The Oxidation Of Organic Material At Interfaces

This thesis provides an investigation into the oxidation of organic material at interfaces using neutron and x-ray reflectometry. The aim of this research is to investigate the kinetics and mechanisms of the oxidation reactions of surface active, organic molecules at the air/liquid or buried solid/liquid interfaces. Oxidation reactions have been researched in both a biological and atmospheric context. Determination of the bimolecular rate constant of the oxidation of a saturated phospholipid bilayer at a quartz/water interface has elucidated kinetic information around cloud nucleation. Hydroxyl radicals were formed as an oxidant in situ via illumination of hydrogen peroxide in solution. Loss of material has been modelled over time and the resulting rate constant is used to postulate an atmospheric lifetime of atmospheric organic material on aerosol. Also in an atmospheric context, neutron reflectometry has been used to observe lipids of variable saturation exposed to aqueous ozone at a buried quartz/water interface. Data analysis for the ozone initiated oxidation of saturated and unsaturated bilayers involved a novel method for determination of the amount of material present at the interface. Information around the specificity of ozone and comparison to previous work also allowed reasonable mechanistic ideas to be postulated. In a biological context, pulmonary surfactant has been oxidised at an air/water interface with hydroxyl radicals and studied using x-ray and neutron reflectometry. Oxidation of pulmonary surfactant by hydroxyl radicals has been analysed in a structural context to show how similar layers may be damaged in the body. Also in a biological context, mimics of gram negative biological outer membranes were oxidised at a buried sapphire/water interface and observed using neutron reflectometry, also by hydroxyl radicals produced via UV. The breakdown of a complex membrane system is modelled and the mechanism of the oxidation is discussed. Finally, beam damage to soft matter from x-rays has been quantified, additives were included in the solution subphase under a monolayer of phospholipids to attempt to suppress by potentially changing the mechanism of damage. Radiation dosage is investigated in comparison to changes exhibited in modelled monolayers to provide an indication of radiation damage as radiation dosage increases.