Understanding the Differing Fluid Phase Behavior of Cyclohexane + Benzene and Their Hydroxylated or Aminated Forms

Abstract

The three binary mixtures cyclohexane + benzene, cyclohexanol + phenol, and cyclohexylamine + aniline exhibit qualitatively different vapor-liquid phase behavior, that is, azeotropic with a pressure maximum, azeotropic with a pressure minimum, and zeotropic, respectively. Employing molecular modeling and simulation, the COSMO-SAC model, and a cubic equation of state, the root of these effects is studied on the basis of phase equilibria, excess properties for volume, enthalpy, and Gibbs energy as well as microscopic structure. It is found that cyclohexane + benzene is characterized by more pronounced repulsive interactions, leading to pressure maximum azeotropy and a positive excess Gibbs energy. Functionalizing the aliphatic and aromatic rings with one amine group each introduces attractive hydrogen bonding interactions of moderate strength that counterbalance such that the mixture becomes zeotropic. The hydroxyl groups introduce strong hydrogen bonding interactions, leading to pressure minimum azeotropy and a negative excess Gibbs energy.

Molecular Insight into the Liquid Propan-2-ol + Water Mixture

“…The geometric parameters, that is, bond lengths and internal angles, as well as the magnitudes of the point charges were taken from the quantum chemical data base of the National Institute of Standards and Technology (NIST) . The model parameters were obtained with an optimization procedure that was reported in prior works, , that is, the Z matrix of propan-2-ol was calculated to obtain bond lengths as well as the angles θ i , j , k between three and ϕ i , j , k , l between four successive sites. All angles, the LJ parameters of the oxygen and methanetriyl sites, as well as the magnitude of the point charges were kept constant during the fitting procedure.…”

“…There has thus been a continuous effort with respect to the development of force fields that are able to cover a wide variety of physical properties, − employing, among others, quantum chemical calculations. In this work, a rigid nonpolarizable force field for propan-2-ol (CAS-number 67-63-0, or henceforth IPA) was developed, which was optimized to experimental vapor–liquid equilibrium (VLE) and transport property data following a procedure that is similar to the one outlined in ref . This force field was tested together with the well-known TIP4P/2005 water force field on its ability to predict macroscopic excess and transport properties of their mixture under ambient conditions of temperature and pressure.…”

Thermodynamics of supercritical carbon dioxide mixtures across the Widom line

“…The molecular models for CO 2 , 58 hydrogen, 59 methane, 60 ethane, 60 isobutane, 61 benzene, 62 toluene 62 and naphthalene 63 were parameterized with an optimization procedure based on quantum chemical calculations and fitting to experimental vapor–liquid equilibrium (VLE) data in previous work of our group. 61,64,65 The molecular models for benzene and toluene were additionally fitted to self-diffusion coefficient data. 62 The parameters of the employed force fields are listed in the MolMod database.…”

ms2: A molecular simulation tool for thermodynamic properties, release 4.0

“…Therein, α, β = x, y, z are Cartesian coordinates, m i and v α i are mass and velocity of molecule i, k and l are the indices of the n interaction sites constituting a molecular model, r α ij is the site-site distance and u ij the potential energy of the sitesite interaction. The present implementation averages over the three off-diagonal elements J xy p , J xz p , J yz p of the stress tensor (42). Procedures for sampling the transport properties can be employed concurrently with both approaches.…”