Influence of the functional group on vapor-liquid phase behavior: a study of cyclohexane + benzene and their hydroxylated or aminated forms

Azeotropy is an important topic for phase equilibria and has been experimentally and theoretically studied since 1802. Although this phenomenon has widely been addressed by experimentalists, there are many efforts that allow... [ view full abstract ]

Azeotropy is an important topic for phase equilibria and has been experimentally and theoretically studied since 1802. Although this phenomenon has widely been addressed by experimentalists, there are many efforts that allow for a detailed explanation of its occurrence on the molecular scale. In that sense, we have selected the perspective of molecular interactions offered by molecular simulation for explain the root of azeotropic behavior of three deliberately selected binary mixtures. These three binary mixtures, i.e. cyclohexane + benzene, cyclohexanol + phenol and cyclohexylamine + aniline, exhibit qualitatively different vapor-liquid phase behavior, i.e. azeotropic with pressure maximum, azeotropic with pressure minimum and zeotropic, respectively. Molecular simulation is employed for determining vapor-liquid equilibria of the pure substances and the binary mixtures. For that purpose, three new force fields were devised for cyclohexylamine, phenol and aniline on the basis of quantum mechanical data and force fields previously published by our group. It is found that the new force fields are able to describe the phase diagrams of the pure substances. Furthermore, these force fields adequately describe the vapor-liquid equilibria diagrams of the binary mixtures. The excess properties and the microscopic structure were sampled in the entire molar fraction range along the saturated liquid line and it a relationship between the shape of the phase diagrams and the sign and magnitude of the excess properties for the studied binary mixtures was found. E.g., 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 is zeotropic. The hydroxyl groups introduce strong hydrogen bonding interactions, leading to pressure minimum azeotropy and a negative excess Gibbs energy. The results were compared with experimental information from the literature and with a recent version of the COSMO-SAC model.