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

Abstract

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 x(CHCl3) = 0.77 is in fair agreement with the experimental value of x(CHCl3)expt = 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.

Microstructures of negative and positive azeotropes

“…Acetone-chloroform mixtures have been studied, although not always at the azeotropic composition, by analysis of thermodynamic excess functions, 7,8,[10][11][12][13][14][15][16][17][18] NMR, [24][25][26][27] Raman, 28,29 FT-IR, 27,28,30 NIR, 31 terahertz time-domain 32 and inelastic neutron spectroscopy 29 as well as Monte Carlo simulations. 22 There is general consensus that hydrogen bonding interactions are likely to be present. For example, a peak at 82 cm -1 in the inelastic neutron spectrum has been assigned to the anti-translational mode of the 1:1 hydrogen bonded complex.…”

“…who used a scalable chloroform potential to reproduce the measured vapour pressure. 22 We tested the potentials used in their study as reference potentials for EPSR, but obtained a significantly poorer fit to our neutron diffraction data compared to using the empirical potentials. Figure 3(d) shows the OA-HC spatial density function (SDF) which highlights the volume where the OA-HC pair-correlation function takes the highest values.…”

Properties of the Liquid–Vapor Interface of Acetone–Water Mixtures. A Computer Simulation and ITIM Analysis Study

“…1 Although numerous computer simulation studies of the bulk liquid phase of neat acetone [2][3][4][5][6] as well as of its mixtures with water [7][8][9][10][11][12][13][14][15][16][17][18] and other cosolvents, 9,[19][20][21][22][23][24][25] being sometimes in supercritical state 12,22 have been reported in the past decades, little is known about the behavior of acetone at interfaces. In fact, although the properties of the acetone molecules adsorbed at the surface of ice, 26,27 mixed acetone-water nanoclusters, 28 the liquid-vapor interface of neat acetone [29][30][31] and acetone-methanol mixtures of various compositions 32 have already been investigated by computer simulation methods several times, and the liquid-vapor interface of neat acetone 29,33 as well as of acetone-water mixtures 34,35 have also been studied, although scarcely, by surface sensitive experimental methods, we are not aware of any detailed computer simulation investigation of the liquidvapor interface of acetone-water mixtures.…”

Properties of the intrinsic surface of liquid acetone, as seen from computer simulations

“…Further, as a potential Hacceptor, acetone is miscible in any ratio with water as well as other hydrogen bonding liquids, such as small alcohols. Because of its importance the properties of acetone have been investigated in detail in different systems by a variety of experimental methods [1][2][3][4][5][6][7][8][9][10][11][12], and these investigations were well complemented by numerous computer simulation studies of neat bulk liquid acetone [13][14][15][16][17], mixtures of acetone in various proportions with water [18][19][20][21][22][23][24][25][26][27], with other solvents [28][29][30][31][32][33] and solvent mixtures [34], adsorption layer of acetone at the surface of ice [35,36], and aqueous acetone nanoclusters [37].…”

“…To avoid any arbitrariness of the results due to the particular choice of the force field used, we repeated all the calculations using two different potential models of acetone, i.e., a four site one belonging to the TraPPE force field, which describes the CH 3 groups as united atoms [29], and a ten site one using all-atom representation, which belongs to the CHARMM27 force field [56]. These models differ considerably in their dipole moment values: whilst the TraPPE model corresponds to the dipole moment of 2.50 D, in accordance with the majority of the acetone potentials as well as with the experimental gas phase value of 2.88 D, [57,58] the CHARMM27 model takes polarization effect into account in an average way, having a molecular dipole moment of 3.68 D. Besides the surface orientation of the molecules we address here questions concerning the separation and width of the subsequent subsurface molecular layers, molecular scale roughness of the liquid surface, dynamics of exchange of the molecules between the surface layer and the bulk phase, and the extent, in terms of molecular layers, to which the vicinity of the vapour phase influences the properties of liquid acetone.…”