Grotthuss Molecular Dynamics Simulations for Modeling Proton Hopping in Electrosprayed Water Droplets

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Excess protons in water exhibit unique transport via Grotthuss diffusion along hydrogen-bonded water wires.
• Quantum mechanical methods capture this mechanism but are computationally expensive for large systems.
• Standard molecular dynamics (MD) simulations cannot model the bond dissociation/formation required for Grotthuss diffusion.

Purpose of the Study:

• To develop a hybrid simulation technique combining MD with proton hopping to model Grotthuss diffusion.
• To accurately simulate proton transport in large, complex systems like charged water droplets.
• To investigate proton behavior in electrospray ionization contexts.

Main Methods:

• Developed a Grotthuss MD technique integrating atomistic MD (Gromacs, TIP4P/2005 water) with modeled proton hopping events (H3O+ + H2O <=> H2O + H3O+).
• Simulated proton hopping in
• bursts
• separated by Brownian diffusion periods, mimicking ab initio MD findings.
• Applied the Grotthuss MD technique to highly charged water droplets (5 nm radius) simulating electrospray ionization conditions.

Main Results:

• The Grotthuss MD technique accurately reproduced the literature proton diffusion coefficient.
• Simulations of charged water droplets showed rapid solvent evaporation and proton ejection, consistent with experimental data (81% of Rayleigh limit).
• The simulated droplet behavior aligns with phase Doppler anemometry measurements.

Conclusions:

• The developed Grotthuss MD technique effectively bridges the gap between high-level quantum methods and standard MD for simulating proton transport.
• This method enables the study of large proton-containing systems, particularly in electrospray ionization.
• Future applications include modeling proton-dominated systems where previous simulations used metal cations.