Application of TraPPE-UA force field for determination of vapor–liquid equilibria of carboxylate esters

Abstract

The transferability of Lennard–Jones parameters in the united-atom force field known as “transferable potentials for phase equilibria” (TraPPE-UA) is assessed through vapor–liquid equilibria calculations performed on carboxylate esters. In the TraPPE-UA force field, non-bonded interactions are governed by a Lennard–Jones plus fixed point charge functional form. Partial charges are borrowed from the optimized potentials for liquid simulations (OPLS) force field [Briggs, Nguyen, Jorgensen, J. Phys. Chem. 95 (1991) 3315]. No reparameterization of pseudo-atoms occurs in this work. Instead, the molecules of interest are built from pseudo-atoms parameterized in previous installments of the TraPPE-UA force field. Configurational-bias Monte Carlo simulations in the grand canonical ensemble, combined with histogram-reweighing techniques, are used to determine the vapor–liquid coexistence curves, vapor pressures and critical points of the esters methyl acetate, ethyl acetate, methyl propionate and vinyl acetate. Pressure-composition diagrams are calculated for methyl acetate + ethyl acetate at 313.15 K and methyl acetate + methanol at 323.15 K. Average deviations in the saturated liquid densities and critical temperatures from experiment vary from 1.8% (methyl acetate) to 4.2% (vinyl acetate), while the critical densities for all four esters are predicted to within 0.8%. The pressure-composition diagrams for methyl acetate + ethyl acetate and methyl acetate + methanol agree qualitatively with experiment, but quantitative differences exist due to the overprediction of the pure component vapor pressures. For the methyl acetate + methanol system the predicted azeotropic composition of x AcOMe = 0.66 is in good agreement with the experimental value of x AcOMe expt = 0.65 . Analysis of the microstructure of the methyl acetate + methanol mixture shows that the addition of methyl acetate has little effect on the self-association of methanol molecules through hydrogen bonding.