The ionic-liquid(IL)/gas interface plays a crucial role in applications such as gas capture and multiphase catalysis. Understanding how the ionic-liquid chemical composition affects the surface structure would facilitate the efficient design of ILs for particular applications. We report a systematic study of surface composition of ionic liquids using a novel, spectroscopically based experimental technique illustrated schematically in the figure. Gas-phase superthermal O(3P) atoms are generated by photolysis. They react selectively with alkyl chains in the interface, acting as an extremely surface-sensitive chemical probe for the presence of abstractable H atoms. The OH radicals produced in the reaction are detected by laser-induced fluorescence (LIF) in the gas phase, allowing us to quantify differences in surface coverage by alkyl chains. This approach has been employed to study the interfacial reactivity of a widely studied and exploited family of ILs containing the 1-alkyl-methylimidazolium cation with different chain lengths and with a series of representative anions. Our results show that the alkyl chains on the cation occupy preferentially the outer surface layer, resulting in a composition that is substantially different from that of the bulk. This preference is strongly dependent on alkyl chain length. The alkyl surface enrichment has also been found to depend on the counteranion in a way that can be predicted from its steric bulk. Figure 1. <img src='https://acs-archive-assets2.s3.amazonaws.com/241nm/uploads/71862_sf-acs-pic-2.jpg' alt='' />

Probing the surface of ionic liquids through collisions with gas-phase atoms

248th National Meeting (2014)

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