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Molecular Modeling of Phase Behavior and Microstructure of Acetone−Chloroform−Methanol Binary Mixtures

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journal contribution
posted on 20.10.2005, 00:00 by Ganesh Kamath, Grigor Georgiev, Jeffrey J. Potoff
Force fields based on a Lennard-Jones (LJ) 12-6 plus point charge functional form are developed for acetone and chloroform specifically to reproduce the minimum pressure azeotropy found experimentally in this system. Point charges are determined from a CHELPG population analysis performed on an acetone−chloroform dimer. The required electrostatic surface for this dimer is determined from ab initio calculations performed with MP2 theory and the 6-31g++(3df,3pd) basis set. LJ parameters are then optimized such that the liquid−vapor coexistence curve, critical parameters, and vapor pressures are well reproduced by simulation. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the phase diagrams for the binary mixtures acetone−chloroform, acetone−methanol, and chloroform−methanol. The force fields developed in this work reproduce the minimum pressure azeotrope in the acetone−chloroform mixture found in experiment. The predicted azeotropic composition of xCHCl3 = 0.77 is in fair agreement with the experimental value of = 0.64. The new force fields were also found to provide improved predictions of the pressure−composition behavior of acetone−methanol and chloroform−methanol when compared to other force fields commonly used for vapor−liquid equilibria calculations. NPT simulations were conducted at 300 K and 1 bar for equimolar mixtures of acetone−chloroform, acetone−methanol, and methanol−chloroform. Analysis of the microstructure reveals significant hydrogen bonding occurring between acetone and chloroform. Limited interspecies hydrogen bonding was found in the acetone−methanol or chloroform−methanol mixtures.