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].

OH/OD product state distributions arising from the reaction of gas-phase O(3P) atoms at the surface of the liquid hydrocarbon squalane C30H62/C30D62 have been measured. The O(3P) atoms were generated by 355 nm laser photolysis of NO2 at a low pressure above the continually refreshed liquid. It has been shown unambiguously that the hydroxyl radicals detected by laser-induced fluorescence originate from the squalane surface. The gas-phase OH/OD rotational populations are found to be partially sensitive to the liquid temperature, but do not adapt to it completely. In addition, rotational temperatures for OH/OD(v'=1) are consistently colder (by 34+/-5 K) than those for OH/OD(v'=0). This is reminiscent of, but less pronounced than, a similar effect in the well-studied homogeneous gas-phase reaction of O(3P) with smaller hydrocarbons. We conclude that the rotational distributions are composed of two different components. One originates from a direct abstraction mechanism with product characteristics similar to those in the gas phase. The other is a trapping-desorption process yielding a thermal, Boltzmann-like distribution close to the surface temperature. This conclusion is consistent with that reached previously from independent measurements of OH product velocity distributions in complementary molecular-beam scattering experiments. It is further supported by the temporal profiles of OH/OD laser-induced fluorescence signals as a function of distance from the surface observed in the current experiments. The vibrational branching ratios for (v'=1)/(v'=0) for OH and OD have been found to be (0.07+/-0.02) and (0.30+/-0.10), respectively. The detection of vibrationally excited hydroxyl radicals suggests that secondary and/or tertiary hydrogen atoms may be accessible to the attacking oxygen atoms.

The inelastic scattering of gas-phase OH radicals from a liquid hydrocarbon and a liquid perfluorinated polyether (PFPE) has been investigated. The surfaces examined were the potentially reactive, branched hydrocarbon squalane (C 30 H 62 , 2,6,10,15,19,23-hexamethyltetracosane) and the inert PFPE Krytox 1506 (F-[CF(CF 3 )-CF 2 O] 14ave -CF 2 CF 3 ). Superthermal OH was formed by 355-nm laser photolysis of a low pressure of HONO above the liquid surface. Laser-induced fluorescence (LIF) was used to determine the relative yields and nascent translational and rotational distributions of OH (V′ ) 0). The time-of-flight profiles from both liquids can be resolved, at least empirically, into two components. The dominant, faster component is consistent with direct, inelastic scattering. It has a higher average translational energy from PFPE than from squalane. This faster OH also has a higher Boltzmann-like rotational temperature for PFPE (655 ( 45 K) than for squalane (473 ( 27 K), in both cases considerably hotter than the incoming OH. For both liquids, there is also a slower component, with characteristics consistent with a thermalized, trapping-desorption mechanism. This is a higher proportion for squalane (0.22 ( 0.02) than for PFPE (0.09 ( 0.01). These results are consistent with squalane being the "softer" surface, exhibiting more efficient momentum transfer than PFPE, and more able to temporarily trap OH. Relative to PFPE, around half (0.49 ( 0.04) of the OH molecules that collide with squalane are lost, presumably due to reaction forming H 2 O. These results are compared with previous studies of the scattering of inert gas species from both squalane and PFPE. The reactive branching fraction of OH on squalane is discussed in the context of previous observations of enhanced reactivity at the gas-liquid interface.

The reactivity of photolytically generated, gas-phase, ground-state atomic oxygen, O((3)P), with the surfaces of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([NTf(2)]) ionic liquids has been investigated. The liquids differ only in the length of the linear C(n)H(2n+1) alkyl side chain on the cation, with n = 2, 4, 5, 8, and 12. Laser-induced fluorescence was used to detect gas-phase OH v' = 0 radicals formed at the gas-liquid interface. The reactivity of the ionic liquids increases nonlinearly with n, in a way that cannot simply be explained by stoichiometry. We infer that the alkyl chains must be preferentially exposed at the interface to a degree that is dependent on chain length. A relatively sharp onset of surface segregation is apparent in the region of n = 4. The surface specificity of the method is confirmed through the nonthermal characteristics of both the translational and rotational distributions of the OH v' = 0. These reveal that the dynamics are dominated by a direct, impulsive scattering mechanism at the outer layers of the liquid. The OH v' = 0 yield is effectively independent of the bulk temperature of the longest-chain ionic liquid in the range 298-343 K, also consistent with a predominantly direct mechanism. These product attributes are broadly similar to those of the benchmark pure hydrocarbon liquid, squalane, but a more detailed analysis suggests that the interface may be microscopically smoother for the ionic liquids.