Richard L. Rowley

Model channel ion currents in NaCl-extended simple point charge water solution with applied-field molecular dynamics.

Using periodic boundary conditions and a constant applied field, we have simulated current flow through an 8.125-A internal diameter, rigid, atomistic channel with polar walls in a rigid membrane using explicit ions and extended simple point charge water. Channel and bath currents were computed from 10 10-ns trajectories for each of 10 different conditions of concentration and applied voltage. An electric field was applied uniformly throughout the system to all mobile atoms. On average, the resultant net electric field falls primarily across the membrane channel, as expected for two conductive baths separated by a membrane capacitance. The channel is rarely occupied by more than one ion. Current-voltage relations are concentration dependent and superlinear at high concentrations.

Diffusion and viscosity equations of state for a Lennard-Jones fluid obtained from molecular dynamics simulations

Equilibrium molecular dynamics simulations were performed for a Lennard-Jones fluid at 171 conditions spanning the range 0≤p+≤1.0 and 0.8≤T+≤4.0. The Einstein or, mean-squared-displacement (MSD) formula was used to compute the self-diffusion coefficient and a recently suggested, modified MSD equation was used to compute the shear viscosity at each condition. Analytical equations for the self-diffusion and viscosity coefficients were then fitted to the simulated data as polynomial functions ofp+ andT+. The resultant smoothing equations correlate the simulated data quite well and agree with argon experimental data within the uncertainty of the data.

Thermophysical properties of HFC-125

Abstract Thermophysical properties of hydrofluorocarbon (HFC)-125 (pentafluoro-ethane) have been measured. These measurements include vapor pressures, critical temperature, critical pressure, critical density, pressure-volume-temperature (PVT) data, liquid and vapor heat capacities, isobaric enthalpy differences, liquid and vapor viscosities, and liquid and vapor thermal conductivities. The BWRS equation of state was used to correlate the PVT and vapor pressure data, and predict enthalpy departures. The measurement results have been compared to data from other sources.

Molecular dynamics calculations of the electrochemical properties of electrolyte systems between charged electrodes

We investigate the interfacial electrochemical properties of an aqueous electrolyte solution with discrete water molecules in slab geometry between charged atomistic electrodes. Long-range intermolecular Coulombic interactions are calculated using the particle–particle–particle–mesh method with a modification to account for the slab geometry. Density distribution profiles and potential drops across the double layer are given for 0, 0.25, and 1 M aqueous electrolyte solutions each at 0, ±0.1, ±0.2, and ±0.3 C/m2 electrode surface charges. Results are compared qualitatively with experimental x-ray scattering findings, other computer simulation results, and traditional electrochemistry theory. The interfacial fluid structure characteristics are generally in good qualitative agreement with the conclusions obtained in some integral equation theories and in the experimental x-ray study. The potential in the simulations shows an oscillatory behavior near the electrode, which theories that do not include the mole...

Comparison of charged sheets and corrected 3D Ewald calculations of long-range forces in slab geometry electrolyte systems with solvent molecules

Two methods of calculating long-range intermolecular potentials are compared for an approximately 3 M aqueous electrolyte solution confined between two charged surfaces. We investigate the ionic density profiles using the charged-sheets method and the corrected three-dimensional (3D) Ewald method at two different system sizes and also compare the Coulomb forces directly. The corrected 3D Ewald method is recommended for the calculation of long-range potentials in systems of this nature because it is less system size dependent than the charged-sheets method and the resultant forces are more consistent with periodic boundaries. However, the charged-sheets method for estimating long-range potentials in Coulombic systems may be useful for certain applications, and the corrected 3D Ewald method also shows some system size dependence.

Determination of a methane intermolecular potential model for use in molecular simulations from ab initio calculations

The possibility of obtaining an accurate site-site potential model suitable for use in molecular dynamics (MD) simulations of methane from ab initio calculations has been explored. Counterpoise-corrected (CPC), supermolecule, ab initio energies at the MP2/6-311+G(2df,2pd) level were computed for eleven relative orientations of two methane molecules as a function of C-C separation distance. C-C, C-H, and H-H interaction parameters in a pairwise-additive, site-site potential model for rigid methane molecules were regressed from the ab initio energies, and the resultant model accurately reproduced the ab initio energies. The model suggests that C-H attractions are dominant in weakly binding the methane dimer. CPC energies for methane trimers, tetramers, and a pentamer were also calculated at the same level. The results indicate that the n-mer energy per pair of interactions monotonically converges with increasing n, but that the assumption of pairwise additivity commonly used in MD simulations is reasonably ...

