Cooking Residues Can Cling on to Indoor Pollutants

Synchrotron and neutron studies show cooking oil residues deposited on windows can harbour toxic chemicals

Aerosols contribute to ambient air pollution and, via processes such as cloud droplet formation, affect air quality and climate. Organic compounds often play a major role in aerosol composition, varying with time, location, season, and environment. Cooking food releases fatty acids into the air, and cooking emissions contribute around 10% to UK emissions of PM2.5 (aerosols up to two and a half microns wide). These aerosol particles can be deposited onto surfaces such as windows. A thin, layered film of material builds up over time, creating a persistent crust that is only slowly broken down by other chemicals in the atmosphere. In work recently selected as the best 2022 paper in Environmental Science: Atmospheres, a team led by researchers at the University of Birmingham created ultra-thin films, just a few tens of nanometres in thickness, approximating real-world pollution samples. Using X-ray and neutron techniques to study the nanoscale composition of the films and the changes in their surface structures as they aged, the researchers found that the self-organised layered structure can trap toxic pollutants and form a barrier that can prevent their breakdown. They also showed that the film’s surface becomes rougher and attracts more water from humidity, an effect with implications for the formation and lifetime of aerosols in the atmosphere.

It will be no surprise to anyone that cooking indoors releases particles that can make your kitchen windows dirty. Organic emissions from cooking include fatty acids, such as oleic acid, which are too stable to break down in the atmosphere. Oleic acid is also abundant in marine aerosol emissions, and the organic materials coating aerosol particles affect urban air quality. Indoors, atmospheric aerosols are deposited on surfaces, such as window glass, forming films.

In laboratory experiments, oleic acid is often used as a proxy, allowing researchers to study reactive organic aerosol systems. As a fatty acid surfactant, oleic acid can decrease the surface tension of a liquid, affecting an aerosol particle’s ability to take up water and form a cloud droplet. Like soap, oleic molecules have both water-attracting and water-repelling sections, allowing them to form lyotropic liquid crystals (LLCs). At a relative humidity of around 50%, they can also form layered stacks. These distinct molecular arrangements have different viscosity and diffusivity, both factors affecting the ageing of atmospheric particles.

In this study, the research team created ultra-thin oleic acid-sodium oleate films on solid silicon substrates and subjected them to oxidation and humidity changes that simulate atmospheric ageing. They used a combination of optical microscopy, neutron reflectometry (NR) (at ISIS and ILL) and grazing-incidence small-angle X-ray scattering (GI-SAXS) on Diamond’s I22 beamline to study changes in the surface structure of the films as they aged.

NR can probe the structure of interfaces, providing depth-resolved structural information and data on the thickness, roughness and density of each interfacial layer. GI-SAXS is a closely-related surface technique used to probe the surface morphology of materials. 

Commonly used to determine the self-assembly and self-organisation of thin films at the nanoscale, GISAXS can measure X-ray scattering over a larger angular range and off-specular scattering (in the y-direction). In this work, it allowed the research team to determine the orientation of the layered stacks.

Dr Nick Terrill is the Principal Beamline Scientist for I22. He says: 

This was a challenging experiment to set up, the first of its kind on I22, and involved a close collaboration to develop the sample environment, a humidity chamber that allowed us to do these experiments in situ. Most of the work we do on I22 looks at bulk samples, so it was an exciting development for us to look at the surface of these materials and use I22 to collect high-quality GI-SAXS data.

Read more on Diamond website

Image: Graphical abstract from the paper

Engineered wall paint could kill corona viruses

Investigation of aerosols on titanium dioxide shows promising routes to surface and air disinfection

Common wall paint could potentially be modified to kill the Corona virus and many other pathogens. This is an important finding of a study from a research team including DESY scientists on the virus-killing effect of titanium dioxide (TiO2), a ubiquitous white pigment that is found in paints, plastic products and sunscreens. TiO2 also has many other important applications relevant to environmental sustainability and renewable energy. The international team led by Heshmat Noei from the DESY NanoLab reports its results in the journal Applied Materials & Interfaces published by the American Chemical Society (ACS).

“Titanium dioxide is widely used as a pigment to whiten a wide range of products,” explains Noei. “But it is also a powerful catalyst in many applications such as air and water purification and self-cleaning materials. Therefore, we saw it as a promising candidate for a virus inactivating coating.” Teaming up with the group of virologist Ulrike Protzer and Greg Ebert from the research centre Helmholtz Munich and the Technische Universität München, the scientists tested titanium dioxide´s power against the corona virus. “We were the first to apply corona viruses on a titanium dioxide surface and investigate what happens,” says Noei.

Hard X-ray photoelectron spectroscopy at the PETRA III beamline P22 at DESY provides the necessary high chemical and elemental sensitivity to resolve subtle chemical changes. The research team investigated the contact process on the surface and was able to clarify that the amino acids of the corona virus spike protein attach to the titanium dioxide surface, trapping the virus and preventing it from binding to human cells. “We found that the virus adsorbs to the titanium dioxide surface and cannot detach again and will eventually be inactivated by dehydration and be denatured,” explains the paper´s main author Mona Kohantorabi from the DESY NanoLab. “Moreover, the titanium dioxide catalyses the inactivation of the virus by light. For our study we used ultraviolet light, which triggered the inactivation of the virus within 30 minutes, but we believe the catalyst can be further optimised to accelerate the inactivation and, more importantly, work under standard indoor lighting. We believe it could then be used as an antiviral coating for walls, windows and other surfaces for instance in hospitals, schools, airports, elderly homes and kindergardens.”

Read more on the DESY website

Image: An image taken with an atomic force microscope from the investigation: The SARS-CoV-2 particles (light) adsorb on the titanium dioxide surface. There, structural proteins are inactivated by denaturation and oxidation by light irradiation.

Credit: DESY Nanolab, Mona Kohantorabi