Neil F. Giles

Experimental and molecular-dynamics simulated excess enthalpies and solubilities of neopentane in supercritical carbon dioxide

Abstract Excess enthalpies were measured as a function of composition for mixtures of neopentane in supercritical CO2 at two temperatures (310 and 313.15 K) slightly above the critical temperature of pure CO2 and at three pressures (6.29, 8.36, and 10.44 MPa) spanning the pure CO2 critical pressure. The excess enthalpies were extremely pressure dependent, ranging from very large exothermic values at the lowest pressure to endothermic values at the highest pressure. Excess enthalpies at these same conditions were calculated from molecular-dynamics simulations treating neopentane as a spherical Lennard-Jones fluid and CO2 as both a single- and two-site Lennard-Jones fluid. Both models correctly predict the strong pressure dependence of the excess enthalpy. The simulated and experimental values agree well over the entire range of conditions studied if the Lennard-Jones unlike energy parameter is regressed from a single experimental datum.

Thermodynamic properties of mixing for SnCl4 dissolved in supercritical CO2 : a combined experimental and molecular dynamics study

Abstract Giles, N.F., Oscarson, J.L., Rowley, R.L., Tolley, W.K. and Izatt R.M., 1992. Thermodynamic properties of mixing for SnCl4 dissolved in supercritical CO2: a combined experimental and molecular dynamics study. Fluid Phase Equilibria, 73: 267-284. Enthalpies and volumes of mixing were measured as a function of composition for mixtures of SnCl4 in supercritical CO2 at two temperatures (313.15 and 348.15 K) above the critical temperature of pure CO2 and at four pressures (6.29, 8.36, 10.44 and 12.50 MPa) spanning the critical pressure of CO2. Molecular dynamics simulations were performed at the same conditions, modeling SnCl4 as a Lennard-Jones fluid and CO2 as a two-site fluid interacting with Lennard-Jones potentials. Quite simple molecular interaction models are capable of reproducing the large pressure dependence observed in both the mixing volume and enthalpy data in the critical region of pure CO2. Additionally, Widoms method was used to obtain the excess free energies of the mixtures. Simulated vapor-liquid equilibrium for conditions below the mixture critical locus is in qualitative agreement with phase boundaries obtained from the experimental heat of mixing data.

Molecular-Dynamics Simulation of Mutual Diffusion in Nonideal Liquid Mixtures

The mutual-diffusion coefficients, D12, of n-hexane, n-heptane, and n-octane in chloroform were modeled using equilibrium molecular-dynamics (MD) simulations of simple Lennard-Jones (LJ) fluids. Pure-component LJ parameters were obtained by comparison of simulations to experimental self-diffusion coefficients. While values of “effective” LJ parameters are not expected to simulate accurately diverse thermophysical properties over a wide range of conditions, it was recently shown that effective parameters obtained from pure self-diffusion coefficients can accurately model mutual diffusion in ideal, liquid mixtures. In this work, similar simulations are used to model diffusion in nonideal mixtures. The same combining rules used in the previous study for the cross-interaction parameters were found to be adequate to represent the composition dependence of D12. The effect of alkane chain length on D12 is also correctly predicted by the simulations. A commonly used assumption in empirical correlations of D12, that its kinetic portion is a simple, compositional average of the intradiffusion coefficients, is inconsistent with the simulation results. In fact, the value of the kinetic portion of D12 was often outside the range of values bracketed by the two intradiffusion coefficients for the nonideal system modeled here.

Molecular-dynamics simulations of excess enthalpies in mixtures of supercritical carbon dioxide and neopentane

Abstract Heats of mixing of neopentane with CO2 near the pure CO2 critical point were measured and found to exhibit a strong state dependence which cannot be adequately modelled using classical equations of state or empirical models with state-independent parameters. Molecular dynamics simulations of spherical, Lennard-Jones (LJ) molecules were used to model the measured heats of mixing. Pure-component potential parameters were obtained from pure-component density and enthalpy data. The results indicate that the LJ model with effective parameters is capable of modeling the observed change from exothermic to endothermic heats of mixing using a state-independent value of the cross interaction energy, ϵ12. The Lorentz-Berthelot combining rule often used to estimate ϵ12 does not adequately model the observed heats of mixing.

Comparison of the thermodynamics of mixing of titanium tetrachloride and tin tetrachloride with supercritical carbon dioxide

Previous studies show that tin tetrachloride and titanium tetrachloride mix readily with supercritical CO{sub 2}. Comparison of these systems demonstrates significant differences in heats of mixing, mixture densities, and vapor-liquid equilibria. Although part of the difference may be attributable to the different densities of the pure solutes, there is some experimental and theoretical evidence that the intermolecular interactions between SnCl{sub 4} and CO{sub 2} are stronger than those between TiCl{sub 4} and CO{sub 2}. These differences are consistent with the presence in tin of completed d and f orbitals which may affect interaction between SnCl{sub 4} and CO{sub 2}.