Dynamics of collisions of OH radicals with organic liquid surfaces

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This proposal concerns the chemical reactions that take place at the boundary between a gas and a liquid.A lot is already known about what happens when molecules react in gases. Because the molecules are spaced relatively far apart, when they do collide each pair interacts effectively in isolation. Reactions of gases at the surfaces of solids are more complex because of the much larger number of atoms involved. However, this is simplified by the solid's rigidity, which normally prevents the gases from penetrating below the outer layer of atoms. Solid structures also tend to be regular, making it easier to treat them theoretically.Contrast this with reactions at the boundary between a gas and a liquid. Much less is known about what happens there. At an atomic scale, the surface is much looser and softer, and the boundary is much less sharp. Molecules attacking from the gas may be able to penetrate to different depths, with varying densities of surrounding molecules. Because there are no regular repeating units, a large number of atoms need to be treated theoretically.We will study a particular class of gas-liquid reactions using a new experimental method that we have developed. We will create OH radicals, one of the key species in combustion and atmospheric chemistry, and collide them with a range of organic liquids. The liquids will contain different functional groups, from saturated (alkanes) and unsaturated (alkenes) hydrocarbons, to oxidised (aldehydes, ketones, carboxylic acids) molecules. It is known that the mechanisms of OH reactions with these types of molecules in the gas phase differ fundamentally. For alkanes, the OH pulls an H atom directly from a single C-H unit. In contrast, OH adds to C=C double bonds in alkenes, forming energized intermediates that require a collision with another molecule to be stabilised. The reactions with oxidised molecules are distinct again, because of the special 'hydrogen-bonding' forces between OH and oxidised sites. We aim to discover what consequences these distinct mechanisms have on the reactivity of OH at different liquid surfaces. We will do this by detecting the escaping OH using laser-spectroscopy. This reveals not only how much OH has reacted (by difference from the scattering from an inert liquid), but also what form of internal (rotational and any vibrational) energy the escaping OH carries away. The information content will be enhanced by the important technical development of creating a well-directed 'molecular beam' of OH, revealing how fast and in what direction the scattered molecules are moving. Overall, this will give a particularly complete signature of the OH that escapes. The experimental results, complemented by computational 'molecular dynamics' modelling of the structure of the liquid surfaces, will allow us to address a number of intriguing questions. How much of the OH makes a direct encounter, with one, or at most a few 'bounces' at the outer layers, coming off in a well-defined direction? In contrast, how much becomes temporarily trapped, leaving in a random direction having given up most of its energy? How does the balance between these outcomes, and between either and chemical reaction, depend on how fast the OH is moving initially? Crucially, how do they vary between different liquids with distinct reaction mechanisms?The answers to these questions are currently unknown. This makes them fundamentally interesting. They are also practically important. One relevant example is reactions at the surfaces of microscopic aerosol particles in the atmosphere. Even trace levels of organic molecules tend to accumulate on the outer surfaces of aqueous droplets. Their oxidation, by OH and other species, is an important step in the processing of organic pollutants. It also has climatic consequences, e.g. by affecting the ability of the droplets to take up further water and act as cloud-condensation nuclei .