How water wets and self-hydrophilizes nanopatterns of physisorbed hydrocarbons

TL;DR

This study demonstrates that water can effectively hydrophilize hydrophobic hydrocarbon nanopatterns on surfaces through rearrangement and displacement of molecules, combining experiments and simulations to reveal the underlying mechanisms.

Contribution

It provides new insights into how water interacts with and modifies the wettability of physisorbed hydrocarbon nanopatterns, highlighting a self-hydrophilization process.

Findings

Water displaces alkane monolayers, reducing hydrophobic coverage.

Nanopattern morphology correlates with water wettability.

Rearrangement of alkane layers upon water contact is confirmed by simulations.

Abstract

Weakly bound, physisorbed hydrocarbons could in principle provide a similar water-repellency as obtained by chemisorption of strongly bound hydrophobic molecules at surfaces. Here we present experiments and computer simulations on the wetting behavior of water on molecularly thin, self-assembled alkane carpets of dotriacontane (n-C32H66 or C32) physisorbed on the hydrophilic native oxide layer of silicon surfaces during dip-coating from a binary alkane solution. By changing the dip-coating velocity we control the initial C32 surface coverage and achieve distinct film morphologies, encompassing homogeneous coatings with self-organised nanopatterns that range from dendritic nano-islands to stripes. These patterns exhibit a good water wettability even though the carpets are initially prepared with a high coverage of hydrophobic alkane molecules. Using in-liquid atomic force microscopy, along with molecular dynamics simulations, we trace this to a rearrangement of the alkane layers upon contact with water. This restructuring is correlated to the morphology of the C32 coatings, i.e. their fractal dimension. Water molecules displace to a large extent the first adsorbed alkane monolayer and thereby reduce the hydrophobic C32 surface coverage. Thus, our experiments evidence that water molecules can very effectively hydrophilize initially hydrophobic surfaces that consist of weakly bound hydrocarbon carpets.