Solvated Ion Evaporation from Charged Water Nanodroplets

Abstract

The behavior of electrified droplets in an 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. Even the morphology, the structure, and the dynamics of aqueous nanodroplets containing ionic solutes are poorly understood. 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. Droplets up to 6.5 nm in diameter were studied using potentials for the peptides and water that accounted for their internal degrees of freedom. Validity testing of the model indicated good agreement between the calculated radial distribution functions for water and corresponding neutron diffraction data. The self-diffusion coefficients and the enthalpy of evaporation derived from the model also gave good agreement with the experimental values. 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, amplitude protrusions higher than in the case of pure water droplets developed. These protrusions served as the intermediate stage preceding ion ejection. The evaporated ions detached from the droplet with a solvation shell of approximately 10 water molecules per ion. Our data were coherent with the solvated ion evaporation model for droplets close to the Rayleigh limit. To the best of our knowledge, these are the first molecular dynamics calculations on realistic charged nanodroplets to give insight into their structure and fission dynamics.