Tailoring the Wetting Behavior of Self-Assembled Monolayers by Surface Charge

The wetting of (mixed) self-assembled monolayers (SAMs) is characterized as a function of surface charge. Using a combination of molecular dynamics simulations and contact angle measurements, we unravel the arrangement of dipoles and hydrogen bonds at the interface. Our basis simulation model features alkyl functionalization of an aluminum oxide surface via n-alkyl-phosphonic acid [C18H37–PO(OH)2] molecules, which represents a commonly used hydrophobic SAM surface readily accessible to contact angle measurements. We then probed the effect of charging the alkyl-terminated SAMs to unravel the interplay of local hydrophobicity and the overall dipole arrangement within the droplets. Strikingly, we find that moderate positive charging retains the hydrophobic character and potentially makes the SAMs even more hydrophobic than their uncharged counterpart. We attribute this phenomenon to the hindering of dangling hydrogen atoms at the water-alkyl contact, as a consequence of dipole rearrangements. While our simulation models predict a contact angle increase by 10° as an upper estimate for boosting hydrophobicity, experimental analyses of mixed SAMs using imidazole/alkyl-terminated species showed roughly constant wetting behavior for moderately positive charging.