Molecular dynamics simulation of liquid ethylene glycol and its aqueous solution

Molecular dynamics simulations were used to study pure liquid ethylene glycol (EG) and EG-water binary mixtures as a function of the molar fraction. Calculations were performed in the isothermal and isobaric (NpT) ensemble. New improvement in the OPLSS-AA-SEI force field was obtained optimizing the Lennard-Jones parameters and charges taking part in 1-4 interactions. With the new force field density and heat of vaporization of pure liquid ethylene glycol were obtained in good agreement with experimental data. The value of the O-C-C-O dihedral was also monitored in the course of force field optimization process. The new optimized force field was named OPLSA-AA-SE-M. Concerning the pure liquid simulation, features observed on the radial distribution functions (RDF) show characteristics of intermolecular and intramolecular hydrogen bonding. The RDF integrations show that each EG molecule in the pure liquid has an average coordination number of five molecules in its first coordination shell. Therefore, the average oxygen-hydrogen contact number due to hydrogen bonding is seven: two of them arising from intramolecular and five from interactions with neighboring molecules. Dihedral angle distributions were calculated, showing the predominance of EG molecules with the O-C-C-O dihedral corresponding to gauche conformations. In the study of water-ethylene glycol binary mixtures, the average structural parameters observed for EG molecules are very similar to the ones found in the pure liquid simulation. RDF analyses also present characteristic features of hydrogen bonding. Particularly, no significant changes were observed on the dihedral angle distributions. The structures of EG-EG and EG-water dimers calculated with OPLS-AA-SEI-M force field and DFT at B3LYP 6-311 g (3df,3pd) level are in very good agreement.