Characterization of Fluorocarbon Monolayer Surfaces for Direct Force Measurements

Abstract

We have prepared hydrophobic surfaces by silylating surfaces of molten glass in three different ways. Heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethoxysilane (FTE) was either (i) spread on the air−water interface, allowed to polymerize and then deposited as an LB film (at surface pressures of 10, 20, and 35 mN/mdesignated LB10, LB20, and LB35), (ii) allowed to react with the silica surface in a CHCl3 solution (FTE/CHCl3), or (iii) allowed to react with the silica in the undiluted liquid state (FTE/neat). The surfaces thus prepared were scanned by atomic force microscopy; their chemical compositions were analyzed by X-ray photoelectron spectroscopy; wettability studies with water were performed; and adhesion or pull-off forces between two such surfaces in humid air and water were determined. The FTE/neat surface was significantly less stable and less hydrophobic than the other surfaces, although an AFM scans indicated comparable smoothness. Considerable amounts of excess material could be removed from this surface by rinsing with ethanol or water. The FTE/CHCl3 surfaces and the LB10 surfaces were the smoothest, with a mean roughness of ∼0.14 nm, whereas LB20 and LB35 were rougher and showed randomly distributed bulges protruding 2.5−3 nm above the surfaces. All surfaces appeared amorphous and the coverage was similar (90−100%) for all LB surfaces, but lower for FTE/CHCl3 (∼80%), which also showed some loss on rinsing. LB10 was the most hydrophobic, with advancing and receding contact angles of water of 123 and 96°, respectively, that were stable with repeated immersion and retraction. FTE/CHCl3 was less hydrophobic and showed larger hysteresis θa = 107°, θr = 60°. The measured pull-off force in humid air was slightly larger for LB10 than for FTE/CHCl3. The pull-off forces in water for LB10 and FTE/CHCl3 were initially similar, but with immersion time the value for LB10 increased and stabilized at a much larger value, whereas that for FTE/CHCl3 remained constant. We conclude that LB deposition at a low surface pressure yields an amorphous surface that is smooth and homogeneous and has optimal hydrophobicity and good stability, whereas deposition at higher pressures give rougher surfaces with more excess material. Nevertheless, there are indications of small amounts of excess material that are slowly removed by water immersion even for deposition at low surface pressures. Adsorption from CHCl3 gives smooth surfaces with large amounts of loosely held material that contributes to a larger contact angle hysteresis and lesser hydrophobicity.