Hydrogen-Bond Heterogeneity Boosts Hydrophobicity of Solid Interfaces

Area of Science:

• Surface Science
• Physical Chemistry
• Computational Materials Science

Background:

• Chemically heterogeneous surfaces exhibit complex hydrophobicity.
• Nonlinearities in hydrophobicity as a function of composition are suggested by prior studies.

Purpose of the Study:

• To systematically explore the nonlinear hydrophobicity of binary mixtures using molecular simulations.
• To characterize hydrophobicity via contact angle and desorption pressure.

Main Methods:

• Molecular simulations using the mW coarse-grained potential for water.
• Studying binary mixtures of hydrophilic and hydrophobic sites at different scales.
• Calculating water contact angles on flat interfaces and desorption pressures from nanopores.

Main Results:

• Linear dependence of contact angle observed when components coordinate similarly with water (Cassie model).
• Unprecedented nonlinear deviations occur when only hydrophilic sites form hydrogen bonds, increasing contact angle and vapor pressure.
• Maximum enhancement observed with 35% randomly scattered hydrogen-bonding molecules; effect sensitive to heterogeneity length-scale.

Conclusions:

• Hydrogen bonding capabilities of surface components significantly influence hydrophobicity nonlinearities.
• A modified Cassie model accounting for different microrugosity can explain the observed phenomena.
• Findings are relevant for understanding water behavior on complex surfaces, especially in confined geometries.