Charged droplet dynamics in the submicrometer size range

The highly charged droplets produced by electrospray are of significant importance in analytical chemistry, particularly in mass spectrometry. In spite of this, little is known about the mechanisms of charge loss for highly charged droplets in the submicrometer size range. In this study, electropsrayed droplet charge loss dynamics in the submicrometer size range were examined for the first time using tandem differential mobility analysis (TDMA) of aerosol particles originally enclosed in methanol-water droplets. After compete droplet evaporation, the remaining aerosol particles carried the nonejected charges from droplets; thus, particle electrical mobility spectra are reflective of the electrical mobility spectra of droplets of similar size. Bayesian data inversion was used to compare measured electrical mobility spectra to models for droplet losing charge by Coulombic fission and by single ion emission. Data inversion showed that, at a size of ∼40 nm, a transition of the mechanism of charge loss occurred. Methanol-water droplets larger than this size lost charge through Coulombic fissions at the Rayleigh limit with an effective surface tension of 0.050 N m_-1 while droplets in the 10-40 nm size range lost charge through ion emission, maintaining a constant electric field of 1.1 V nm _-1 on their surface during ion emission. This transition was observed experimentally for the first time in this study and is in good agreement with theoretical predictions from prior study of ion emitting nanodroplets The data presented here provide the necessary link between studies of highly charged micrometer sized droplet and those of highly charged liquid clusters.