Molecular Simulation For Pure Supercritical Fluids And Their Binary Mixtures

Monte Carlo and molecular dynamics simulations were used to investigatea variety of supercritical fluids: carbon dioxide, ethanol and the classicalcosolvent -supercritical carbon dioxide systems. Many different force fieldswere tested in this work by simulating the physical properties of the fluids. Indetail, the following topics have been discussed: 1. By using the canonical ensemble Monte Carlo simulation with newdeveloped EPM-M serial models, the structural and PVT properties ofsupercritical carbon dioxide (ρ=100-900 kg/m3,T=313,333,353 K) werestudied. In addition, since the new EPM2-M carbon dioxide molecule has asmall dipole moment, the dielectric constant simulation results are inexcellent agreement with experimental data. 2. The NPT ensemble Monte Carlo method was used to simulate puresupercritical ethanol with two different united-atom force fields (OPLS-UAand TraPPE-UA) in range of 473 K～623 K and 10 MPa～70 MPa. Thestructure and hydrogen bond network were studied with the TraPPE-UAmodel. The results showed that even at 623 K, hydrogen bond interactionsbecome rather weak but still exist and the simulation data quantitatively agreewith NMR data. Furthermore, the hydrogen bond networks of threerepresentative self associating pure fluids: water, methanol and ethanol werecompared. 3. Molecular dynamics simulation was used to investigate the structural,dynamic and dielectric properties of the well known supercritical carbondioxide -cosolvent binary mixtures. TraPPE-UA force field was chosen tomodel cosolvent fluids and EPM2 model was used to simulate carbon dioxide.The simulation results of dielectric constant agree with experiments at lowcosolvent concentration, and at high concentration region, the results arehigher than experimental data. The diffusion constants for carbon dioxide arelarger compared with to those obtained for cosolvent. From structural analysis,it can be seen that alcohol molecules aggregated to form clusters and thehydrogen bond interaction was stronger in comparison with the hydrogenbonds in pure alcohols. Meanwhile, acetone-carbon dioxide mixtures did notshow such a strong aggregation phenomenon.