Computational IR spectroscopy of interfacial water at fluorinated and non-fluorinated hydrophobic surfaces

Abstract

We report ab-initio simulations of the interface between water and fluorinated and non-fluorinated hydrocarbon self assembled monolayers (SAMs) and compare with the prototypical interfacial system, the vapor-water interface. The thickness of the microscopic depletion layer between SAMs and water is larger for the fluorinated SAM, consistent with the larger contact angle of fluorinated SAMs. We calculate the infrared absorption spectrum of interfacial water, which displays a prominent sharp peak at around 3700 $\mathrm{cm}^{-1}$, signaling the presence of dangling OH bonds. We describe the vibrational properties of dangling OH bonds by a harmonic model and show that spectral line shifts reflect OH-dangling-bond interactions with the surface and line widths report on the rotational lifetimes of dangling OH configurations.