Influence of Molecular and Supramolecular Structure on the Gas−Liquid Interfacial Reactivity of Hydrocarbon Liquids with O(³P) Atoms

Abstract

We have investigated the interfacial reactivity of gas-phase O( 3 P) atoms with a representative range of longchain liquid hydrocarbons. These consisted of two branched molecules, squalane (C 30 H 62 , 2,6,10,15,19,23hexamethyltetracosane) and pristane (C 19 H 40 , 2,6,10,14-tetramethylpentadecane), and three linear ones, n-docosane (C 22 H 46 ), n-tetracosane (C 24 H 50 ) and n-octacosane (C 28 H 58 ). This represents the first systematic investigation of reactions of this type for molecules other than squalane. The O( 3 P) atoms were generated by 355-nm laser photolysis of a low pressure of NO 2 above the liquid surface. The nascent gas-phase OH radical products were detected by laser-induced fluorescence (LIF). Measurements for the linear hydrocarbons were constrained by their vapor pressures to single temperatures slightly (∼1 K) above their respective melting points. Pristane was studied at the lowest temperature practically achievable. Squalane was compared as a reference at the full set of temperatures. Appearance profiles for all of the liquids showed similar characteristic differences between OH V′)0 and 1. LIF excitation spectra were obtained for each of the vibrational levels at both the rising edge and peak of the appearance profiles. We conclude that the observed variations in rotational temperatures are consistent with dual contributions to the reaction mechanism for all the liquids, involving both direct escape and trapping-desorption components of the observed OH, as has previously been proposed for squalane. The relative yields of OH showed some surprising dependences on the liquid, including an unexpectedly strong variation with linear hydrocarbon chain length. These cannot all be explained by the relative reactivity of primary, secondary, and tertiary H-C units. We discuss the possibility that the known "surface freezing" phenomenon for linear hydrocarbons may play a role.

Dynamics of the Reaction of O(3P) Atoms with Alkylthiol Self-assembled Monolayers

“…Within this period, all the SAM samples gave reproducible LIF signals when all other parameters were carefully maintained constant (especially laser energies, gas pressure, and distance of the wheel/SAM specimen from the laser axis). This naturally makes the experiments we report here considerably more challenging than our previous work on liquids, [43][44][45][46][47][48][49]62 where much longer signal acquisition times were possible.…”

“…43,44,[46][47][48][49]62 A schematic of the revised apparatus is shown in Figure 1. It is a dual-chamber design, with the subsidiary, turbopumped chamber allowing SAM specimens to be stored rapidly under vacuum on removal from their solutions.…”

“…[43][44][45][46][47][48][49] We use laser-induced fluorescence (LIF) spectroscopy to detect the hydroxyl radicals formed at the liquid surface, yielding information on their translational and internal state distributions. This is a development of the earlier work of McCaffery, who was the first to use LIF spectroscopy to detect I 2 inelastically scattered from a range of liquid surfaces.…”

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

“…The LIF-based approach has been applied to squalane (2,6,10,15,19,23-hexamethyltetracosane), widely used as a standard in this field, and other long-chain hydrocarbons and to related self-assembled monolayers. [43][44][45][46][47][48][49][50] The molecular beam approach with mass spectrometric detection was also first applied to squalane and extended to other related surfaces. 9,51-54 5 RAS-LIF detects exclusively the OH products of hydrogen abstraction as they scatter into the gas phase.…”

Chemical Reactivity as a Probe of Ionic-Liquid Surfaces

“…In our experiments, we use a laser-photolysis based method to study this reaction, monitoring the relative OH product yields from a range of [RMIM] [Im] salts. This method has been pioneered successfully in related studies of the reactions of O( 3 P) atoms with hydrocarbon liquids 29,30,31,32,33,34 , and has proven to be a sensitive probe of the liquid surface structure.…”