A team of researchers from the Universities of Birmingham, Bath, Liverpool, and Imperial College London are continuing their research into cooking emissions at the ISIS Neutron and Muon Source to determine how a droplet’s molecular arrangement could provide valuable insight into the development of potential models to advise future government policies on pollutants in the atmosphere and air quality.
Using our INTER beamline, the team members from Birmingham and Bath previously found that dirty windows can harbour harmful pollutants under protective films of fatty acids from cooking emissions and that these can persist for a long time. The fatty acids emitted during cooking are highly stable and not easily broken down in the atmosphere when these molecules are self-organised. That means that, when they hit a solid surface such as a window, the fatty acids form a self-organised thin film that builds up over time and will only be very slowly broken down by other chemicals in the atmosphere. he team studied laboratory 'proxies', engineered in the lab to act as a model of 'real world' samples. These were spun into super-thin films of pollution, just a few tens of nanometres in thickness. They found that the self-organised arrangement within the films in repeating molecular sheets, known as a lamellar phase, made it difficult for smaller molecules, such as ozone, to access the reactive parts of the fatty acids within these structures. Once deposited and exposed to ozone, the surfaces of the films became less smooth and increasingly likely to take up water, an effect which also has implications for the formation and lifetime of aerosols in the atmosphere.
Now, joined by colleagues from Liverpool and Imperial, the team are working on levitating proxies of cooking emissions from oleic acid (olive oil) and ageing them to react with gases we might find in the atmosphere. Within the atmosphere are aerosol particles, which are the reason cloud droplets form. Water condenses around the aerosol particles which then form into clouds. We also breathe in these droplets, and they may affect our health. The advantage of using levitation is that you can probe an individual droplet in the environment it exists in normally (as atmospheric particles are not usually squashed on a microscope slide).
Dr Adam Milsom from University of Birmingham explains, “We want to look at how the molecules arrange themselves in these particles, using neutron scattering to see how the molecules change their arrangement with oxidation. So, we can witness the chemical reaction, and what happens to their ability to take on water before and after this ageing process happens. This would then have implications for the atmosphere. Once oxidised, these droplets become larger.”
The levitation work began with Professor Christian Pfrang as an RCUK Academic Fellow at University of Reading initially funded by the Royal Society. Dr Adam Milsomthen joined him as a PhD student, and now a research fellow. They used sound to levitate the droplets, using the same force that you feel when you hear loud music, to keep that droplet up.
Six years on, this research continues and again took the team to Diamond Light Source where X-rays instead of neutrons. he technique there complements their work at ISIS, where they gain valuable additional information via the use of deuterated materials. They are also using the lasers at Central Laser Facility to examine the of oleic acid and mixtures of related compounds of atmospheric interest.
viscous. Small molecules such as water and reactive gases take much longer to dissolve through a viscous, semi-solid, droplet compared to a well-mixed liquid droplet. he team are now also examining the optical texture of these particles as a complementary method of examining aerosol processes. The way droplets are broken down in the atmosphere is via reactive gases that can infiltrate droplets. It is that can slow down this process, which was one of the original motivations for starting this research.
Adam further explains “We can change the humidity in a chamber to a certain level and can then compare how much water they take up once it is oxidised, to when it has not been aged. During our work on windows, we found that the films did take up more water when we aged them but here with the help of neutrons, we can specifically select certain sorts of molecules and figure out how fast things happen within the droplet. If you know how much a droplet will take up water at a certain humidity, you can then translate that into models for the atmosphere, and ultimately link up to advice for policies for governments and industries.”
The full paper can be found at: Molecular Self-Organization in Surfactant Atmospheric Aerosol Proxies | Accounts of Chemical Research (acs.org)