Collision dynamics as a probe of gas-liquid and related interfaces

This thesis presents a body of work aimed at increasing the current understanding of the dynamics of collisions at the gas-liquid and related interfaces. In particular, collisions of open-shell radicals (oxygen atoms in the electronic ground state and hydroxyl radicals) with hydrocarbon surfaces have been investigated. Translationally hot radicals were scattered from liquid hydrocarbons and related surfaces. The products of inelastic scattering and hydrogen abstraction reactions (OH radicals) were detected, with translational and internal-state resolution, by laser-induced fluorescence (LIF). The radicals were used as a ‘chemical probe’ of the interface, providing unique information on the structure and reactivity of the surface, and how this relates to the bulk composition. Key results include the establishment of the relative reactivity towards oxygen atoms of liquid hydrocarbons and alkylthiol SAMs and the penetration depth of O atoms into the SAM surfaces. This was achieved through systematic studies involving custom-synthesised site-selectively deuterated SAMs. It was found that hydrogen abstraction can occur deeper within the monolayer than previously believed but still emerge as OH (or OD) without reacting further to form water. The reactivity of O(3P) atoms towards a technologically important family of ionic liquids ([Cnmim][Im]) was measured. It was found that the reactivity increased non-linearly as a function of alkyl chain length, to an extent which far exceeded any anticipated increase in reactivity based on stoichiometry alone. The interface in this case was found to differ greatly from the bulk composition, with preferential occupation by the alkyl units at the surface. The energy transfer and reactive uptake of OH radicals at a variety of hydrocarbon surfaces was investigated. There was significant transfer of the initial translational energy to all the surfaces as well as substantial translational-to-rotational energy conversion. This energy conversion was dependent on the functional groups present in the liquids as was the reactive uptake of OH. Reactive uptake coefficients (γ) were obtained for alkane, alkene and carboxylic acid-containing organic liquids. The results are discussed in terms of the relevance to the aging of organic particles in the troposphere

New insights into the genetic etiology of Alzheimer's disease and related dementias

Characterization of the genetic landscape of Alzheimer's disease (AD) and related dementias (ADD) provides a unique opportunity for a better understanding of the associated pathophysiological processes. We performed a two-stage genome-wide association study totaling 111,326 clinically diagnosed/'proxy' AD cases and 677,663 controls. We found 75 risk loci, of which 42 were new at the time of analysis. Pathway enrichment analyses confirmed the involvement of amyloid/tau pathways and highlighted microglia implication. Gene prioritization in the new loci identified 31 genes that were suggestive of new genetically associated processes, including the tumor necrosis factor alpha pathway through the linear ubiquitin chain assembly complex. We also built a new genetic risk score associated with the risk of future AD/dementia or progression from mild cognitive impairment to AD/dementia. The improvement in prediction led to a 1.6- to 1.9-fold increase in AD risk from the lowest to the highest decile, in addition to effects of age and the APOE ε4 allele

Reactive scattering as a chemically specific analytical probe of liquid surfaces

In this Perspective, we highlight some recent progress in the reactive scattering of "chemical probe" species such as atoms or small radicals from liquid surfaces. We emphasize in particular the evolution of this area from purely dynamical studies of the scattering mechanism. The mechanistic understanding that has now been gained is sufficiently mature to allow the same methods to be used as an effective analytical tool. The use of this approach to measure liquid-surface composition and structure is illustrated through the scattering of O(3P) atoms from a common, imidazolium-based family of ionic liquids. © 2010 American Chemical Society

Dynamics of the reaction of O(3P) atoms with alkylthiol self-assembled monolayers

We have studied the dynamics of the reactions of O(3P) atoms with alkylthiol self-assembled monolayers (SAMs). Superthermal O(3P) atoms, with a fairly broad distribution of laboratory-frame kinetic energies (mean = 16 kJ mol-1, fwhm = 26 kJ mol-1),were generated by 355 nm photolysis of NO2 introduced at a low pressure above the SAM surface. Nascent OH v' = 0 products were detected by laser-induced fluorescence. SAMs of two different alkyl chain lengths, C6 and C18, were studied. The existence of SAM layers, and their robustness under our experimental conditions during the relevant measurement period, were confirmed by scanning-tunneling microscopy (STM). Reaction at the SAM surface was verified as the authentic source of the hydroxyl radicals using a perdeuterated C6D13-SAM sample. The OH appearance profiles as a function of photolysis-probe delay, and the rotational-state distributions at their peaks, were compared with those of liquid squalane (C30H62, 2,6,10,15,19,23- hexamethyltetracosane). The reactivity of the SAMs and of squalane was found to be comparable. We conclude that the O(3P) atoms must be able to access the more reactive secondary hydrogen atoms along the alkyl chains of the SAMs. We find no perceptible differences in reactivity or product energy disposal between the two SAM chain lengths. Both produce a substantial fraction of the OH with relatively high velocities, which must result from direct, impulsive reaction. There is also a slower component, with velocities consistent with a thermal, ng-desorption mechanism. The proportion of this component appears to be lower for SAMs than for qualane. This would be compatible with the expected greater smoothness of the SAM surface at the molecular scale. We find little evidence for significant rotational excitation of the OH products, although the details of any correlation between translational and rotational energy release require further investigation. We compare our results with the limited available prior theoretical modeling of O(3P) + SAM systems. © 2009 American Chemical Society