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

By mixing ionic liquids (ILs), it is possible to fine-tune their bulk and interfacial structure. This alters their physical properties and solvation behavior and is a simple way to prepare a collection of ILs whose properties can be tuned to optimize a specific application. In this study, mixtures of perfluorinated and alkylated ILs have been prepared, and links between composition, properties, and nanostructure have been investigated. These different classes of ILs vary substantially in the flexibility and polarizability of their chains. Thus, a range of useful structural and physical property variations are accessible through mixing that will expand the library of IL mixtures available in an area that to this point has received relatively little attention. In the experiments presented herein, the physical properties and bulk structure of mixtures of 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [C8MIM][Tf2N] and 1-(1H,1H,2H,2H-perfluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C8MIM-F13][Tf2N] have been prepared. The bulk liquid structure was investigated using a combination of small-angle X-ray and neutron scattering (SAXS and SANS, respectively) experiments in combination with atomistic molecular dynamics simulations and the measurement of density and viscosity. We observed that the addition of [C8MIM-F13][Tf2N] to [C8MIM][Tf2N] causes changes in the nanostructure of the IL mixtures that are dependent on composition so that variation in the characteristic short-range correlations is observed as a function of composition. Thus, while the length scales associated with the apolar regions (polar non-polar peak—PNPP) increase with the proportion of [C8MIM-F13][Tf2N] in the mixtures, perhaps surprisingly 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 both in the direct anion···cation contact length scale and the nature (and hence volume) of the chains appended to the imidazolium cation.


Fit parameters
Table S1: Fit parameters used to fit the SAXS data using a combination of Lorentzian peaks and an Ornstein-Zernicke model to determine correlation length (CL) In addition, some additional experiments were conducted using the DL-SAXS instrument at Diamond, allowing access to scattering at smaller q, but the data (DOI: 10.15124/31b8a7ba-8465-4e92-a114-1101207b183d) did not yield any information not already in hand from the in-house instrument.

SANS D16 ILL: Grenoble, France
SANS was carried at the Institut Laue-Langevin (ILL, Grenoble, France) on the D16 instrument, a cold neutron diffractometer, which uses a highly orientated pyrolytic graphite monochromator to focus the beam along the vertical axis.This gave access to a neutron wavelength of 4.47 Å.The nine crystals which make up the monochromator are orientated to maximise the incident neutron flux by focussing the beam to the sample.The detector angle g was varied (13, 29, 45 and 61 o ) in order to access the desired q range, with the samples tilted 10 o away from the detector in the Z axis.The distance of the MWPC detector relative to the sample was fixed, at 950mm from the sample.This 2D 3 He detector (MILAND) is made of 320 X by 320 Y wires spaced by 1 mm providing a pixel resolution of 1 mm × 1mm and a detection area of 320 × 320 mm.The resulting q range was 0.085-1.6Å -1 .

Fit parameters
Table S2: Fit parameters used to fit the SANS data for [d17-C8MIM]1-x[C8MIM-F13]x[Tf2N] using a combination of Lorentzian peaks and an Ornstein-Zernicke model to determine correlation length (CL)

Fit parameters
Table S4: Fit parameters used to fit the SANS data for [d17-C8MIM]1-x[C8MIM-F13]x[Tf2N] using a combination of Lorentzian peaks and an Ornstein-Zernicke model to determine correlation length (CL)

Figure S5 :
Figure S5: Temperature dependence of the viscosity (η) of neat [C8MIM-F13][Tf2N] Experimental values are represented by the markers, the dashed line indicates the Arrhenius model and the solid line indicates the VFT model.

Table S1 :
Density, excess density, molar volume and excess molar volume data at 293.

Table S5 :
Fit parameters used to fit the SANS data for for [h-C8MIM]1-x[C8MIM-F13]x[Tf2N] using a combination of Lorentzian peaks and an Ornstein-Zernicke model to determine correlation length (CL)