Ordered Water Layer on the Macroscopically Hydrophobic Fluorinated Polymer Surface and Its Ultrafast Vibrational Dynamics

Hydrophobic-like water monolayers have been predicted at the metal and some polar surfaces by theoretical simulations. However, direct experimental evidence for the presence of this water layer at surfaces, particularly at biomolecule and polymer surfaces, is yet to be validated at room temperature. Here we observe experimentally that an ordered molecular water layer is present at the hydrophobic fluorinated polymer such as polytetrafluoroethylene (PTFE) surface by using sum frequency generation vibrational spectroscopy. The macroscopic hydrophobicity of PTFE surface is actually hydrophilic at the molecular level. The macroscopically hydrophobic character of PTFE is indeed resulting from the hydrophobicity of the ordered twodimension (2D) water layer, in which cyclic water tetramer structure is found. The water layer at humidity of <= 40% has a vibrational relaxation time of 550 +/- 60 fs. The vibrational relaxation time in the frequency range of 3200-3400 cm(-1) shows remarkable difference from the interfacial water at the air/H2O interface and the lipid/H2O interface. No discernible frequency dependence of the vibrational relaxation time is observed, indicating the homogeneous dynamics of OH groups in the water layer. These insights into the water layer at the macroscopically hydrophobic surface may contribute to a better understanding of the hydrophobic interaction and interfacial water dynamics.

Automated summary

Experimental evidence shows the presence of an ordered molecular water layer at hydrophobic fluorinated polymer surfaces like PTFE, which differs from the macroscopic hydrophobicity of the surface and exhibits unique vibrational and structural properties. The water layer at PTFE surface has a distinctive cyclic water tetramer structure and a vibrational relaxation time without frequency dependence, indicating homogeneous dynamics of OH groups in the water layer.