The evolution of surface structure during simulated atmospheric ageing of nano-scale coatings of an organic surfactant aerosol proxy

Adam Milsom, Adam M. Squires, Maximilian W.A. Skoda, Philipp Gutfreund, Eleonore Mason, Nicholas J. Terrill, Christian Pfrang

Research output: Contribution to journalArticlepeer-review

4 Citations (SciVal)

Abstract

Atmospheric aerosol particles can be coated with organic materials, impacting aerosol atmospheric lifetime and urban air quality. Coatings of organic materials are also found on indoor surfaces such as window glass. Oleic acid is a fatty acid surfactant that is abundant in cooking and marine aerosol emissions. Under ambient conditions it can self-assemble into lamellar bilayers (stacks) with its sodium salt. We found that nano-scale oleic acid-sodium oleate films spin-coated onto solid silicon substrates form a mixed-phase area of lamellar stacks and amorphous films. The coatings were subjected to simulated atmospheric ageing (ozonolysis and humidity changes) while the surface structure was followed by neutron reflectometry. We found that the orientation of lamellar stacks, which is known to affect the diffusivity of small molecules through them, was sensitive to humidity both in oxidised and pristine films. Lamellar bilayer stacks in oxidised films acquired ∼11-fold more water under humid conditions (>80% relative humidity) compared to the unoxidised film, demonstrating a significant increase in film hygroscopicity after oxidation. Lamellar stacks, consisting only of starting materials, persisted at the end of simulated atmospheric ageing. These findings for atmospherically relevant nano-scale films corroborate previous work on micrometre-scale layers, thus demonstrating that fatty acid self-assembly could significantly increase the atmospheric lifetime of these molecules. The persistence of such semi-solid surfactant arrangements in the atmosphere has implications for the climate as well as urban and indoor air pollution.

Original languageEnglish
Pages (from-to)964-977
Number of pages14
JournalEnvironmental Science: Atmospheres
Volume2
Issue number5
Early online date25 May 2022
DOIs
Publication statusPublished - 1 Sept 2022

Bibliographical note

Funding Information:
AM acknowledges funding by NERC through the SCENARIO DTP (NE/L002566/1) and the research grant (NE/T00732X/1) for his postdoctoral fellowship; AM is also grateful for support from the NERC CENTA DTP. Jacob Boswell (University of Bath) is acknowledged for helping out at the ISIS beamtime and Ben Woden is acknowledged for helping to calibrate the ozonisers. Andrew Nelson (Australian Nuclear Science and Technology Organisation) is acknowledged for advice on the use of refnx for model fitting. We acknowledge the Research England funded TALENT: Technician Led Equipment Fund for enabling the offline GI-SAXS measurements. Steven Huband (University of Warwick) is acknowledged for carrying out these offline GI-SAXS measurements. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beamtime allocation from the Science and Technology Facilities Council. This work was carried out with the support of the Diamond Light Source (DLS), instrument I22 (proposal NT23096). We are grateful to the Institut Laue-Langevin (ILL) for awarding us beamtime on the FIGARO reflectometry instrument. The computations described in this paper were performed using the University of Birmingham's BlueBEAR HPC service, which provides a High Performance Computing service to the University's research community.

ASJC Scopus subject areas

  • Analytical Chemistry
  • Chemistry (miscellaneous)
  • Environmental Chemistry
  • Pollution

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