Microscopic Structure and Dynamics of Interfacial Water at Fluorinated vs Nonfluorinated Surfaces -- Insights from Ab-Initio Simulations and IR Spectroscopy

Abstract

Per- and polyfluoroalkyl substances are a class of synthetic chemical compounds widely used as coatings to lower surface energies. Yet the microscopic mechanisms of their weak interaction with water and organic compounds remain poorly understood. Here, we perform large-scale density-functional-theory molecular dynamics simulations to investigate water at self-assembled monolayers (SAMs) of fluorinated and non-fluorinated hydrocarbons. We analyze the interfacial water structure and compare it to the prototypical hydrophobic air-water interface. The interfacial water structure at both SAMs closely resembles that at the air-water interface, featuring a distinct depletion layer and a two-dimensional hydrogen-bond network parallel to the surface. Computed anisotropic infrared spectra reproduce key experimental signatures observed in surface-enhanced infrared absorption spectroscopy (SEIRAS), including the presence of free OH vibrations directly probing the local surface-water interactions. Notably, while the free OH stretch at the hydrocarbon SAM-water interface exhibits a red shift relative to the air-water interface, indicative of weak binding, the fluorinated SAM-water interface displays a weakly blue-shifted free OH mode, in agreement with experiment. This frequency behavior, which defies common interpretations based on the vibrational Stark effect, indicates that dispersive rather than electrostatic interactions dominate the interaction between water and SAMs. Analysis of spectral line shapes further shows that the reorientation dynamics of water molecules are significantly slower near the fluorinated surface, as commonly observed at hydrophilic surfaces. This indicates that fluorinated surfaces, despite being macroscopically more hydrophobic than their unfluorinated counterparts, exhibit spectroscopic characteristics that neither qualify it as hydrophobic nor hydrophilic.

Figures (7)

• Figure 1: A-C: Simulation snapshots from DFT-MD simulations of the three model interfaces: air-water, HSAM-water containing n-hexane molecules (D, left), and FSAM-water containing the semifluorinated compound CH$_3$CH$_2$(CF$_2$)$_3$CF$_3$ (D, right). To keep the SAMs in place, carbon atoms at the bottom of the SAMs are restricted to a hexagonal close lattice indicated in panels E and F by blue circles. G-I: Density profiles in the vicinity of all three interfaces as a function of the distance to the Gibb's dividing surface of the water calculated according to Eq. \ref{['eq:theory:gibbs_dividing']}. The thicknesses of the depletion layers $\delta$, defined in Eq. \ref{['eq:sams:depletion_layer']}, are given in the legends.
• Figure 2: Interfacial water structure at different interfaces from DFT-MD simulations. For comparison, FF-MD results are shown in Sec. S3 of the supplementary material. A-C: Molecular number density profiles of water molecules as a function of the distance from the centers of mass to the WCI. For all three interfaces the first minimum of the density profile is found at the same distance to the WCI, $z_\mathrm{fhl} - z_\mathrm{WCI}=3.0~\mathrm{\AA}$, marked with a vertical dashed line. D-I: Joint probability angular distributions of the orientations of molecular water dipole moments $\theta_\mathrm{dip}$ and OH bonds $\theta_\mathrm{OH}$ normalized to their respective homogeneous distributions (Eq. \ref{['eq:homogeneous_dist']} in the methods section). Data is shown for two distinct slabs, as defined by the positions of the molecules' center of mass, with boundaries indicated by dashed vertical lines in panels A-C. Positive values of $\cos \theta_\mathrm{dip}$ and $\cos \theta_\mathrm{OH}$ indicate that the molecules' dipole or the OH vector points towards the bulk phase, respectively. J: Example orientations of water molecules for various combinations of $(\cos \theta_\mathrm{dip}, \cos \theta_\mathrm{OH})$. K: Profile of the number of donated hydrogen bonds per water molecule, binned with respect to the center of mass.
• Figure 3: Vibrational spectra of water in the first hydration shell at various interfaces. A-B: Parallel and perpendicular component of interfacial spectra according to Eqs. \ref{['eq:sams:chi_fhl_par']}-\ref{['eq:sams:chi_fhl_perp']}. C-D: Close up of the OH stretch vibrational region. E-F: Self contributions to interfacial spectra according to Eq. \ref{['eq:self_corr']} (methods). Arrows indicate the signal corresponding to the vibration of free OH groups.
• Figure 4: Difference SEIRA spectra of the water-stretching region after adsorption of a monolayer of (A) 1-hexanethiol and (B) perfluorohexanethiol. Spectral features from the gold–water interface and the liquid bulk water displaced by the monolayer volume appear as negative bands. Vibrational features from the monolayer and the newly created interfacial water structure appear as positive bands.
• Figure 5: A: Power spectrum of the OH bond length in the first hydration layer of different interfaces compared to bulk water. B: Power spectrum of the OH bond length for free OH groups using the criterion ($z_\mathrm{COM}<z_\mathrm{WCI}+0.9~\mathrm{\AA}$, $\theta_\mathrm{OH}>120^\circ$). Fits to a single Lorentzian function using Eq. \ref{['eq:sams:lorentzian']} are indicated as solid lines.