Computer Simulations For Supercritical Carbon Dioxide Systems

Ab initio calculations and molecular simulations were employed to investigate molecular interactions and microscopic structures in several common supercritical carbon dioxide systems.1. The famous EPM2-M and TraPPE-EH potentials were modified to accurately describe the detailed microscopic structures of supercritical carbon dioxide system. In addition, three very efficient Monte Carlo algorithms were developed, which could be used to accurately predict the thermodynamic properties and aggregation patterns of polar species in supercritical fluids.2. There were three important molecular interactions in the scCO₂ + ethanol binary system, which were ethanol-ethanol hydrogen bonding, CO₂-ethanol weak hydrogen bonding, and CO₂-ethanol EDA (electron donor-acceptor) bonding, respectively. The effects of concentration, temperature and pressure on the three interactions were also investigated. The detailed distribution of ethanol aggregates in the mixture indicated that the mixtures consisted of a lot of ethanol monomers, cyclic tetramers and cyclic pentamers. Moreover, the dipole moment of these cyclic aggregates was very small, showing that the cyclic aggregates were nonpolar. In the scCO₂ + acetic acid mixture, however, the linear logarithmic relationship was observed between the percentage of the acetic acid aggregates and their size. Moreover, the mixture almost contained the same amount of linear and cyclic acetic acid aggregates. The difference between the two solutions indicates that the structure of polar chemical groups has a great impact on the distribution of aggregates.3. According to the above calculation results, the mechanism for the dissolution of ethanol and acetic acid in supercritical carbon dioxide was proposed. On the one hand, a substantial percentage of polar molecules were dissolved in the nonpolar solvent by forming various cyclic aggregates, i.e. planar micelle-like structure, which could greatly reduce the overall polarity of polar species and thus satisfy the rule of similarity at the microscopic level. On the other hand, the monomers and linear aggregates were dissolved in scCO₂ through the weak hydrogen bonding and EDA interaction between CO₂ and polar molecules.4. The self- and cross-aggregation in the ternary systems of scCO₂ + ethanol + water and scCO₂ + acetic acid + water was intensively studied. On the basis of interactions between different polar species, the mechanism of cosolvent was proposed: first, cosolvent molecules cross-associated with solute molecules through hydrogen bonding to form various nonpolar cross-aggregtes; then, these cross-aggregates were dissolved in scCO₂ through the weak hydrogen bonding and EDA interactions between cosolvent molecules and CO₂ molecules.5. Ab initio method was employed to compare the interactions between CO₂ molecules and several different chemical groups, whereas molecular simulations were performed to consider the influence of solvation effect and self-aggregation of polar chemical groups on molecular interactions. The interaction between the S=O group and CO₂ was the strongest among the chemical groups studied. Moreover, the carbonyl and ester groups also exhibited an affinity for CO₂. The results would shed light on the development of new CO₂-philes.