Understanding the Liquid Structure in Mixtures of Ionic Liquids with Semiperfluoroalkyl or Alkyl Chains

By mixing ionic liquids (ILs), it is possible to fine-tunetheirbulk and interfacial structure. This alters their physical propertiesand solvation behavior and is a simple way to prepare a collectionof ILs whose properties can be tuned to optimize a specific application.In this study, mixtures of perfluorinated and alkylated ILs have beenprepared, and links between composition, properties, and nanostructurehave been investigated. These different classes of ILs vary substantiallyin the flexibility and polarizability of their chains. Thus, a rangeof useful structural and physical property variations are accessiblethrough mixing that will expand the library of IL mixtures availablein an area that to this point has received relatively little attention.In the experiments presented herein, the physical properties and bulkstructure of mixtures of 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide[C8MIM][Tf2N] and 1-(1H,1H,2H,2H-perfluorooctyl)-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide [C8MIM-F-13][Tf2N] have been prepared. The bulk liquid structurewas investigated using a combination of small-angle X-ray and neutronscattering (SAXS and SANS, respectively) experiments in combinationwith atomistic molecular dynamics simulations and the measurementof density and viscosity. We observed that the addition of [C8MIM-F-13][Tf2N] to [C8MIM][Tf2N] causes changes in the nanostructure of the IL mixturesthat are dependent on composition so that variation in the characteristicshort-range correlations is observed as a function of composition.Thus, while the length scales associated with the apolar regions (polarnon-polar peak PNPP) increase with the proportion of [C8MIM-F-13][Tf2N] in the mixtures, perhapssurprisingly given the greater volume of the fluorocarbon chains,the length scale of the charge-ordering peak decreases. Interestingly,consideration of the contact peak shows that its origins are bothin the direct anion & BULL;& BULL;& BULL;cation contact length scale andthe nature (and hence volume) of the chains appended to the imidazoliumcation.

Automated summary

By mixing different classes of ionic liquids (ILs), their properties can be tuned to optimize specific applications. In this study, mixtures of perfluorinated and alkylated ILs were prepared and their composition, properties, and nanostructure were investigated. The results show that by mixing these ILs, various structural and physical property variations can be achieved, expanding the library of IL mixtures available in an area that has received relatively little attention.