Interrogating liquid surfaces via reactive atom scattering

Abstract

An investigation of the structure of various liquid surfaces has been conducted using Reactive Atom Scattering coupled to laser-induced fluorescence spectroscopy (RAS-LIF). In this technique, reactive collisions of gas-phase O(3P) atoms with hydrocarbon-based liquid surfaces produce OH radicals which are probed by laserinduced fluorescence in the gas phase. The relative fraction of the surface occupied by groups containing abstractable hydrogen atoms is quantified by the yield of OH. Therefore, RAS-LIF can be used to study the surface composition of liquids and mixtures containing alkyl groups. In addition, the technique yields information on the fundamental dynamics of gas-liquid interactions. The dynamics of the interfacial H-abstraction reaction between O(3P) and the partially unsaturated hydrocarbon squalene (C30H50) has been studied for the first time. The yield and internal-state distribution of OH produced by this reaction have been characterised by RAS-LIF. In comparison with the fully saturated analogue, squalane (C30H62), the results show that the O(3P) atoms are extremely sensitive to the type of C– H bond where reaction takes place, and the products partially accommodate their energy at the liquid surface. There is an additional channel for O(3P) loss on squalene, inferred to be addition to exposed unsaturated sites at the liquid surface. RAS-LIF has been employed to characterise the surfaces of ionic liquids containing alkyl chains. The O(3P) atoms react selectively with CH2 groups, acting as a quantitative surface-specific probe of the alkyl chains at the surface. The results show that the interfacial composition is different from that of the bulk, with alkyl chains being preferentially exposed at the surface. A series of ionic liquids and mixtures containing 1-alkyl-3-methylimidazolium and 1-alkyl-1-methylpyrrolidinium cations has been investigated. It has been found that alkyl surface coverage depends strongly on the length of the alkyl chain and the chemical identity of the anion, but not on the cation headgroup (imidazolium or pyrrolidinium). Measurements on ionic liquid mixtures show that their surfaces are enriched in cations and anions that are relatively nonpolar, and depleted in polar ions.