Mechanisms for Enhanced Hydrophobicity by Atomic-Scale Roughness

Abstract

It is well known that the close-packed CF 3 -terminated solid surface is among the most hydrophobic surfaces in nature. Molecular dynamic simulations show that this hydrophobicity can be further enhanced by the atomic-scale roughness. Consequently, the hydrophobic gap width is enlarged to about 0.6 nm for roughened CF 3 -terminated solid surfaces. In contrast, the hydrophobic gap width does not increase too much for a rough CH 3 -terminated solid surface. We show that the CF 3 -terminated surface exists in a microscopic Cassie-Baxter state, whereas the CH 3 -terminated surface exists as a microscopic Wenzel state. This finding elucidates the underlying mechanism for the different widths of the observed hydrophobic gap. The cage structure of the water molecules (with integrated hydrogen bonds) around CH 3 terminal assemblies on the solid surface provides an explanation for the mechanism by which the CH 3 -terminated surface is less hydrophobic than the CF 3 -terminated surface.