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

1 Citation (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 Sep 2022

ASJC Scopus subject areas

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

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