more than ${\pi}$-anion interaction. The calculated interaction energy ($E_{INT}$) indicates that $C_{60}$ can form stable complex with each IL molecule. Moreover, the direction of charge transfer occurred from IL to $C_{60}$ during the $C_{60}$-IL interaction. Quantitative models were developed to evaluate the self-diffusion coefficient of $C_{60}$ ($D_{fullerene}$) in bulk ILs. Three interpretative molecular descriptors (heat of formation, $E_{INT}$, and charge) that describe the $C_{60}$-IL interactions and the alkyl side chain length were found to be determinants affecting $D_{fullerene}$.

• Macromolecular Research
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• v.15 no.6
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• pp.565-574
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• 2007

In this study, five representative, commercially available polymers, Ultem 1000 polyetherimide, Kapton polyimide, phenolic resin, polyacrylonitrile and cellulose acetate, were used to prepare pyrolyzed polymer membranes coated on a porous {\alpha}-alumina$ tube via inert pyrolysis for gas separation. Pyrolysis conditions (i.e., final temperature and thermal dwell time) of each polymer were determined using a thermogravimetric method coupled with real-time mass spectroscopy. The surface area and pore size distribution of the pyrolyzed materials derived from the polymers were estimated from the nitrogen adsorption/desorption isotherms. Pyrolyzed membranes from polymer precursors exhibited type I sorption behavior except cellulose acetate (type IV). The gas permeation of the carbon/{\alpha}-alumina$ tubular membranes was characterized using four gases: helium, carbon dioxide, oxygen and nitrogen. The polyetherimide, polyimide, and phenolic resin pyrolyzed polymer membranes showed typical molecular sieving gas permeation behavior, while membranes from polyacrylonitrile and cellulose acetate exhibited intermediate behavior between Knudsen diffusion and molecular sieving. Pyrolyzed membranes with molecular sieving behavior (e.g., polyetherimide, polyimide, and phenolic resin) had a $CO_2/N_2$ selectivity of greater than 15; however, the membranes from polyacrylonitrile and cellulose acetate with intermediate gas transport behavior had a selectivity slightly greater than unity due to their large pore size.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1885-1890
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• 2008

3-D heterogeneous nucleation was simulated by classical molecular dynamics (MD), where the Lennard-Jones (LJ) gas and solid cluster-seed molecules have argon and aluminum properties, respectively. There are three shapes of cluster-seeds, cube, rod, and sphere, and three classes of masses and the simulation took place under nine supersaturation ratios, making a total of 81 calculations. Results show that the dimension of the cluster-seed highly affects the rates of cluster development. In order to analyze heterogeneous nucleation above and below the critical supersaturation ratio, growth rate and liquefaction rate were separately defined to supplement the investigation. Design of experiments (DOE) was used for analysis which displayed that the shape and mass of the cluster-seed are prominent for the growth rate, while the supersaturation ratio is most significant followed by the mass for liquefaction rate. The significance of the supersaturation ratio for overall liquefaction suggests that thermal diffusion is more dominant than mass interactions for this system.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.160.2-160.2
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• 2014

Recently silicon has attracted intense interest as a promising anode material of lithium-ion batteries due to its extremely high capacity of 4200 mA/g (for Li4.2Si) that is much higher than 372 mAh/g (for LiC6) of graphite. However, it seriously suffers from large volume change (even up to 300%) of the electrode upon lithiation, leading to its pulverization or mechanical failure during lithiation/delithiation processes and the rapid capacity fading. To overcome this problem, Si nanowires have been considered. Use of such Si nanowires provides their facile relaxation during lithiation/delithiation without mechanical breaking. To design better Si electrodes, a study to unveil atomic-scale mechanisms involving the volume expansion and the phase transformation upon lithiation is critical. In order to investigate the lithiation mechanism in Si nanowires, we have developed a reactive force field (ReaxFF) for Si-Li systems based on density functional theory calculations. The ReaxFF method provides a highly transferable simulation method for atomistic scale simulation on chemical reactions at the nanosecond and nanometer scale. Molecular dynamics (MD) simulations with the ReaxFF reproduces well experimental anisotropic volume expansion of Si nanowires during lithiation and diffusion behaviors of lithium atoms, indicating that it would be definitely helpful to investigate lithiation mechanism of Si electrodes and then design new Si electrodes.

• Environmental Analysis Health and Toxicology
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• v.23 no.2
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• pp.67-77
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• 2008

It has been observed that the linear relationship between the logarithm of bioconcentration factor (log BCF) of highly hydrophobic chemicals and their log $K_{ow}$ breaks when log $K_{ow}$ becomes greater than 6.0. Consequently, super hydrophobic chemicals were not thought to cause baseline toxicity as a single compound. Researchers often call this phenomenon as "hydrophobicity cutoff" meaning that bioconcentration or corresponding baseline toxicity has a certain cutoff at high log $K_{ow}$ value of hydrophobic organic pollutants. The underlying assumption is that the increased molecular size with increasing hydrophobicity prohibits highly hydrophobic compounds from crossing biological membranes. However, there are debates among scientists about mechanisms and at which log $K_{ow}$ this phenomenon occurs. This paper reviews three hypotheses to explain observed "cutoff": steric effects, kinetic or physiological limitations, and chemical activity cutoff. Although the critical molecular size that makes biological membranes not permeable to hydrophobic organic chemicals is uncertain, size effects in combination with kinetic limitation would explain observed non-linearity between log BCF and log $K_{ow}$. Chemical activity of hydrophobic chemicals generally decreases with increasing melting point at their aqueous solubility. Thus, there may be a chemical activity cutoff of baseline toxicity if there is a critical chemical activity over which baseline effects can be observed.

• Bulletin of the Korean Chemical Society
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• v.34 no.10
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• pp.2925-2930
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• 2013

We present results of molecular dynamics simulations for hydroxide ion in supercritical water of densities 0.22, 0.31, 0.40, 0.48, 0.61, and 0.74 g/cc using the SPC/E water potential with Ewald summation. The limiting molar conductance of $OH^-$ ion at 673 K monotonically increases with decreasing water density. It is also found that the hydration number of water molecules in the first hydration shells around the $OH^-$ ion decreases and the potential energy per hydrated water molecule also decreases in the whole water density region with decreasing water density. Unlike the case in our previous works on LiCl, NaCl, NaBr, and CsBr [Lee at al., Chem. Phys. Lett. 1998, 293, 289-294 and J. Chem. Phys. 2000, 112, 864-869], the number of hydrated water molecules around ions and the potential energy per hydrated water molecule give the same effect to cause a monotonically increasing of the diffusion coefficient with decreasing water density in the whole water density region. The decreasing residence times are consistent with the decreasing potential energy per hydrated water molecule.