AFM measurements of hydrophobic forces between a polyethylene sphere and silanated silica plates- the significance of surface roughness

Abstract

Recently, substantial research effort has been devoted to the study of non-DLVO forces between hydrophobic surfaces. However, the significance of surface roughness in the analysis of these hydrophobic attractive forces has not been given sufficient consideration and research is now in progress to attend to this issue. Fused silica plates covered with adsorbed octadecyltrichlorosilane (OTS) were characterized by water contact angle measurements and atomic force microscopy (AFM). Surfaces with different surface coverages and different contact angles were obtained by variation of the adsorption time. OTS formed patches on the silica surfaces, the lateral size and height of which depended on the adsorption time. Such surfaces exhibit differences in roughness at the sub-nanometer level. Using the AFM colloidal probe technique, forces between a polyethylene sphere and silanated silica surfaces were measured in water. Long-range attractive forces were found, usually referred to as hydrophobic forces. The resulting force vs. distance curves were fitted with a double exponential function. The magnitude of the short-range part of the force curves seems to correlate with water contact angles at silanated silica surfaces. On the other hand, the range of the long-range force correlates with the roughness of the silanated silica surface. These results with silanated silica surfaces were compared with the AFM results for polyethylene and graphite surfaces and on the basis of these experimental efforts, it appears that the nature of these hydrophobic attractive forces is related to surface roughness.

Keywords:

• Silanation of silica
• octadecyltrichlorosilane
• atomic force microscopy (AFM)
• hydrophobic forces
• roughness