The liquid-vacuum interfaces of a series of room-temperature ionic liquids (RTILs) containing the 1-alkyl-3-methylimidazolium cation ([C n mim]) were investigated by reactive-atom scattering (RAS). The length of the alkyl chain (n = 4, 6, 8, and 12) and the anion type (bis(trifluoromethylsulfonyl)imide ([Tf 2 N]), trifluoromethanesulfonate ([OTf]), and tetrafluoroborate ([BF 4 ])) were varied systematically to determine their effects on the preferential occupancy of the surface by alkyl chains. The experiments employed collisions with gas-phase, ground-state oxygen atoms, O( 3 P), generating OH and H 2 O products that revealed the abundance of abstractable H atoms at the liquid surface. Two complementary approaches with different Oatom energies and detection methods were employed: we denote these RAS-laser induced fluorescence (RAS-LIF) and RAS-mass spectrometry (RAS-MS). [C n mim][BF 4 ] RTILs were studied by both methods, giving consistent trends of strongly increasing alkyl coverage with chain length. Even for the longest alkyl chain, n = 12, the surface is not saturated with alkyl chains, with some fraction still occupied by other groups. RAS-LIF results for RTILs with the three different anions, over the range of alkyl chain lengths, showed that their surfaces can be distinguished clearly. Alkyl surface coverage depends sensitively on the anionic volume, indicating that the packing of ions at the surface is driven largely by steric effects. Molecular dynamics simulations of the liquid surfaces support all the experimental findings, including the rationalization of expected quantitative differences between the RAS-LIF and RAS-MS results.

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].

Interactions of ground-state atomic and molecular oxygen, O( 3 P) and O 2 ( 3 Σ g −), with a highly oriented pyrolytic graphite surface were investigated for a broad range of surface temperatures from 1100 K to approximately 2300 K. A molecular beam composed of 89% O atoms and 11% O 2 , with average translational energies of 472.1 and 944.4 kJ mol −1 , respectively, was directed at the surface with an incidence angle, θ i , of 45°. Angle-and velocity-resolved distributions were collected for nonreactively and reactively scattered products with the use of a rotatable mass spectrometer detector. Four scattered products were observed: O, O 2 , CO, and CO 2 . O atoms that exited the surface without reacting exhibited both impulsive scattering (IS) and thermal desorption (TD) components. The primary reaction product observed was carbon monoxide (CO). Carbon dioxide (CO 2 ) was measured only with surface temperatures below 1400 K, and O 2 was attributed to IS of O 2 that was present in the incident beam. Although there is evidence for either Eley−Rideal or hot atom reactions, CO and CO 2 were primarily formed by Langmuir−Hinshelwood (LH) reactions. However, the flux angular distributions of the LH products were significantly narrower than a cosine distribution, and the final energies were much higher than those predicted by the Maxwell−Boltzmann distribution characterized by the surface temperature. These observations indicate that CO and CO 2 that were produced by LH reactions desorb from the surface over a barrier. The desorption barrier of CO was determined by using the principle of detailed balance (where the desorption and adsorption barriers are equal) and was found to increase from 121 ± 5 kJ mol −1 at 1100 K to 155 ± 7 kJ mol −1 at 1300 K. As the surface temperature increased, the fluxes of CO and CO 2 produced by LH mechanisms decreased. Simultaneously, the flux of O atoms that scattered via the TD channel increased, which reduced the surface oxygen coverage at higher temperatures. The combination of reduced O-atom surface coverage and increased desorption barriers for CO suppresses the reactivity of the surface at high temperatures.

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.