Water Self-Dissociation is Insensitive to Nanoscale Environments

Area of Science:

• Physical Chemistry
• Chemical Physics
• Materials Science

Background:

• The effects of nanoconfinement on water dissociation and reactivity are poorly understood, leading to conflicting experimental and simulation results.
• Understanding aqueous chemistry at interfaces, pores, and aerosols is crucial for various scientific disciplines.

Purpose of the Study:

• To investigate the energetics of water dissociation under nanoconfinement.
• To determine the minimum length scales at which bulk water dissociation behavior is maintained.
• To clarify the thermodynamics and mechanism of water autoionization in confined environments.

Main Methods:

• Ab initio simulations were employed to model water dissociation.
• Energetics and free-energy profiles of water autoionization were calculated.
• Simulations were performed on molecular aggregates and nanoporous structures.

Main Results:

• The energetics of bulk water dissociation remain largely unchanged down to nanoscopic scales (approx. 12 molecules or <2nm pores).
• The free-energy barrier for O-H bond breaking is conserved across bulk, nanodroplets, and nanopores (without strong interfacial interactions).
• Dissociation free-energy profiles in nanoscopic systems mimic bulk liquid behavior.

Conclusions:

• Nanoconfinement does not significantly alter water dissociation energetics until very small length scales.
• The fundamental mechanism and thermodynamics of water autoionization are robust across different scales.
• This work provides a definitive understanding of water dissociation, impacting studies on reactivity and self-ionization at interfaces.