Abstract
The formation of multicomponent aerosol particles from precursor solution droplets often involves segregation and surface enrichment of the different solutes, resulting in non-homogeneous particle structures and diverse morphologies. In particular, these effects can have a significant influence on the chemical composition of the particle–vapor interface. In this work, we investigate the bulk/surface partitioning of inorganic ions, Na+, Mg2 +, Ca2 +, Cl− and Br−, in atomiser-generated submicron aerosols using synchrotron radiation based X-ray photoelectron spectroscopy (XPS). Specifically, the chemical compositions of the outermost few nm thick surface layers of non-supported MgCl2/CaCl2 and NaBr/MgBr2 particles are determined. It is found that in MgCl2/CaCl2 particles, the relative abundance of the two species in the particle surface correlates well with their mixing ratio in the parent aqueous solution. In stark contrast, extreme surface enrichment of Mg2 + is observed in NaBr/MgBr2 particles formed from both aqueous and organic solution droplets, indicative of core–shell structures. Structural properties and hydration state of the particles are discussed.
Highlights
The internal structure of aerosol particles formed from precursor solution droplets, e.g. in industrial spray-drying processes[1] or natural sea spray,[2] is often non-uniform
For the X-ray photoelectron spectroscopy (XPS) analysis, the particles were introduced into a vacuum chamber using the ‘‘Multi-Purpose Source Chamber’’ (MPSC) of the PLEIADES beamline, in which a focused particle beam was produced by an aerodynamic lens and a beam skimmer.[12]
We note that in an earlier study carried out using same MPSC set-up, NaCl/NaBr particles were concluded to be devoid of water,[4] here we find that the NaBr particles are not completely dry as seen in spectrum #36
Summary
The internal structure of aerosol particles formed from precursor solution droplets, e.g. in industrial spray-drying processes[1] or natural sea spray,[2] is often non-uniform. As the solvent is gradually evaporated and concentration increases in the finite-sized droplet environment, the different chemical species can segregate into sub-units in adjacent or core–shell configurations.[3,4,5,6,7] As a consequence, certain species, possibly with seemingly negligible concentrations in the parent solution, can become enriched in the particle surface. Such processes should be accounted for when the particle surface properties and thereby the chemical reactivity are considered, yet they remain incompletely understood. Assessed by atomising the Br-salts using an organic solvent (ethanol), and comparing the results to the aqueous case
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