Reactive Scattering Dynamics at the Gas-Liquid Interface: Bridging the Gap between the Gas-Phase and Solution

Anywhere the gas and liquid phases meet, chemistry occurs at the interface. Examples in the natural world include: respiration in living organisms; atmospheric aerosol particles; the surface of the sea on Earth; hydrocarbon particles in the atmosphere of Saturn's moon Titan. The chemistry of these interfaces is also vital in man-made environments as well: combustion of liquid fuels; industrial processes such as multiphase catalysis, gas sequestration and distillation. However, despite their importance, in comparison to the chemistry of reactions in the gas-phase or in solution, reactions at the gas-liquid interface are much less well understood. This project aims to deepen our fundamental understanding of reactions at liquid surfaces through a combination of cutting-edge experiment and theory.

Consider a gas molecule approaching a liquid surface. The first encounter it makes with the surface will be an isolated event; the gas phase molecule will collide with a single molecule of the liquid surface. At this point the encounter is essentially the same as a gas phase collision between two isolated molecules. In the gas-phase, the molecules will then recoil and the encounter will be over. In some cases, collisions at the liquid surface will also result in the gas-phase molecule rebounding back into the gas-phase. However, it may instead go on to collide with further liquid surface molecules, and may even pass through the surface of the liquid and into solution, before eventually returning to the gas-phase. Reactions at the gas-liquid interface thus share characteristics of both the gas and solution phases, and by studying the dynamics of the reactions we can bridge the gap between them. This complements the intensive on-going effort in these hitherto largely separate areas, providing a unifying picture of molecular scattering dynamics.

We will develop a new apparatus for our experiments, based on our previous experience in gas-liquid interfacial scattering, and combine it with high-resolution laser spectroscopy previously applied to study gas-phase dynamics. We will use this to study the reaction of CN radicals with liquid hydrocarbons, which forms HCN. The dynamics of this benchmark reaction process have been previously studied in the gas and solution phases. This reaction is not only of fundamental interest, as the CN radical is an important reactive species in extra-terrestrial atmospheres (e.g. atmosphere of Titan), and liquid hydrocarbon combustion. Simultaneously with the experiments, we will develop new theoretical models of the forces between the atoms present, and use those in calculations to simulate the dynamics of the reactions under experimental conditions. We will compare and combine the results of the experiments and theory to provide the most-detailed ever description of gas-liquid interfacial reaction dynamics. The fundamental insights into dynamics at the gas-liquid interface provided by this work will inform our understanding and modelling of the processes at gas-liquid interfaces in a wide range of environments vital to our society, e.g. atmospheric aerosols, liquid fuel combustion.

The proposed work is fundamental in nature and therefore it is expected that the societal and economic benefits deriving from the results will primarily be realised in the longer term. These benefits will follow from the improved understanding and capacity to model interactions at the gas-liquid interface that will be derived from this work. Such interactions have, in general, very broad applicability across many diverse fields, including, e.g. gas separation and multiphase catalysis; heterogeneous chemistry on atmospheric aerosol particles and at the surface of the sea on Earth, and at different gas-liquid interfaces in extra-terrestrial atmospheres (where CN, the specific target of this work, is itself a known species in some cases, such as the atmosphere of Titan); combustion of liquid fuels (where CN reactions are also known to be involved in the combustion of hydrocarbons); through to biological respiration. A refined understanding of the fundamental processes of gas uptake, energy exchange and reactivity will lead to improved numerical models of these environments. The improved predicative capability that this would enable has potential societal benefit through, e.g. informing policy on emissions control to mitigate the effects of environmental pollutants.

The technical development here of a high-resolution, tuneable IR source coupled with frequency-modulated detection has potential applications in other fields. The more obvious candidates include detection of trace-species concentrations and temperatures in gas-phase environments such as the atmosphere or in combustion.

The computational tools developed here will be exploited more widely through being made available in established molecular simulation packages (e.g., CHARMM and TINKER). These packages are used by the wider dynamics community to study a range of chemical systems beyond the G-L interface. This includes the investigation of molecules of biological interest, including peptides, proteins, small molecule ligands, nucleic acids, lipids, and carbohydrates, in a variety of heterogeneous environments, including solution, crystal, and membrane environments.

There will, in addition, be more immediate societal impact from the output of highly trained personnel with either technical skills in the use of modern laser, vacuum, electronic-data capture and data processing technologies, and at the forefront of high-performance computing methods. They will also have well-developed generic and transferable communication, presentation and problem-solving skills. They will be ideally suited to contribute to the growth or creation of high-technology companies, enhancing innovative capacity and consequently increasing business revenues.

We will also continue to enhance public awareness of science by disseminating the aims and results of this work to the wider public, at a suitable level, through available media including lectures, on-line video presentations and websites. The high-performance computational algorithms will be folded in the 'danceroom Spectroscopy' (dS) framework, an interactive molecular dynamics simulation platform at the frontiers of high-performance computing, real-time 3d imaging, and chemical dynamics. dS has been developed by PI2 over the last few years. It has been the subject of innumerable national and international press stories, and toured around major cultural venues across the UK, Europe, and the US. To date, dS has enabled over 100,000 worldwide to literally step into a real-time molecular dynamics simulation.