An Improved Force Field for the Prediction of the Vapor−Liquid Equilibria for Carboxylic Acids

Abstract

An improved united-atom force field, based on a Lennard-Jones plus point charge functional form, for carboxylic acids is proposed. Point charges are determined from a Mulliken population analysis of ab initio calculations performed at the MP2 level of theory with the 6-31+g(d,p) basis set. The Lennard-Jones well depth and size parameters for the carboxyl carbon interaction site were determined by fitting to single-component vapor-liquid equilibrium data for acetic acid, while the remaining Lennard-Jones parameters were taken from the TraPPE-UA force field. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble were used to determine the vapor-liquid coexistence curves, vapor pressures, and critical points predicted by the new force field. The new force field was found to describe accurately the phase equilibria of acetic acid, with mean unsigned errors in the saturated liquid density of less than 1.6%. The critical temperature and density predicted by the new force field were within 1% of experimental values. Additional calculations were performed in the grand canonical and Gibbs ensembles to determine the coexistence properties predicted by the optimized potentials for liquid simulations united-atom (OPLS-UA), all-atom (OPLS-AA), and CHARMM force fields. Simulations in the isobaric-isothermal ensemble were used to determine selected radial distribution functions predicted by the four force fields at 300 K and 1 bar.

Gibbs ensemble Monte Carlo simulation using an optimized potential model: pure acetic acid and a mixture of it with ethylene

“…The lengths of these bonds were originally determined from experimental data [23] and used in the TraPPE-UA force field. Values for bonded parameters are listed in Table 2.…”

“…Also, the interaction energy of dimer B was always less than that of dimer C. These results show that the method we used was able to accurately describe the interactions of the acetic acid molecules in the acetic acid dimers. Furthermore, the MP2/6-31+G(d,p) method was used in a paper in which the vapor-liquid equilibria of carboxylic acids were predicted [23], so this method would appear to be a relatively reliable method to use in our quantum chemistry calculations.…”

“…The force field also predicted a higher critical temperature than that obtained experimentally. Kamath [23] used the TraPPE force field to predict the vapor-liquid equilibrium of pure acetic acid, but the force field could not predict the vapor pressure accurately. They came to the conclusion that the force field was able to accurately predict the liquid density and critical temperature of acetic acid, whereas it overestimated the vapor pressure of acetic acid by up to 50 %.…”

Extension of the TraPPE-UA force field to the simulation of vapor–liquid phase equilibria of vinyl acetate system

“…Parameters for the CH3*(sp3), Cco_o(sp2), O_c(sp2), Occ(sp3) and CH2_*(sp2) units were taken directly from the TraPPE-UA force field [13][14][15][16]. All geometric data for valence terms of CHo_c(sp2), i.e., bond lengths, angles, bending and torsional potentials, were directly taken from previous installments of the TraPPE-UA [14] and OPLS-UA [9] force fields as listed in Table 1.…”

“…The current research only studied the bond parameters of vinyl acetate, without describing non-bonded parameters of the intermolecular interactions. Kamath et al [12] accurately predicted the vapor-liquid equilibria of various pure linear esters (methyl acetate, ethyl acetate methyl propionate and vinyl acetate) with previous parametrizations which were taken from literature about alkanes, alkenes and carboxylic acids [13][14][15][16] with the TraPPE-UA force field. The vapor-liquid equilibrium behavior of vinyl acetate was also calculated, and its parameters of pseudoatoms were all taken from previous installments of the TraPPE-UA force field.…”