No abstract

The transferable potentials for phase equilibria-united atom (TraPPE-UA) force field for hydrocarbons is extended to primary, secondary, and tertiary alcohols by introducing the following (pseudo-)atoms:  common hydroxyl O and H for all alcohols, α-CH3, α-CH2, α-CH, and α-C for methanol, primary, secondary, and tertiary alcohols, respectively. In the TraPPE-UA force field, the nonbonded interactions of these sites are governed by Lennard−Jones 12−6 potentials and Coulombic interactions of fixed partial charges. The values of these partial charges were borrowed from the optimized potentials for liquid simulations-united atom (OPLS−UA) force field [Jorgensen, W. L. J. Phys. Chem. 1986, 90, 1276]. The Lennard−Jones well depth and size parameters for the new interaction sites were determined by fitting to the single-component vapor−liquid-phase equilibria of a few selected model compounds. Although the well-depth parameters for the α-carbons could be taken directly from the TraPPE-UA parameters for the corresponding pseudoatoms in alkanes, the size parameters required small adjustments to reflect the differences in C−C and C−O bond lengths and the reduced electron density for α-carbons. Coupled−decoupled configurational-bias Monte Carlo simulations in the Gibbs and grand-canonical ensembles were carried out to calculate the one-component vapor−liquid coexistence curves for methanol, ethanol, propan-1-ol, propan-2-ol, butan-2-ol, 2-methylpropan-2-ol, pentan-1-ol, pentane-1,5-diol, and octan-1-ol, and to determine the binary phase diagrams for the mixtures of n-hexane/methanol and n-hexane/ethanol. It was found that the phase equilibria of the pure alcohols are accurately described by the TraPPE-UA force field, with mean unsigned deviations of about 1% from the experimental data for the normal boiling points and the saturated liquid densities. The azeotropic compositions for n-hexane/methanol and n-hexane/ethanol were predicted to be 0.340 at T = 448.15 K and 0.454 at 413.15 K and (in mole fraction of n-hexane), which are in good agreement with the experimental results of 0.288 and 0.440, respectively. Analysis of the structures of the n-hexane/methanol mixtures shows evidence for significant enhancements in the local mole fraction of alcohols in the vicinity of other alcohols. The magnitude of these local enhancements decreases with increasing alcohol concentration, but the change is gradual and no discontinuity was observed at the azeotropic composition.

The extension of the transferable potentials for phase equilibria-united atom (TraPPE-UA) force field to the ether, glycol, ketone, and aldehyde functionalities is presented. New parameters for the ether oxygen, the carbonyl carbon (ketones), the carbonyl methine (aldehydes), and a special intramolecular hydrogen-bond term were fitted to the vapor-liquid coexistence curves for selected one-component systems. Coupleddecoupled configurational bias Monte Carlo simulations in the Gibbs or grand canonical ensemble were used to compute the vapor-liquid coexistence curves for the neat systems of dimethyl ether, ethyl methyl ether, diethyl ether, dipropyl ether, diisopropyl ether, methyl tert-butyl ether, 1,2-ethanediol, 2-methoxyethan-1-ol, 1,2-dimethoxyethane, 1,3-propanediol, acetone, 2-pentanone, 2-octanone, acetaldehyde, pentanal, and octanal. Additional simulations were performed for the binary mixtures of diethyl ether + ethanol and acetone + hexane. Excellent agreement with experimental results was found with the mean unsigned errors being less than 1% for the critical temperatures and about 3% (ethers) and 1% (other) for the normal boiling temperatures. For the mixture of acetone + hexane at 328.15 K, a positive pressure azeotrope was found with x acetone azeo ) 0.71 in satisfactory agreement with the experimental result of 0.64. Additionally, the structures of hydrogenbonded aggregates were investigated for 1,2-ethanediol and 2-methoxyethan-1-ol, where the average hydrogenbond energies were found to be about -20 and -14 kJ mol -1 for inter-and intramolecular hydrogen bonds, respectively.