Ionic Liquid–Vacuum Interfaces Probed by Reactive Atom Scattering: Influence of Alkyl Chain Length and Anion Volume

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)­...

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Published in:Journal of physical chemistry. C 2015-03, Vol.119 (10), p.5491-5505
Main Authors: Tesa-Serrate, Maria A, Marshall, Brooks C, Smoll, Eric J, Purcell, Simon M, Costen, Matthew L, Slattery, John M, Minton, Timothy K, McKendrick, Kenneth G
Format: Article
Language:English
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Summary: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 ([Tf2N]), trifluoromethanesulfonate ([OTf]), and tetrafluoroborate ([BF4])) 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 H2O products that revealed the abundance of abstractable H atoms at the liquid surface. Two complementary approaches with different O atom energies and detection methods were employed: we denote these RAS-laser-induced fluorescence (RAS-LIF) and RAS-mass spectrometry (RAS-MS). [C n mim]­[BF4] 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.
ISSN:1932-7447
1932-7455
DOI:10.1021/jp5126238