Investigation of the Structure of Ethanol-Water Mixtures by Molecular Dynamics Simulation I: Analyses Concerning the Hydrogen-Bonded Pairs

Series of molecular dynamics simulations for ethanolwater mixtures with 2080 mol % ethanol content, pure ethanol, and water were performed. In each mixture, for ethanol the OPLS force field was used, combined with three different water force fields, the SPC/E, the TIP4P-2005, and the SWM4-DP. Water potential models were distinguished on the basis of deviations between calculated and measured total scattering X-ray structure factors aided by ethanolwater pair binding energy comparison. No single water force field could provide the best agreement with experimental data at all concentrations: at the ethanol content of 80% the SWM-DP, for 60 mol % the SWM4-DP and the TIP4P-2005, whereas for the 40 and 20 mol % mixtures TIP4P-2005 water force field provided the closest match. Coordination numbers and hydrogen bonds/molecule values were calculated, revealing that the oxygenoxygen first coordination numbers strongly overestimate the average number of hydrogen bonds/molecule. The center-of-molecule distributions indicate that the ethanolethanol first coordination sphere expands with increasing water concentration while the size of the first waterwater coordination sphere does not change. Various two and three-dimensional distributions were calculated that reveal the differences between simulations with different water force fields. Detailed conformational analyses of the hydrogenbonded pairs were performed; drawings of the characteristic molecular arrangements are provided.