Ejection of solvated ions from electrosprayed methanol/water nanodroplets studied by molecular dynamics simulations

Area of Science:

• Physical Chemistry
• Analytical Chemistry
• Computational Chemistry

Background:

• Solvated ion ejection from nanodroplets is critical for electrospray ionization (ESI).
• Previous molecular dynamics (MD) studies primarily examined aqueous systems, not mixed solvents common in ESI.
• Understanding mixed solvent effects is crucial for optimizing ESI applications.

Purpose of the Study:

• To investigate the behavior of mixed water/methanol nanodroplets containing excess ammonium (NH4+) ions using MD simulations.
• To elucidate the impact of methanol on ion ejection, solvent evaporation, and droplet composition.
• To explore the implications for analyte partitioning and ESI efficiencies.

Main Methods:

• Molecular dynamics (MD) simulations were performed on nanometer-sized droplets composed of water and methanol with excess NH4+ ions.
• Analysis focused on ion ejection mechanisms, solvent evaporation rates, and droplet structural changes over time.
• Comparison with purely aqueous systems and classical ion evaporation models.

Main Results:

• Methanol disrupts the hydrogen-bonding network, accelerating ion ejection and solvent evaporation rates.
• Differential escape rates of water and methanol lead to time-dependent changes in droplet composition and segregation.
• Layered droplet structures with methanol-enriched peripheries form at low methanol concentrations, potentially enhancing analyte partitioning.

Conclusions:

• Methanol significantly alters nanodroplet dynamics in ESI, increasing ion and solvent escape rates.
• Droplet composition changes and segregation phenomena influence analyte behavior and ESI efficiency.
• Ion ejection mechanism aligns with a diffusion-mediated escape over a free energy barrier, consistent with classical models.