Surface charge of electrosprayed water nanodroplets: a molecular dynamics study

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Surface Science

Background:

• Aqueous nanodroplets with excess charge carriers are crucial for electrospray ionization (ESI).
• The location of these charge carriers (ions) within nanodroplets is debated, with surface confinement often assumed.
• Solvation effects may make surface localization energetically unfavorable for ions.

Purpose of the Study:

• To investigate the charge carrier location in sodium ion-containing water nanodroplets near the Rayleigh limit.
• To reconcile the apparent contradiction between simulated ion location and the established surface charge paradigm in ESI.

Main Methods:

• Utilized molecular dynamics simulations to model Na(+)-containing water nanodroplets.
• Performed electrostatic mapping to determine charge distribution within the nanodroplets.
• Analyzed water molecule orientational polarization effects.

Main Results:

• Simulations revealed that Na(+) ions are located within the nanodroplet, not at the surface.
• Despite internal ion location, electrostatic mapping confirmed excess charge resides on the droplet surface.
• Water molecule polarization, particularly at the droplet periphery, accounts for the surface charge effect.

Conclusions:

• The surface charge paradigm in ESI is valid, but does not necessitate ions being at the liquid/vapor interface.
• Internal ion solvation and external water dipole orientation collectively maintain surface charge.
• This finding clarifies the fundamental behavior of charged nanodroplets in ESI processes.