Ejection of Solvated Ions from Electrosprayed Methanol/Water Nanodroplets Studied by Molecular Dynamics Simulations
Elias Ahadi
Lars Konermann
10.1021/ja111492s.s003
https://acs.figshare.com/articles/media/Ejection_of_Solvated_Ions_from_Electrosprayed_Methanol_Water_Nanodroplets_Studied_by_Molecular_Dynamics_Simulations/2639104
The ejection of solvated small ions from nanometer-sized droplets plays a central role during electrospray ionization (ESI). Molecular dynamics (MD) simulations can provide insights into the nanodroplet behavior. Earlier MD studies have largely focused on aqueous systems, whereas most practical ESI applications involve the use of organic cosolvents. We conduct simulations on mixed water/methanol droplets that carry excess NH<sub>4</sub><sup>+</sup> ions. Methanol is found to compromise the H-bonding network, resulting in greatly increased rates of ion ejection and solvent evaporation. Considerable differences in the water and methanol escape rates cause time-dependent changes in droplet composition. Segregation occurs at low methanol concentration, such that layered droplets with a methanol-enriched periphery are formed. This phenomenon will enhance the partitioning of analyte molecules, with possible implications for their ESI efficiencies. Solvated ions are ejected from the tip of surface protrusions. Solvent bridging prior to ion secession is more extensive for methanol/water droplets than for purely aqueous systems. The ejection of solvated NH<sub>4</sub><sup>+</sup> is visualized as diffusion-mediated escape from a metastable basin. The process involves thermally activated crossing of a ∼30 kJ mol<sup>–1</sup> free energy barrier, in close agreement with the predictions of the classical ion evaporation model.
2011-06-22 00:00:00
surface protrusions
Considerable differences
methanol concentration
ion secession
nanodroplet behavior
Solvated ions
ion evaporation model
conduct simulations
energy barrier
droplet composition
ESI efficiencies
Solvated Ions
metastable basin
NH
ion ejection
Molecular Dynamics SimulationsThe ejection
droplets
MD studies
electrospray ionization
ESI applications
analyte molecules
Molecular dynamics