Distinct chemistries explain decoupling of slip and wettability in atomically smooth aqueous interfaces

TL;DR

This study uses molecular dynamics simulations to reveal that the distinct chemistries of hexagonal boron nitride and graphite cause significant differences in fluid slip behavior, despite similar wettability and structure.

Contribution

It demonstrates that polar bonds in hexagonal boron nitride lead to Coulombic interactions, explaining the decoupling of slip and wettability in atomically smooth aqueous interfaces.

Findings

Hexagonal boron nitride exhibits higher fluid slip than graphite.

Polar bonds in boron nitride cause Coulombic interactions with water.

Wettability remains similar due to dispersive forces in both materials.

Abstract

Despite essentially identical crystallography and equilibrium structuring of water, nanoscopic channels composed of hexagonal boron nitride and graphite exhibit an order-of-magnitude difference in fluid slip. We investigate this difference using molecular dynamics simulations, demonstrating that its origin is in the distinct chemistries of the two materials. In particular, the presence of polar bonds in hexagonal boron nitride, absent in graphite, leads to Coulombic interactions between the polar water molecules and the wall. We demonstrate that this interaction is manifested in a large typical lateral force experienced by a layer of oriented hydrogen atoms in the vicinity of the wall, leading to the enhanced friction in hexagonal boron nitride. The fluid adhesion to the wall is dominated by dispersive forces in both materials, leading to similar wettabilities. Our results rationalize recent observations that the difference in frictional characteristics of graphite and hexagonal boron nitride cannot be explained on the basis of the minor differences in their wettabilities.