(708d) Optimized Mie Potentials for Phase Equilibria: Application to Noble Gases, Alkanes, Alkynes and Their Mixtures

Transferable united-atom force fields, based on n-6 Mie potentials are presented for noble gases, branched alkanes, and alkynes. By tuning the repulsive exponent, it is possible to simultaneously reproduce experimental saturated liquid densities and vapor pressures with high accuracy, from the normal boiling point to the critical point. Vapor-liquid coexistence curves for pure fluids are calculated using histogram reweighting Monte Carlo simulations in the grand canonical ensemble, while Gibbs ensemble Monte Carlo is used to calculate binary and ternary mixture phase behavior. All calculations are performed with GOMC[1]. A semi-automated process for optimization of non-bonded force field parameters is presented. For each compound, vapor-liquid-coexistence curves, vapor pressures, heats of vaporization, critical properties, and normal boiling points are predicted and compared to experiment.

For all noble gases, saturated liquid densities and vapor pressures are reproduced to within 1% and 4% of experiment, respectively. For alkynes, experimental saturated liquid densities are reproduced to within 2% average absolute deviation (AAD), except for 1-hexyne, which are reproduced with 3% AAD. Experimental saturated vapor pressures are reproduced to within 3% AAD, except for 1-pentyne, 2-pentyne, and 2-hexyne, where deviations from experimental up to 20% AAD were observed. For branched alkanes, simulations are performed on branched isomers of butane, pentane, hexane, heptane, and octane to assess the transferability of potential parameters. Experimental saturated liquid densities and critical temperatures are reproduced with a median absolute average error of 0.6%, while vapor pressures are reproduced with a median absolute average error of 2.2%. Binary phase diagrams predicted by the Mie potentials for a variety of systems are in close agreement with experiment. Comparisons are made to the predictions of existing force fields, including SPEAD[2] and 2CLJQ[3], NERD[4, 5] and TraPPE[6].

References:

1. GOMC: http://gomc.eng.wayne.edu.

2. Unlu, O., N.H. Gray, Z.N. Gerek, and J.R. Elliott, Transferable Step Potentials for the Straight-Chain Alkanes, Alkenes, Alkynes, Ethers, and Alcohols. Industrial & Engineering Chemistry Research, 2004. 43(7): p. 1788-1793.

3. Vrabec, J., J. Stoll, and H. Hasse, A set of molecular models for symmetric quadrupolar fluids. Journal of Physical Chemistry B, 2001. 105(48): p. 12126-12133.

4. Nath, S.K. and J.J. De Pablo, Simulation of vapour-liquid equilibria for branched alkanes. Molecular Physics, 2000. 98(4): p. 231-238.

5. Nath, S.K. and R. Khare, New forcefield parameters for branched hydrocarbons. The Journal of Chemical Physics, 2001. 115(23): p. 10837-10844.

6. Martin, M.G. and J.I. Siepmann, Novel configurational-bias Monte Carlo method for branched molecules. Transferable potentials for phase equilibria. 2. United-atom description of branched alkanes. Journal of Physical Chemistry B, 1999. 103(21): p. 4508-4517.