Where Do the Ions Reside in a Highly Charged Droplet?

ABSTRACT

Aqueous droplets in atmospheric and electrosprayed aerosols are charged due to presence of multiple ionic species. We examine the ion spatial distribution and the surface electric field in aqueous charged nanodrops by using atomistic modeling and analytical theory. We find that in nanoscopic liquid drops the concentration of simple ions is higher in the outer droplet shells, reduces gradually toward the drop center, and dies-off toward the vapor-droplet interface. The behavior of the ion spatial distribution is supported by a general analytical theory that takes into account a fluctuating droplet interface, an effective screening length of the charges and the finite size of a solvated ion. We compute the electric potential and the electric field near the droplet surface using a multipole expansion. We emphasize the significance of the fluctuations of the normal component of the electric field in ion evaporation via the Born model. In the presence of a highly charged peptide, we find that the peptide is situated mainly in the droplet interior and occasionally near the droplet surface. The simple ions are mainly near the droplet surface. The study provides insight into droplet chemistry and electrospray ionization mass spectrometry findings.