Sung Jin Pai

Ab initio potential energy surface for methane and carbon dioxide and application to vapor-liquid coexistence

A six-dimensional intermolecular potential energy surface for a rigid methane (CH4) and carbon dioxide (CO2) dimer was developed from the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory. A total of 466 grid points distributed to 46 orientations were calculated from the complete basis set limit extrapolation based on up to aug-cc-pVQZ basis set. A modified site-site pair potential function was proposed for rapid representation of the high level ab initio calculations. A nonadditive three-body interaction was represented by the Axilrod-Teller-Muto expression for mixtures with the polarizability and the London dispersion constant of each molecule. Second to fourth virial coefficients of CH4 and CO2 mixtures were calculated using both the Mayer sampling Monte Carlo method and the present potential functions. The virial equation of state derived from these coefficients was used to predict the pVT values and showed good agreement with experimental data below 200 bar at 300 K. The vapor-liquid coexistence curves of pure CH4, CO2 and their mixtures were presented with the aid of Gibbs ensemble Monte Carlo simulations. The predicted tie lines agreed with the experimental data within the uncertainties up to near the critical point.

Electrochemical Properties for Solid Polymer Electrolyte/Salt Systems in Lithium Secondary Batteries

We establish a new thermodynamic framework to describe the electrochemical properties of solid polymer electrolyte/salt systems. The Pitzer-Debye-Huckel expression is introduced to the modified double lattice-nonrandom model to take into account long-range electrostatic forces between molecules. After developing full descriptions for the electrochemical properties of a poly(ethylene oxide) (PEO)/NaCF 3 SO 3 system, the open-circuit voltage (OCV), diffusion coefficient, and ionic conductivity are calculated and compared with experimental data. We predict OCVs and diffusion coefficients of PEO/ZnBr 2 and PEO/ZnI 2 systems with the parameters obtained from phase diagrams and ionic conductivity data. The quantitative description according to the proposed model is in good agreement with the experimental data.

Theoretical consideration of osmotic pressure in aqueous protein/salt systems based on extended hard core Lennard-Jones potential

A simple and analytical pair potential function was developed to represent the osmotic pressures in aqueous protein/salt systems under various conditions. Based on a hard core Lennard-Jones (HCLJ) potential model, the new potential function considers various interactions by extending the attractive Lennard-Jones potential. A temperature-dependent coefficient term was introduced to take into account the specific properties of given materials. Comparison of the new potential function with the HCLJ model in hydrocarbon and water systems showed that consideration of the temperature dependence in the potential function was effective, especially for strong polar systems such as water. To predict the osmotic pressures of aqueous lysozyme/(NH(4))(2)SO(4) solutions of various ionic strength and pH, the energy parameters of lysozyme were correlated with the experimental cloud point temperature. The proposed model agreed fairly well with the experimental osmotic pressure data with only previously obtained parameters.