  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.
