Linking the structures, free volumes, and properties of ionic liquid mixtures

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Title: Linking the structures, free volumes, and properties of ionic liquid mixtures
Authors: Brooks, NJ
Castiglione, F
Doherty, CM
Dolan, A
Hill, AJ
Hunt, PA
Matthews, RP
Mauri, M
Mele, A
Simonutti, R
Villar-Garcia, IJ
Weber, CC
Welton, T
Item Type: Journal Article
Abstract: The formation of ionic liquid (IL) mixtures has been proposed as an approach to rationally fine-tune the physicochemical properties of ILs for a variety of applications. However, the effects of forming such mixtures on the resultant properties of the liquids are only beginning to be understood. Towards a more complete understanding of both the thermodynamics of mixing ILs and the effect of mixing these liquids on their structures and physicochemical properties, the spatial arrangement and free volume of IL mixtures containing the common [C4C1im]+ cation and different anions have been systematically explored using small angle X-ray scattering (SAXS), positron annihilation lifetime spectroscopy (PALS) and 129Xe NMR techniques. Anion size has the greatest effect on the spatial arrangement of the ILs and their mixtures in terms of the size of the non-polar domains and inter-ion distances. It was found that differences in coulombic attraction between oppositely charged ions arising from the distribution of charge density amongst the atoms of the anion also significantly influences these inter-ion distances. PALS and 129Xe NMR results pertaining to the free volume of these mixtures were found to strongly correlate with each other despite the vastly different timescales of these techniques. Furthermore, the excess free volumes calculated from each of these measurements were in excellent agreement with the excess volumes of mixing measured for the IL mixtures investigated. The correspondence of these techniques indicates that the static and dynamic free volume of these liquid mixtures are strongly linked. Consequently, fluxional processes such as hydrogen bonding do not significantly contribute to the free volumes of these liquids compared to the spatial arrangement of ions arising from their size, shape and coulombic attraction. Given the relationship between free volume and transport properties such as viscosity and conductivity, these results provide a link between the structures of IL mixtures, the thermodynamics of mixing and their physicochemical properties.
Issue Date: 11-Jul-2017
Date of Acceptance: 11-Jul-2017
ISSN: 2041-6520
Publisher: Royal Society of Chemistry
Start Page: 6359
End Page: 6374
Journal / Book Title: Chemical Science
Volume: 8
Issue: 9
Copyright Statement: This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Sponsor/Funder: Commission of the European Communities
Funder's Grant Number: 226438
Keywords: Science & Technology
Physical Sciences
Chemistry, Multidisciplinary
XE-129 NMR
Publication Status: Published
Appears in Collections:Chemistry
Biological and Biophysical Chemistry
Catalysis and Advanced Materials
Faculty of Natural Sciences

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