To mix or not to mix: charge and polarity effects on alkyl/fluoroalkyl compound miscibility

The (im)miscibility of hydrocarbons and perfluorocarbons is well known, depends on the chain lengths involved and has been exploited widely in many different areas of chemistry. One area where mixing hydrocarbon- and fluorocarbon-containing moieties is of current interest is in ionic liquids (ILs), where physicochemical properties may be tuned via the preparation of mixtures. Recent work has shown that mixtures of the methylimidazolium ILs [C_nMIM][Tf₂N] and [C_nMIM-F_2m+1][Tf₂N] (m = n-2) are miscible when a perfluorooctyl chain is used (n = 10), which is unexpected at this chain length. In order to explore the influence of electrostatic attraction between ions in this observed miscibility, related, neutral N-substituted imidazoles C_nIm and C_nIm-F_2m+1 employing hydrocarbon and semiperfluorocarbon chains, and their mixtures, were prepared. The mixtures were miscible at room temperature for n = 8 and immiscible for n = 10. The miscible mixtures were investigated by surface tension, small-angle X-ray and neutron scattering methods and by atomistic molecular dynamics simulations. The data show that while the mixtures [C₈Im]_1-x[C₈Im-F₁₃]_x are continuously miscible, some preferential aggregation of alkyl and fluoroalkyl chains is nonetheless present, mirroring the behaviour observed for the IL mixtures [C₁₀MIM]_1-x[C₁₀MIM-F₁₇]_x[Tf₂N], yet in contrast to the shorter-chain mixtures [C₈MIM]_1-x[C₈MIM-F₁₃]_x[Tf₂N] where no aggregates are seen. As such, it has been possible to draw some conclusions concerning the ability of electrostatic interactions between the ions to suppress the otherwise expected immiscibility of the alkyl and fluoroalkyl chains.