Molecular features of hydration layers probed by atomic force microscopy.

Area of Science:

• Surface Science
• Physical Chemistry
• Computational Biophysics

Background:

• Structurally-ordered water layers form on surfaces in aqueous environments.
• Atomic force microscopy (AFM) is a common technique for probing these hydration layers.
• The manifestation of hydration layers in AFM experiments requires further elucidation.

Purpose of the Study:

• To investigate hydration layers on hydrophilic and hydrophobic surfaces using AFM.
• To understand how water density, molecular orientation, free energy, and tip forces behave.
• To analyze the influence of tip-surface distance on hydration layer behavior.

Main Methods:

• Molecular dynamics simulations were employed to model water-surface interactions.
• Simulations focused on nanoscale tip-surface interactions with varying tip-surface distances.
• Analysis included water density, molecular orientation, free energy, and tip force calculations.

Main Results:

• Oscillations in force-distance curves were observed, attributed to confined water layer transitions (mono-, bi-, tri-layers).
• Hydrophobic tip-surface interactions led to water evaporation and non-oscillating hydrophobic forces due to dewetting.
• Water molecular dipoles aligned parallel to the surface at close tip proximity, irrespective of surface hydrophilicity.

Conclusions:

• AFM force-distance curves are significantly influenced by the layered structure of water.
• Dewetting transitions in hydrophobic environments alter force-distance relationships in AFM.
• Water molecular orientation near surfaces shows a consistent alignment pattern under confinement.