(102c) Hydriphobic Attraction between Silanated Silica Surfaces of Different Hydrophobicity

Despite the recent progress in understanding hydrophobic attraction, the origin and nature of the attraction have not been completely clarified so far. Particularly, the relationship between surface hydrophobicity and the forces acting on surfaces has not been fully understood.

In this study, we have conducted force measurements between hydrophobic surfaces of different hydrophobicity using atomic force microscopy in order to investigate how surface hydrophobicity affects the resulting interaction forces. Silica particles and silicon wafers were hydrophobised with octadecyltrichlorosilane. Interactions between a hydrophobic particle with a contact angle of 108° and a silicon wafer with various hydrophobicity were evaluated. The force curves obtained between the hydrophobic particle and a completely hydrophilic silicon wafer fitted well to DLVO theoretical predictions, indicating that no additional forces were present between the surfaces. On the other hand, when the silicon wafer was slightly hydrophobised, with a contact angle of 40°, an additional attractive force which was longer-ranged than van der Waals attraction was observed between the surfaces. This suggests that, when one surface is highly hydrophobic, the hydrophobic attraction can occur even when the other surface is weakly hydrophobic. We also found that the range of the additional attractive force depends weakly on the contact angle, whereas the adhesion force steeply increases with increasing hydrophobicity of the wafer.