How water wets and self-hydrophilizes nanopatterns of physisorbed hydrocarbons
Hypothesis: Weakly bound, physisorbed hydrocarbons could in principle provide a similar water-repellency as obtained by chemisorption of strongly bound hydrophobic molecules at surfaces. Experiments: Here we present experiments and computer simulations on the wetting behaviour 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. Findings: 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.
Atomic force microscopy
Molecular dynamics simulation
More Funding Information
UGV acknowledges support of Fondecyt 1180939. TPC acknowledges the support of Fondecyt Iniciación 11160664, PCI MPG190023. PH acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) within the priority program SPP 2171, “Dynamic wetting of flexible, adaptive and switchable surfaces”, Projektnummer 22879465.