Evidences for a Null Molar Volume Contribution by Hydroxyl Groups in Ammonium Bistriflimide-Based Ionic Liquids

In this work, we report on the thermophysical and transport properties of seven ammonium bistriflimide ionic liquids. Aiming to study the effect of the manipulation of the ammonium cation on distinct properties of ionic liquids, we have considered two sets of bistriflimide-based ionic liquids with (i) three quasi-similar molar weight cations with distinct functional groups, [N_{2 1 1 3OH}]⁺, [N_{2 1 1 2O1}]⁺, and [N_{2 1 1 4}]⁺ (ii) four cations with increasing number of 2-hydroxyethyl groups and/or alkyl side chain, [N_{2 1 2OH 2OH}]⁺, [N_{1 2OH 2OH 2OH}]⁺, [N_{2 2OH 2OH 2OH}]⁺, and [N_{4 2OH 2OH 2OH}]⁺. The most relevant result is the strong evidence that the replacement of a methyl group by a hydroxymethyl group in the cation has an almost-null contribution to the overall molar volume of the ionic liquid. Molecular dynamics simulations have been performed to explain from a structural perspective the obtained results. Most of the effect comes from the fact that the intrinsic van der Waals volumes of -CH₃ and -CH₂OH are not that different (14.7 and 17.8 cm³·mol^-1); the remainder of the effect comes from the fact that hydrogen bonding between hydroxyl groups or between them and the charged moieties of the ionic liquid can lead to further contractions of the free volume that annuls the modest intrinsic volume contribution of the OH group.