Accessibility navigation


Night-time oxidation of surfactants at the air–water interface: effects of chain length, head group and saturation

Sebastiani, F., Campbell, R. A., Rastogi, K. and Pfrang, C. (2018) Night-time oxidation of surfactants at the air–water interface: effects of chain length, head group and saturation. Atmospheric Chemistry and Physics, 18 (5). pp. 3249-3268. ISSN 1680-7316

[img]
Preview
Text (Open Access) - Published Version
· Available under License Creative Commons Attribution.
· Please see our End User Agreement before downloading.

5MB
[img]
Preview
Text (Open Access) - Accepted Version
· Available under License Creative Commons Attribution.
· Please see our End User Agreement before downloading.

11MB

To link to this item DOI: 10.5194/acp-18-3249-2018

Abstract/Summary

Reactions of the key atmospheric night-time oxidant NO3 with organic monolayers at the air–water interface are used as proxies for the ageing of organic-coated aqueous aerosols. The surfactant molecules chosen for this study are oleic acid (OA), palmitoleic acid (POA), methyl oleate (MO) and stearic acid (SA) to investigate the effects of chain length, head group and degree of unsaturation on the reaction kinetics and products formed. Fully and partially deuterated surfactants were studied using neutron reflectometry (NR) to determine the reaction kinetics of organic monolayers with NO3 at the air–water interface for the first time. Kinetic modelling allowed us to determine the rate coefficients for the oxidation of OA, POA and MO monolayers to be (2.8 ± 0.7) × 10−8 cm2 molecule−1 s−1, (2.4 ± 0.5) × 10−8 cm2 molecule−1 s−1 and (3.3 ± 0.6) × 10−8 cm2 molecule−1 s−1, respectively. The corresponding uptake coefficients were found to be (2.1 ± 0.5) × 10−3, (1.7 ± 0.3) × 10−3 and (2.1 ± 0.4) × 10−3. For the much slower NO3-initiated oxidation of the saturated surfactant SA we found a loss rate of (5 ± 1) × 10−12 cm2 molecule−1 s−1 which we consider to be an upper limit for the reactive loss, and estimated an uptake coefficient of (5 ± 1) × 10−7. Our investigations demonstrate that NO3 will contribute substantially to the processing of unsaturated surfactants at the air–water interface during night-time given its reactivity is ca. two orders of magnitude higher than that of O3. Furthermore, the relative contributions of NO3 and O3 to the oxidative losses vary massively between species that are closely related in structure: NO3 reacts ca. 400 times faster than O3 with the common model surfactant oleic acid, but only ca. 60 times faster with its methyl ester MO. It is therefore necessary to perform a case-by-case assessment of the relative contributions of the different degradation routes for any specific surfactant. The overall impact of NO3 on the fate of saturated surfactants is slightly less clear given the lack of prior kinetic data for comparison, but NO3 is likely to contribute significantly to the loss of saturated species and dominate their loss during night-time. The retention of the organic character at the air–water interface differs fundamentally between the different surfactant species: the fatty acids studied (OA and POA) form products with a yield of ∼ 20% that are stable at the interface while NO3-initiated oxidation of the methyl ester MO rapidly and effectively removes the organic character (≤ 3% surface-active products). The film-forming potential of reaction products in real aerosol is thus likely to depend on the relative proportions of saturated and unsaturated surfactants as well as the head group properties. Atmospheric lifetimes of unsaturated species are much longer than those determined with respect to their reactions at the air–water interface, so that they must be protected from oxidative attack e.g. by incorporation into a complex aerosol matrix or in mixed surface films with yet unexplored kinetic behaviour.

Item Type:Article
Refereed:Yes
Divisions:Interdisciplinary centres and themes > Chemical Analysis Facility (CAF)
Interdisciplinary centres and themes > Walker Institute
Faculty of Life Sciences > School of Chemistry, Food and Pharmacy > Department of Chemistry
ID Code:72068
Publisher:Copernicus Publications

Downloads

Downloads per month over past year

University Staff: Request a correction | Centaur Editors: Update this record

Page navigation