Structure, morphology, and dynamics of charged nanodroplets

ANYL 234

Akos Vertes, Ioan Marginean, and Vasiliy Znamenskiy. Department of Chemistry, George Washington University, 725 21-st Street, N.W, Washington, DC 20052
The behavior of electrified droplets in atmospheric environment and the mechanism of ion formation in electrospray ionization are the subject of continuing debate. Experimental evidence to decide between the various models of ion formation (e.g., ion evaporation, Coulomb explosion and charge residue model) is not readily available and is especially scarce for nanometer-sized droplets. Classical molecular dynamics simulations were used to explore the effect of ions on the shape and structure of these droplets. We also followed the gas-phase formation of hydronium and glycine homologue ions from the disintegrating nanodroplets. Our results showed that the ions were distributed in concentric layers within the droplet. This is a departure from the expectation that ions inside the droplet follow a monotonic radial distribution close to the surface because of the Coulomb repulsion and/or hydrophobic forces. Due to the presence of ions in the droplet, both overall shape deformations and enhanced surface fluctuations were observed. Charge reduction at the Rayleigh limit proceeded through the formation of transient surface protrusions. For droplets containing ions, higher amplitude protrusions developed than in the case of pure water droplets. These protrusions served as the intermediate stage preceding ion ejection. The evaporated ions detached from the droplet with a solvation shell of approximately ten water molecules per ion.