Structures of binary mixtures of ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with primary alcohols: The role of hydrogen-bonding

Abstract

The structure of ionic liquids (ILs) with shorter alkyl tail(s) of the constituent ions is predominantly governed by strong electrostatic interactions which are, in most cases, accompanied by specific interactions such as hydrogen bonding. Neat 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide Im1,4+/NTf2− IL possesses both of these interactions. However, in many conventional solvents such as short tail length primary alcohols, the structure is primarily determined by hydrogen bonding interactions. In this study, we aim to understand how the structure of Im1,4+/NTf2− changes with the addition of different amount of alcohol by employing atomistic molecular dynamics simulations. Specifically, we have chosen Im1,4+/NTf2− + 1-propanol and Im1,4+/NTf2− + 1-butanol mixtures as the pure components of both the mixtures possess complex interactions, including hydrogen bonding. The mixtures' composition-dependent structures were examined through simulated X-ray scattering structure functions (S(q)s), their partial components along with radial and spatial distribution functions. It is observed that there is significant hydrogen bonding between the anion and alcohols along with strong electrostatic interactions between the cation head and anion. For the entire composition of the mixtures studied, while alcohol's hydroxyl group is found near the cation head and the anion's oxygen atoms, the nonpolar tail of alcohol molecules tend to align with the tails of either the cation or other alcohol molecules. It is interesting to note that on adding even small amount of the IL (10%) to alcohol, the low q peak observed for the neat alcohols disappears. The X-ray scattering S(q)s reveal that the long-range charge ordering structure of the Im1,4+/NTf2− IL begin to build up at xIL = 0.3 but at larger length scales. The transitional structural changes in the both the mixtures take place for 0.3 ≤ xIL ≤ 0.8. The neat IL structure is virtually regained at and above xIL = 0.8.