When 20% Is Enough: Counterintuitive Contact Angle Maxima on Chemically Heterogeneous Hydrophobic/Hydrophilic...

Solid-liquid-gas three-phase systems are central to chemistry, biology, physics, engineering, and even botany. Representative examples range from gas-evolving reactions at (photo)electrodes to hydrophobic interactions between proteins. Although these processes occur on the nanoscale, they are often interpreted using empirical principles extrapolated from macroscopic laws. Here we show, by means of atomistic simulations, that regularly distributed atomic- or molecular-scale chemical heterogeneities can give rise to counterintuitive behavior: the contact angle of an heterogeneous hydrophobic surface is maximized when the surface is decorated with a ∼20% atomic-level hydrophilic particle (with fixed interaction force). Similarly, a maximum in the contact angle is observed, at fixed hydrophilic particles geometry, when the difference in interaction force between the hydrophobic and hydrophilic spots is Δ ∼ 40%. We demonstrate that such atomistic-level heterogeneities can pin the solid-liquid-gas contact line. These findings have several implications. First, they rationalize the high hydrophobicity of materials that nonetheless contain a significant fraction of hydrophilic sites, Cu₂(tebpz), a metal-organic framework whose pores host Cu nodes, being a notable example. Second, if our results are experimentally confirmed for more general, nonregularly patterned surfaces, they will add a new evidence for the need of overcome the asymptotic homogenization, widely used for describe heterogeneous surfaces from individual component, such as protein surfaces with amino-acid hydrophobicity scales or nanoporous materials.