Effect of Nanostructure on the Properties of Water at the Water−Hydrophobic Interface:  A Molecular Dynamics Simulation

Abstract

The local structure of water near hydrophobic surfaces of different surface topographies has been analyzed by molecular dynamics simulation. An alkane crystal has been taken as the parent model for a hydrophobic surface. Surface structures were created by placing pits into it, which were half a nanometer deep and several nanometers wide. Around all structures, the water has a lower density, less orientational ordering, fewer water-water hydrogen bonds, and fewer surface contacts than for a flat unstructured surface. This indicates that the structured surfaces are more hydrophobic than the flat surface. Of the structures investigated, pits with a diameter of approximately 2.5 nm were effective in increasing the hydrophobic character of the surface.

Molecular Dynamics Simulation of Water Near Nanostructured Hydrophobic Surfaces: Interfacial Energies

“…[20,21] The model of our n-eicosane crystal has been described in refs. [16,17]. The crystal structure is triclinic (a = 67.68, b = 83.98, c = 2.544 nm) and close to the experimental crystal (a = 68.28, b = 85.78, c = 2.743 nm).…”

“…The motivations for performing the calculations are basically two. Firstly, we supplement our previous article [16,17] which found enhanced hydrophobicity for structured hydrophobic surfaces having different indentations (hexagonal, stripes, triangular) compared to flat surfaces. Although indentations are easier to manufacture technically, it is not clear if they are as efficient as protrusions for increasing the hydrophobicity of the surface.…”

“…[16]. We also report herein the differences in interfacial free-energy between a planar surface, a surface with a hole and a surface with a protrusion.…”

“…The details of the simulation setup are discussed in ref. [16]. We have used the YASP simulation package [25] for the molecular dynamics (MD) simulations.…”

“…The spring constants were chosen as 2000 kJ mol À1 À2 . These lengths were chosen to maintain the crystal structure of n-eicosane [16,17] under constant pressure simulations. This rigidification was necessary, not so much for the native planar alkane slab, but to prevent surface reconstruction of structured alkane surfaces, which also contained shorter alkane chains.…”