Issue 12, 2016

Ion-water wires in imidazolium-based ionic liquid/water solutions induce unique trends in density

Abstract

Ionic liquid/water binary mixtures are rapidly gaining popularity as solvents for dissolution of cellulose, nucleobases, and other poorly water-soluble biomolecules. Hence, several studies have focused on measuring the thermophysical properties of these versatile mixtures. Among these, 1-ethyl-3-methylimidazolium ([emim]) cation-based ILs containing different anions exhibit unique density behaviours upon addition of water. While [emim][acetate]/water binary mixtures display an unusual rise in density with the addition of low-to-moderate amounts of water, those containing the [trifluoroacetate] ([Tfa]) anion display a sluggish decrease in density. The density of [emim][tetrafluoroborate] ([emim][BF4])/water mixtures, on the other hand, declines rapidly in close accordance with the experimental reports. Here, we unravel the structural basis underlying this unique density behavior of [emim]-based IL/water mixtures using all-atom molecular dynamics (MD) simulations. The results revealed that the distinct nature of anion–water hydrogen bonded networks in the three systems was a key in modulating the observed unique density behaviour. Vast expanses of uninterrupted anion–water–anion H-bonded stretches, denoted here as anion–water wires, induced significant structuring in [emim][Ac]/water mixtures that resulted in the density rise. Conversely, the presence of intermittent large water clusters disintegrated the anion–water wires in [emim][Tfa]/water and [emim][BF4]/water mixtures to cause a monotonic density decrease. The differential nanostructuring affected the dynamics of the solutions proportionately, with the H-bond making and breaking dynamics found to be greatly retarded in [emim][Ac]/water mixtures, while it exhibited a faster relaxation in the other two binary solutions.

Graphical abstract: Ion-water wires in imidazolium-based ionic liquid/water solutions induce unique trends in density

Supplementary files

Article information

Article type
Paper
Submitted
15 Jan 2016
Accepted
12 Feb 2016
First published
12 Feb 2016

Soft Matter, 2016,12, 3032-3045

Ion-water wires in imidazolium-based ionic liquid/water solutions induce unique trends in density

D. Ghoshdastidar and S. Senapati, Soft Matter, 2016, 12, 3032 DOI: 10.1039/C6SM00117C

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