Surprising Water Slipping Performance on Highly Hydrophilic PEG‐Derived Monolayer‐Covered Surfaces: Preparation, Mechanism, and Application

ABSTRACT

Many scientists would have no doubt that water droplets tend to be firmly adhered to hydrophilic surfaces. Contrary to this common belief, we have succeeded in preparing surfaces that are statically very hydrophilic but exhibit surprising water slipping performance. These surfaces were prepared through a chemisorption of ethoxy‐terminated polyethylene glycol (PEG) organosilane (Si‐PEG‐Si) and subsequent alkali‐treatment. We prepared Si‐PEG‐Si‐derived monolayers by varying Si‐PEG‐Si concentrations (10 ⁻¹ –10 ⁻⁸  mol/L), reaction time (5–48 h), and temperature (room‐temperature or 50°C) and monitored changes in static/dynamic wetting behaviors. The optimal condition (10 ⁻⁵  mol/L, 18 h and room‐temperature) achieved the best surfaces with the lowest static water contact angle ( θ _S , ≈35.4°), contact angle hysteresis (Δ θ , ≈2.6°) and sliding angle ( α , ≈8.7°). Subsequent alkali‐treatment further increased hydrophilicity ( θ _S , ≈17.7°) without any marked degradation of dynamic wetting behaviors (Δ θ , ≈2.2° and α , ≈7.3°). Based on sum‐frequency generation spectroscopy, we concluded that the water‐thin layer containing water‐swollen PEG chains worked as a self‐lubricating layer for enhancing the movement of water droplets, while increasing hydrophilicity. Moreover, Si‐PEG‐Si‐derived monolayer‐covered paper filters showed excellent filtration performance of oil/water mixtures with a stable separation‐efficiency because of their water‐driven oleophobicity, resulting in the high purity of the extracted water over multiple separation cycles.