Hydrogen bonding in ethanol–water and trifluoroethanol–water mixtures studied by NMR and molecular dynamics simulation

Abstract

1H NMR measurements were conducted on aqueous mixtures of ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) at 25.0 and −10.0°C with varying alcohol mole fraction xalc. At 25.0°C the TFE hydroxyl proton and water protons in TFE–water mixtures could not be individually observed on the NMR time scale due to the fast exchange between both protons. By cooling at −10.0°C, the peaks of the alcohol hydroxyl and water protons became distinguishable in both aqueous EtOH and TFE mixtures over the entire xalc range. The alcohol hydroxyl and water protons in both alcohol mixtures at −10.0°C were gradually deshielded with decreasing xalc, suggesting the increase in the number of the hydrogen bonds in the mixtures. In particular, the NMR results showed that the hydrogen bonds between the alcohol hydroxyl group and water strengthen in both mixtures below xalc≈0.2. Moreover, the water structure by hydrogen bonding among water is enhanced in the mixtures below the same mole fraction. To clarify the contribution of the alcohol hydroxyl hydrogen and oxygen atoms in the hydrogen bonding for both mixtures, molecular dynamics (MD) simulations were made on aqueous EtOH and TFE systems at −10.0°C. The hydrogen bonding abilities of the EtOH hydroxyl hydrogen and oxygen atoms are comparable with each other. The ability of the TFE hydroxyl hydrogen is higher than EtOH, while that of the TFE oxygen is much lower than EtOH. The present results suggested that the higher hydrophobicity of TFE compared to EtOH is mainly attributed to the two lone pairs of the oxygen with the lower electron donicity.