Correlation lengths in nanoconfined water and transport properties

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Confinement significantly alters the properties of water at the nanoscale.
• Understanding nanoconfined water (NCW) is crucial for various applications, yet its complex behavior remains incompletely understood.

Purpose of the Study:

• To investigate the influence of confinement on the static and dynamic correlation lengths of nanoconfined water (NCW).
• To analyze the viscous properties, including anisotropy and viscoelasticity, of NCW as a function of hydrophobic plate separation distance (d).
• To identify and characterize novel spectral features in the frequency-dependent viscosity of NCW.

Main Methods:

• Molecular dynamics simulations were employed to study NCW confined between hydrophobic plates.
• Analysis of the transverse component of the mean square stress to determine spatial decay and static correlation length.
• Calculation of stress-stress time correlation functions to derive the dynamic correlation length.
• Examination of frequency-dependent viscosity spectra.

Main Results:

• Disparate static (0.75 nm) and dynamic (0.35 nm) correlation lengths were observed in NCW, indicating confinement effects.
• A novel peak in the frequency-dependent viscosity at ~50 cm⁻¹ was identified, absent in bulk water, persisting even at 3 nm separation.
• The transverse mean square stress showed slow spatial decay beyond ~1.8 nm, a previously unreported phenomenon.

Conclusions:

• The study reveals distinct static and dynamic behaviors in NCW, with dynamic correlation length limited by orientational relaxation sensitivity to confinement.
• The newly observed viscosity peak suggests unique intermolecular interactions or vibrational modes in confined water.
• Further exploration of the diffusion-viscosity relationship in NCW is warranted.