Hydrogen bond, electron donor-acceptor dimer, and residence dynamics in supercritical CO(2)-ethanol mixtures and the effect of hydrogen bonding on single reorientational and translational dynamics: A molecular dynamics simulation study.

The hydrogen bonding and dynamics in a supercritical mixture of carbon dioxide with ethanol as a cosolvent (X(ethanol) approximately 0.1) were investigated using molecular dynamics simulation techniques. The results obtained reveal that the hydrogen bonds formed between ethanol molecules are significantly more in comparison with those between ethanol-CO(2) molecules and also exhibit much larger lifetimes. Furthermore, the residence dynamics in the solvation shells of ethanol and CO(2) have been calculated, revealing much larger residence times for ethanol molecules in the ethanol solvation shell. These results support strongly the ethanol aggregation effects and the slow local environment reorganization inside the ethanol solvation shell, reported in a previous publication of the authors [Skarmoutsos et al., J. Chem. Phys. 126, 224503 (2007)]. The formation of electron donor-acceptor dimers between the ethanol and CO(2) molecules has been also investigated and the calculated lifetimes of these complexes have been found to be similar to those corresponding to ethanol-CO(2) hydrogen bonds, exhibiting a slightly higher intermittent lifetime. However, the average number of these dimers is larger than the number of ethanol-CO(2) hydrogen bonds in the system. Finally, the effect of the hydrogen bonds formed between the individual ethanol molecules on their reorientational and translational dynamics has been carefully explored showing that the characteristic hydrogen bonding microstructure obtained exhibits sufficiently strong influence upon the behavior of them.