Sub-nanometer Confinement Suppresses Autoionization of Water

Area of Science:

• Physical Chemistry
• Nanotechnology
• Materials Science

Background:

• Water's behavior in confined nanoscale environments is crucial for nanofluidics, membranes, and biological processes.
• Acid-base equilibria and proton transport are key mechanisms in these systems.
• Sub-nanometer confinement alters molecular interactions, challenging standard chemical assumptions.

Purpose of the Study:

• To investigate water autoionization in quasi-two-dimensional monolayers within sub-nanometer slit pores.
• To understand the molecular mechanisms behind confinement-induced changes in water chemistry.
• To provide principles for controlling aqueous reactivity at the nanoscale.

Main Methods:

• Density-corrected density functional theory (DFT).
• Machine-learned interatomic potentials.
• Simulations of water confined in sub-nanometer slit pores.

Main Results:

• Extreme confinement suppresses water autoionization, increasing the effective pKw by over two units.
• Hydroxide ion destabilization at interfaces is the primary cause.
• Restricted hydrogen bonding, hindered reorientation, and disrupted Grotthuss transport contribute to suppression.

Conclusions:

• Confinement dramatically alters fundamental aqueous chemistry at the molecular level.
• Understanding these effects is vital for designing functional nanomaterials and understanding biological systems.
• This study offers a molecular basis for tuning reactivity in confined aqueous environments.