Non-equilibrium molecular dynamics simulation of the shear viscosity of liquid methanol: adaptation of the Ewald sum to Lees-Edwards boundary conditions

The Ewald sum method is commonly used in equilibrium simulations of polar fluids to enhance convergence of long-range Coulombic forces within modest-sized cubic simulation cells. In this work, we derive a form of the standard Ewald sum technique for use with non-equilibrium molecular dynamics (NEMD) simulations of viscosity that make use of the Lees–Edwards boundary conditions. This generalized Ewald sum can be used for any parallelepiped simulation cell. The method was tested by performing NEMD simulations at various temperatures and densities for simulated liquid methanol. The results were in excellent agreement with experimental data for methanol and Green–Kubo simulations of the viscosity using the standard cubic-cell Ewald sum. A simple truncation of the polar interactions at 10 A was found to produce errors of over 200% in the simulated viscosities. Values obtained with the polar interactions turned off (i.e. using only dispersion forces) were generally 40–60% below the experimental values. These re...

Molecular-dynamics simulations of ion size effects on the fluid structure of aqueous electrolyte systems between charged model electrodes

The effect of ion size on the structure of aqueous electrolyte solutions between charged nonpolarizable surfaces or electrodes is investigated using molecular-dynamics simulations of discrete water molecules and ions confined to a slab geometry. Long-range intermolecular Coulombic interactions are calculated using the particle–particle–particle–mesh method with a modification to account for the slab geometry. Density distribution and potential profiles are reported for 1 M aqueous electrolyte solutions with ±0.1 C/m2 electrode surface charge at the electrode surfaces. Five different models for the ions are studied. The models can be characterized as (1) ions of equal size, (2) smaller cations, (3) larger anions, (4) smaller cations and larger anions, and (5) ions representing aqueous NaCl. Compared to the equal-size ion reference case, smaller cation size decreases the contact adsorption at the cathode, but interestingly anion size tends to moderate this effect somewhat. Whereas there is no contact adsorp...

Excess enthalpies between 293 and 323 K for constituent binaries of ternary mixtures exhibiting partial miscibility

Abstract Excess molar enthalpies have been measured with an isothermal flow calorimeter for 10 binary mixtures and 4 ternary mixtures at p = 101.325 kPa between 293.15 and 323.15 K. The measured binary values, with the addition of readily available literature values for (benzene + cyclohexane), form a complete set of the consolute or miscible binaries comprising 5 ternary systems that exhibit type 1 (only one non-consolute binary) liquid-liquid equilibrium over this temperature range. Ternary excess enthalpies were also measured in the one-phase region for 4 of these 5 mixtures from which effective excess enthalpies for the non-consolute binary were obtained. Excess enthalpies for one partially miscible binary were measured directly. The results form a complete set of constituent binary excess enthalpies for the 5 partially miscible ternary mixtures. They have been fitted with a rational-function equation at each measured temperature. In view of the Gibbs-Helmholtz identity relating excess enthalpies and Gibbs free energies, these results, in conjunction with liquid-liquid equilibrium data, will be useful in testing liquid-mixture models and their capabilities for describing both phase behavior and excess enthalpies with one set of parameters.

Predicting the viscosity of alkanes using nonequilibrium molecular dynamics: Evaluation of intermolecular potential models

Nonequilibrium molecular dynamics (NEMD) viscosity simulations of branched and linear alkanes at liquid densities were performed using both united-atom (UA) and all-atom (AA) intermolecular potential models in order to study the relative efficacy of the models in predicting fluid viscosity. Both models were used in conjunction with fixed bond lengths and bond angles, but different torsional potentials were investigated. The commonly used Ryckaert–Bellemans intermolecular potential model, which accurately predicts viscosities for short straight-chain alkanes, produced values for branched and long-chain alkanes that were significantly below experimental values. Likewise, a more complex UA model that uses transferrable site potentials and is commonly used to simulate thermodynamic properties also under predicted viscosities for branched and long-chain molecules. The UA models were also found to be density dependent, substantially under predicting viscosity at high liquid densities for all model fluids tested...