  Experimental investigations of hydrophobic/water interfaces often return
controversial results, possibly due to the unknown role of gas accumulation at
the interfaces. Here, during advanced atomic force microscopy of the initial
evolution of gas-containing structures at a highly ordered pyrolytic
graphite/water interface, a fluid phase first appeared as a circular wetting
layer ~0.3 nm in thickness and was later transformed into a cap-shaped
nanostructure (an interfacial nanobubble). Two-dimensional ordered domains were
nucleated and grew over time outside or at the perimeter of the fluid regions,
eventually confining growth of the fluid regions to the vertical direction. We
determined that interfacial nanobubbles and fluid layers have very similar
mechanical properties, suggesting low interfacial tension with water and a
liquid-like nature, explaining their high stability and their roles in boundary
slip and bubble nucleation. These ordered domains may be the interfacial
hydrophilic gas hydrates and/or the long-sought chemical surface
heterogeneities responsible for contact line pinning and contact angle
hysteresis. The gradual nucleation and growth of hydrophilic ordered domains
renders the original homogeneous hydrophobic/water interface more heterogeneous
over time, which would have great consequence for interfacial properties that
affect diverse phenomena, including interactions in water, chemical reactions,
and the self-assembly and function of biological molecules.
