Scattering dynamics of hyperthermal oxygen atoms on ionic liquid surfaces: [emim][NTf₂] and [C12mim][NTf₂]

Collisions of hyperthermal oxygen atoms, with an average laboratory-frame translational energy of 520 kJ mol_-1, on continuously refreshed ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([emim][NTf₂]) and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C₁₂mim][NTf₂]), were studied with the use of a beam-surface scattering technique. Time-of-flight and angular distributions of inelastically scattered O and reactively scattered OH and H₂O were collected for various angles of incidence with the use of a rotatable mass spectrometer detector. For both O and OH, two distinct scattering processes were identified, which can be empirically categorized as thermal and nonthermal. Nonthermal scattering is more probable for both O and OH products. The observation of OH confirms that at least some reactive sites, presumably alkyl groups, must be exposed at the surface. The ionic liquid with the longer alkyl chain, [C₁₂mim][NTf₂], is substantially more reactive than the liquid with the shorter alkyl chain, [emim][NTf2], and proportionately much more so than would be predicted simply from stoichiometry based on the number of abstractable hydrogen atoms. Molecular dynamics models of these surfaces shed light on this change in reActivity. The scattering behavior of O is distinctly different from that of OH. However, no such differences between inelastic and reactive scattering dynamics have been seen in previous work on pure hydrocarbon liquids, in particular, the benchmark, partially branched hydrocarbon, squalane (C₃₀H₆₂). The comparison between inelastic and reactive scattering dynamics indicates that inelastic scattering from the ionic liquid surfaces takes place predominantly at nonreactive sites that are effectively stiffer than the reactive alkyl chains, with a higher proportion of collisions sampling such sites for [emim][NTf ₂] than for [C₁₂mim][NTf₂]. © 2010 American Chemical Society.