The Study of the Stepwise Hydroxyl Radical-Mediated Oxidation of Alkyl Surfactants at the Air-Water Interface
<p>This thesis presents work on the heterogeneous chemistry of surfactant monolayers adsorbed at the air-water interface with gas phase hydroxyl radicals (OH). A novel hydroxyl radical source utilizing a low-temperature plasma known as the dielectric barrier discharge source (DBDS) is designed...
| Summary: | <p>This thesis presents work on the heterogeneous chemistry of surfactant monolayers adsorbed at the air-water interface with gas phase hydroxyl radicals (OH). A novel hydroxyl radical source utilizing a low-temperature plasma known as the dielectric barrier discharge source (DBDS) is designed, characterized, and implemented as a high concentration OH generator (approximately 1000 fold higher than ambient conditions) in these experiments. Millimeter-sized water droplets containing a monolayer of surface-adsorbed species are exposed to this OH source until a designated level of oxidation. Field-induced droplet ionization mass spectrometry (FIDI-MS), which has been previously proven to be a surface-selective ionization technique, is used as an analytical method to determine the identity and relative quantity of oxidation products at the interface by sampling progeny droplets expelled upon the application of a pulsed electric field. Chapter 2 establishes the utility of the DBDS and FIDI-MS setup by oxidizing the 12-carbon n-alkyl surfactant dodecyltrimethylammonium (DTA<sup>+</sup>). Mechanistic details can be determined through the evolution of high mass oxidation products through a nearly complete conversion of the parent monolayer. Carbonyl, hydroxyl, peroxyl, and small amounts of fragmentation products can be discerned from the collision-induced dissociation mass spectra and through hydrogen-deuterium exchange experiments. Psuedo-first order kinetics can also be observed in this system, suggesting a Langmuir-Hinshelwood mechanism of OH-initiated oxidation. Chapter 3 extends the study of oxidation of hydrocarbon surfactants by looking at neighbor chain competition among a bis-quaternary ammonium gemini surfactants, a species that contains two long alkyl chains. Specifically, the experiment assesses the ability of two non-identical alkyl chains to compete with each other for the impinging gas phase hydroxyl radical. The increased reactivity of singly oxidized chains is evaluated by the oxidation of a gemini surfactant parent with identical alkyl chains. Chapter 4 investigates the oxidation of α,ω-surfactants, those that contain polar functional group separated by methylene spacers, discussing deviations of this class of surfactants with those that contain only one head group (i.e. DTA<sup>+</sup>). Chapter 5 demonstrates the DBDS and FIDI-MS system for use as a peptide footprinting technique for the amphiphilic species substance P. Localization of the site of oxidation can be done through FIDI-MS/MS analysis.</p> |
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