Nanobubbles and Their Precursor Layer at the Interface of Water Against a Hydrophobic Substrate

Neutron reflectivity experiments on the interface of pure D2O against thin films of perdeuterated polystyrene (d-PS) spin-coated onto silicon blocks were performed to study the intrinsic structure of the interface of water against hydrophobic substrates. The experiments reveal nonvanishing scattering contrast at the polymer/water interface, although the two materials (d-PS and D2O) have closely similar scattering length densities. Organic (nondeuterated) contaminants or macroscopic air bubbles trapped at the polymer/water interface can be ruled out as the origin of this observation. From a systematic study of this system, it is concluded that the source of the nonvanishing contrast is a depletion of water in the boundary layer against the hydrophobic surface. It is conjectured that this depletion layer represents a precursor layer of submicroscopic gas bubbles recently observed by Tyrrell and Attard. The existence of such gas nanobubbles in the present system is confirmed by atomic force microscopy (AFM) of the surface of d-PS coatings in contact with bulk water. The thickness of the precursor gas layer as determined by neutron reflectometry is 2−5 nm, depending on the level of air saturation of the water sample and on the time elapsed after contacting it with the hydrophobic surface.