Abstract
To shed light on recently explored long-range surface forces generated by subsurface-confined water, the structural characteristics of water molecules penetrating into nanoporous homogeneous and nanograded siloxane plasma polymer films (PPFs) over the time scale of 24 hours are studied by surface-enhanced IR spectroscopy (SEIRAS). Chemically graded PPFs, with embedded hydrophobic-to-hydrophilic gradient, are found to significantly change the average interfacial water orientation due to a unique nanoporous morphology and silanol group coordination. Diffusion of water through the hydrophobic SiO:CH matrix creates an evolution of the coordination of matrix silanol groups, which are eventually deprotonated as soon as the hydration network connects to the aqueous environment. This occurs after ~6 hours of water immersion and coincides with the change of average interfacial water orientation. Both effects are present on hydrophobic samples, but are significantly amplified by the presence of the subsurface vertical amphiphilic gradient (Vgrad), whereas enhanced water uptake in oxygen-plasma modified graded PPFs is covering such effects.
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