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 ethanol–water mixtures with 20–80 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 ethanol–water 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 oxygen–oxygen first coordination numbers strongly overestimate the average number of hydrogen bonds/molecule. The center-of-molecule distributions indicate that the ethanol–ethanol first coordination sphere expands with increasing water concentration while the size of the first water–water 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 hydrogen–bonded pairs were performed; drawings of the characteristic molecular arrangements are provided