Two models try to describe the mechanism of ion formation in electrospray ionization:

  • Charge Residue Model (Dole, 1968) is based on continued division of supercharged droplets until a single ion remains in the residue
  • Ion Evaporation Model (Iribarne, Thomson, 1976) states that the ions are directly ejected from the supercharged droplet

Experimental evidence to decide between these models is not readily available and is especially scarce for nanometer sized droplets. We used classical molecular dynamics simulations to study the disintegration of nanodroplets of various charge states. Molecular dynamics calculations were conducted on realistic nanodroplets containing hydronium and/or glycine homologue ions to directly observe the prevailing charge reduction mechanism.

The CHARMM22 all-atom potential function was used for the peptides in combination with the TIP3P water model. The number of water molecules was varied from ~1,000 to ~5,000. For each droplet size, the number of ions was selected in the vicinity of the Rayleigh limit. This charge density led to at least one ion formation event during the first 100 ps of simulation.

Our simulations showed that both models are valid under different conditions. If the charge on the droplet exceeded the Rayleigh limit, a Coulomb explosion took place. Generally, a few ions evaporated from droplets during the simulation time if the charge was lower than the Rayleigh limit. The following image illustrates the ion evaporation mechanism:

The thermal fluctuations induce the formation of protrusions on the surface of any droplet. Formation of protrusions is accelerated by the presence of ions inside the droplet. Some ions were pushed by electrostatic repulsion from the droplet along or into the protrusion toward lower potential points. When the repulsion between the rest of the droplet and the separated ion exceeded the cohesive forces within the protrusion, the solvated ion detached.