AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Surface-specific Chemical Reactions for Atmospheric Surfactants Observed Directly with Synchrotron-based XPS
NONNE PRISLE, Gunnar Öhrwall, Josephina Werner, Olle Björneholm, University of Helsinki, Helsinki, Finland
Abstract Number: 598 Working Group: Aerosol Chemistry
Abstract Atmospheric organic aerosols from a wide range of environments often comprise surface active compounds, i.e. molecules that concentrate in the surfaces of aqueous solutions. A major class of atmospheric surfactants is the fatty acids and their carboxylate anions. We have investigated aqueous solutions of such straight-chain carboxylic acid sodium salts with carbon numbers from 1-10, mixed with different inorganic salts commonly found in atmospheric aerosols. These solutions are considered representative model systems for aqueous aerosols found in the atmosphere, e.g. marine aerosols with concentrations relevant for CCN activation. Aqueous solution surfaces were probed directly using synchrotron-based X-ray photoelectron spectroscopy (XPS) on a liquid jet. XPS is a highly surface sensitive and chemically selective technique and thus ideal for probing the chemical environment of surfactants adsorbed at the air-water interface. XPS measurements were performed at the Swedish national synchrotron facility MAX IV Laboratory, Lund University.
We verify by direct observation that carboxylate anions are significantly partitioned to the aqueous surface, confirming significant surface activity as expected from their influence on bulk aqueous surface tension. Specifically in mixtures with weakly acidic ammonium ions, we find that surface-adsorbed carboxylates are protonated to a degree that is orders of magnitude greater than expected from solution bulk properties. We have investigated the variation of this enhanced protonation with surfactant concentration, organic-inorganic mixing ratio, and pH, in order to establish the underlying mechanism. The observed changes in chemical composition of the aqueous surfaces occur without corresponding changes in solution bulk properties and may influence chemical reactivity and physical properties of the surface. Surface specific composition and properties are particularly important for heterogeneous atmospheric processes occurring on the very large surfaces of sub-micron aerosols.