Hydrophobicity of Self-Assembled Monolayers of Alkanes: Fluorination, Density, Roughness, and Lennard-Jones Cutoffs

Abstract

The interplay of fluorination and structure of alkane self-assembled monolayers and how these affect hydrophobicity are explored via molecular dynamics simulations, contact angle goniometry, and surface-enhanced infrared absorption spectroscopy. Wetting coefficients are found to grow linearly in the monolayer density for both alkane and perfluoroalkane monolayers. The larger contact angles of monolayers of perfluorinated alkanes are shown to be primarily caused by their larger molecular volume, which leads to a larger nearest-neighbor grafting distance and smaller tilt angle. Increasing the Lennard-Jones force cutoff in simulations is found to increase hydrophilicity. Specifically, wetting coefficients scale like the inverse square of the cutoff, and when extrapolated to the infinite cutoff limit, they yield contact angles that compare favorably to experimental values. Nanoscale roughness is also found to reliably increase monolayer hydrophobicity, mostly via the reduction of the entropic part of the work of adhesion. Analysis of depletion lengths shows that droplets on nanorough surfaces partially penetrate the surface, intermediate between Wenzel and Cassie−Baxter states.

Critical Packing Density of Water-Mediated Nonstick Self-Assembled Monolayer Coatings

“…Recently, there have been theories developed to predict the transitions from hydration repulsion to adhesion based on the contact angles of asymmetric hydrophilic and hydrophobic surfaces. − Whether these theories (originally developed for the partially charged polar surfaces) may be extended to the surfaces with high charge densities such as the calcites explored in this work remains an open question and is very interesting for future study.…”

Computing the Work of Solid–Liquid Adhesion in Systems with Damped Coulomb Interactions via Molecular Dynamics: Approaches and Insights

“…Only in several control systems full long-range Coulomb interactions were realized via the PPPM method with a precision of 10 –6 instead of using eq 3 . 43 Regarding the treatment of L-J interactions, it has been demonstrated that the contact angle, that is, the work of solid–liquid adhesion, is also sensitive to the L-J cutoff length, 44 where best agreement with experimental measurements was achieved by treating the L-J dispersive term as long-range, equivalent to that of Coulomb. Indeed, many force fields tend to underestimate surface tensions, and considering long-range dispersion forces often brings those values closer to the experimental ones, 45 which creates a predicament as most force fields were optimized under an assumption of short-range L-J interactions.…”

Surface Roughness Explains the Observed Water Contact Angle and Slip Length on 2D Hexagonal Boron Nitride

“…As opposed to the excellent agreement of our WCA predictions with experimental data using a finite LJ cut-off of 12.0 Å, recent work by Carlson et al has demonstrated the importance of considering long-range LJ interactions to accurately determine the interfacial properties of surfaces. 71 Therefore, to understand whether the LJ cut-off impacts the hBN surface's wettability, we fixed the electrostatic interaction cut-off at 12.0 and 24.0 Å for pristine hBN and hBN with exposed edges (0.058 nm/nm 2 ), respectively, and varied the LJ cut-off from 12.0 to 30.0 Å, while determining the work of adhesion. The corresponding work of adhesion values for bulk hBN and hBN with exposed edges are tabulated in the Supporting Information, Section S5.…”