Hydrophobic monolayer preparation by Langmuir–Blodgett and chemical adsorption techniques

Alkylsiloxane and perfluoroalkylsiloxane monolayers are prepared on siliceous surfaces using the techniques of Langmuir–Blodgett deposition and solid–liquid chemical adsorption. Acid-catalyzed hydrolysis and polycondensation reactions provide two-dimensional siloxane networks at the liquid–vapor interface, which can be compressed to mean molecular areas of ∼22 and ∼32 Å2 for pendent hydrocarbon and fluorocarbon chains, respectively. Subsequent Langmuir–Blodgett transfer onto glass substrates at moderate surface pressures leads to compact monolayers for single-component precursors, while mixed alkyl- and perfluoroalkylsilanes produce nonhomogeneous films characterized by transfer ratios greater than unity. As an alternate monolayer preparation technique, silane polymerization was performed directly on siliceous surfaces via a chemical adsorption mechanism. XPS analysis of a chemically adsorbed 1H,1H,2H,2H-perfluorodecylsiloxane film confirms a single adsorbed monolayer thickness in which the pendent fluoroalkyl chains align nonperpendicularly with respect to the surface. The surface free energy was determined to be 11.4 dyn cm−1 based on static contact angle measurements. AFM imaging shows the presence of surface defects due to oligomer deposition during the drying process. The use of solubilized trichloro-based silane coupling agents under anhydrous conditions is shown to produce surfaces with a minimal number of surface defects. The presence of undissolved silane material in the bulk solution significantly increases the number of surface defects.

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