Roles of hydrogen bonding interactions and hydrophobic effects on enhanced water structure strength in aqueous alcohol solutions

Abstract

The structure and dynamics of water in aqueous alcohol solutions were explored using two-dimensional Raman correlation spectroscopy (2D Raman-COS) combined with the density functional theory (DFT). The spectral changes in the H-O-H bending and O:H stretching modes demonstrated that ethanol and n-propanol induced an enhancement of the water structure compared to methanol. The extent of this effect was related to the length of the alkyl chain. Comparative studies with aqueous ethylene glycol solution revealed that an enhanced water structure stemmed mainly from hydrophobic effects rather than hydrogen bonding (H-bonding) interactions. Alcohol-induced water-specific structural transitions were further analyzed using 2D Raman-COS, which showed that the free OH and strong H-bond structure of water respond preferentially to changes in alcohol content, inducing a transition in the weak H-bond structure of water. In addition, the 2D Raman-COS results indicated that the CH3 stretching mode of alcohol responds preferentially to variations in water content compared to other C-H vibrational modes. Finally, the details of the alcohol-induced water structural transitions were calculated using DFT. The 2D Raman-COS combined with DFT calculations provided insight into alcohol-induced water structural transitions and can be easily extended to other studies of water-organic chemistry.