Solvent and cosolute dependence of Mg surface enrichment in submicron aerosol particles

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⁺, Mg^{2 +}, Ca^{2 +}, 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 Mg^{2 +} 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.