Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure, and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This article presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cmim][TfN] and [Cmim][TfN]). Physical property measurements (viscosity, conductivity, and density) reveal that these IL mixtures are not well described by simple mixing laws, implying that their structure and dynamics are strongly composition dependent. Small-angle X-ray and neutron scattering measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [Cmim][TfN], the bulk of the IL is composed of small aggregates of [Cmim] ions in a [Cmim][TfN] matrix, which is driven by nanosegregation of the long alkyl chains and the polar parts of the IL. As the proportion of [Cmim][TfN] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and nonpolar domains is formed. Reactive atom scattering-laser-induced fluorescence experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, in contrast to previous suggestions, that the [Cmim] ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [Cmim][TfN].

The vacuum–liquid interfaces of a number of ionic-liquid mixtures have been investigated using a combination of RAS-LIF, selected surface tension measurements, and molecular dynamics simulations.

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The atomic-level description of liquid interfaces has lagged behind that of solid crystalline surfaces because existing experimental techniques have been limited in their capability to report molecular structure in a fluctuating liquid interfacial layer. We have moved toward a more detailed experimental description of the gas–liquid interface by studying the F-atom scattering dynamics on a common ionic liquid, 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide. When given contrast by deuterium labeling, the yield and dynamical behavior of reactively scattered HF isotopologues can resolve distinct signatures from the cation butyl, methyl, and ring groups, which help to quantify the relative populations of cation conformations at the liquid–vacuum interface. These results demonstrate the importance of molecular organization in driving site-specific reactions at the extreme outer regions of the gas–liquid interface.

The liquid–vacuum interfaces of a series of ionic liquids (ILs) containing 1-alkyl-1-methylpyrrolidinium ([C n mpyrr]+) cations of different alkyl chain lengths have been studied by reactive-atom scattering with laser-induced fluorescence detection (RAS-LIF) and molecular dynamics (MD) simulations. A direct, quantitative comparison has been performed between [C n mpyrr]+ and the previously better-characterized 1-alkyl-3-methylimidazolium ([C n mim]+) ILs with the same chain lengths, n, and common anion, bis­(trifluoromethylsulfonyl)­imide ([Tf2N]−). Both RAS-LIF experiments, using O­(3P) as the projectile and monitoring OH yield, and MD simulations indicate that the coverage of the surface by alkyl chains is almost independent of the identity of the cation headgroup. Moreover, the potentially abstractable H atoms of the saturated pyrrolidinium ring do not contribute appreciably to the experimental OH yield. In both these senses, therefore, the headgroup is “hidden” from the probe particles approaching from vacuum. More predictably, the alkyl coverage depends strongly and nonstoichiometrically on the length of the alkyl chain, n, for either cation. These results imply the presence of an alkyl-rich layer on the surface formed by preferential orientation of the cations to expose their chains to the vacuum phase. We suggest that the lack of dependence of the packing density of this layer on cation type results from compensating effects of charge density and steric blocking.