Surface Composition of Free Mixed NaCl/Na₂SO₄ Nanoscale Aerosols Probed by X-ray Photoelectron Spectroscopy

Abstract

The local chemical surface composition of unsupported mixed solid NaCl/NaSO aerosols ( d ∼ 70 nm) is studied by X-ray photoelectron spectroscopy. The solid aerosols are generated by drying aqueous droplets containing mixtures of the two salts in different mole fractions. The mole fraction of these salts is found to deviate at the solid aerosol surface significantly from the initial droplet composition. The minority species in the droplets are found to be enhanced at the surface of the solid mixed aerosols. This surface enhancement is rationalized in terms of the nucleation/crystallization process, where the salts evidently do not cocrystallize, rather than each salt forms pure crystal moieties. Characteristic variations of the surface ion concentration as a function of the mole fraction of the salts in the initial droplet are observed in the nanometer size regime. This is unlike core-shell architectures previously found in mixed micron salt aerosols, indicating that aerosol models derived from micron-sized aerosols are evidently not fully reliable to describe the surface composition of nanosized aerosols. Furthermore, surface enhancement of the minority component in mixed NaCl/NaSO aerosols is also different from previous results on surface segregation of mixed NaCl/NaBr aerosols, where one of the anionic species is surface segregated for all mole fractions, which was explained in terms of the ability of the involved salts to cocrystallize and forming solid solutions. The present results rather indicate that mixed NaCl/NaSO aerosols do not cocrystallize. Electron microscopy of deposited mixed salt aerosols reveals mostly a cubic structure of pure NaCl aerosols, whereas mixed salt aerosols are found to show a grainy structure composed of multiple small crystals which supports the present findings obtained from photoelectron spectroscopy.

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

“…Whether the two species are segregated or mixed throughout the particles is not apparent since both cases can similarly produce the observed spectra. The observed uniform distribution can result from cocrystallization, separate crystal domains (where slight enrichment of the minority component should occur, 6 which is not clearly manifested in the data), or if the particle surface is not fully crystalline. The presence of both Ca 2 + and Mg 2 + with “uniform” abundances would also be expected if the particles were still in liquid state, although as discussed above, we estimate that there should not be much excess water in the particles.…”

“…ref. 4 and 6). The use of both methods is motivated by their complementary nature, but also to provide discussion on their accuracy.…”

“…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. 3–7 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.…”

“…A variety of different structural phenomena in freestanding salt nanoparticles has been uncovered in works by different groups: Antonsson et al 4 observed surface enrichment of Br in freestanding NaBr/NaCl particles. In NaCl/Na 2 SO 4 particles, Antonsson et al 6 found that the inability of the species to co-crystallise resulted in surface enhancement of the minority species. Unger et al 7 observed core–shell type structures in artificial sea spray aerosol (SSA) particles, discussing also that substrate deposition may affect the particle structure, specifically the formation of core–shell structures vs. adjacent crystal moieties (full vs. partial surface coverage, respectively).…”

Surface Properties and Porosity of Silica Particles Studied by Wide-Angle Soft X-ray Scattering

“…The experimental setup has been described previously. ,,, Therefore, only a short overview is given here. Silica particles (samples 1 – 3 ) were dispersed in ethanol at a concentration of 1 g/L, and the dispersions were treated directly before the measurements for 15 min in an ultrasonic bath (Sonorex model RK 100H, Bandelin).…”