Impact of hydrophilic/hydrophobic surface chemistry on hydration forces in the absence of confinement

The oscillatory force profile, observed in liquids due to molecular ordering at interfaces, has been extensively investigated by means of atomic force microscopy, but it remains unclear whether molecular ordering is present at the tip apex. Using a displacement-sensitive, low-noise atomic force microscope (AFM) operated in dynamic mode, with a tip of radius < 1 nm, we have investigated the force profile between two approaching surfaces of the same or different hydrophilic and hydrophobic character. By directly comparing different surface chemistry interactions, we have been able to elucidate whether an oscillatory force profile is due to structured water layers adjacent to the surface, the tip, or a combination of the two. We have found that an oscillatory force profile is observed when the surface is hydrophilic in nature, irrespective of whether the tip is hydrophilic or hydrophobic. When the surface is hydrophobic, an oscillatory force profile is not measured, but rather a monotonic repulsive or a short-range attractive force is observed for interactions with a hydrophilic or a hydrophobic tip, respectively. Thus, we attribute the measurement of an oscillatory force profile, in the absence of lateral confinement effects, solely to water layers adjacent to a hydrophilic surface rather than the structuring of water at the tip apex. This is the first direct evidence that solvation forces occur solely as a result of water layers adjacent to the substrate.