Characterization of fluorocarbon monolayer surfaces for direct force measurements

We have prepared hydrophobic surfaces by silylating surfaces of molten glas s in three different ways. Heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethox ysilane (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/m-designated LB10, LB20, and LB35), (ii) allowed to react with the silica surface in a CHCl3 solution (FTE/CHCl3), or (iii) allowed to rea ct with the silica in the undiluted liquid state (FTE/neat). The surfaces t hus prepared were scanned by atomic force microscopy; their chemical compos itions were analyzed by X-ray photoelectron spectroscopy; wettability studi es with water were pel formed; 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 amount s of excess material could be removed from this surface by rinsing with eth anol or water. The FTE/CHCl3 surfaces and the LB10 surfaces were the smooth est, with a mean roughness of similar to 0.14 nm, whereas LB20 and LB35 wer e 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 (similar to 80%), wh ich also showed some loss on rinsing. LB10 was the most hydrophobic, with a dvancing and receding contact angles of water of 123 and 96 degrees, respec tively, that were stable with repeated immersion and retraction. FTE/CKCl3 was less hydrophobic and showed larger hysteresis theta(a) = 107 degrees, t heta(r) = 60 degrees. 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 p ressure yields an amorphous surface that is smooth and homogeneous and has optimal hydrophobicity and good stability, whereas deposition at higher pre ssures give rougher surfaces with more excess material. Nevertheless, there are indications of small amounts of excess material that are slowly remove d by water immersion even for deposition at low surface pressures. Adsorpti on from CHCl3 gives smooth surfaces with large amounts of loosely held mate rial that contributes to a larger contact angle hysteresis and lesser hydro phobicity.