Unexpected Hydrophobicity on Self-Assembled Monolayers Terminated with Two Hydrophilic Hydroxyl Groups

TL;DR

This study reveals that a self-assembled monolayer terminated with only two hydrophilic hydroxyl groups can unexpectedly exhibit hydrophobic behavior due to ice-like hydrogen bonding structures, challenging traditional views.

Contribution

It demonstrates, for the first time, that a hydroxyl-terminated monolayer can be hydrophobic because of specific hydrogen bonding arrangements, providing new insights into surface hydrophobicity mechanisms.

Findings

Contact angle of 82 degrees on hydroxyl-terminated SAM.

Formation of hexagonal-ice-like hydrogen bonds reduces water-surface interactions.

Hydrophobicity arises despite the surface being terminated with hydrophilic groups.

Abstract

Current major approaches to access surface hydrophobicity include directly introducing hydrophobic nonpolar groups/molecules into surface or elaborately fabricating surface roughness. Here, for the first time, molecular dynamics simulations show an unexpected hydrophobicity with a contact angle of $82^o$ on a flexible self-assembled monolayer terminated only with two hydrophilic OH groups ($(OH)_2\!-\!SAM$). This hydrophobicity is attributed to the formation of a hexagonal-ice-like H-bonding structure in the OH matrix of $(OH)_2\!-\!SAM$, which sharply reduces the hydrogen bonds between surface and water molecules above. The unique simple interface presented here offers a significant molecular-level platform for examining the bio-interfacial interactions ranging from biomolecules binding to cell adhesion.