SELF-ASSEMBLED MONOLAYERS ON MICA FOR EXPERIMENTS UTILIZING THE SURFACE FORCES APPARATUS

Strongly bound, closely packed self-assembled monolayers of condensed alkyltriethoxysilanes were formed on mica and other substrates with a packing density of about 20 Angstrom(2)/molecule. Techniques to exploi t these monolayers in the surface forces apparatus (SFA) are described . The length of the alkyl chain of the amphiphiles was 18 carbons (met hyl functionality at the terminus) or 20 and 22 carbons (vinyl functio nality at the terminus). The thickness of both monolayers, determined independently by ellipsometry and by in situ SFA measurement, was cons istent with models in which the alkyl chains were directed away from t he surface in all-trans conformations. The average tilt angle of the a lkyl chains, determined by Fourier transform infrared spectroscopy, wa s 13 +/- 5 degrees from the surface normal. The surface energies of th e monolayers, determined by in situ SFA measurement of the pull-off fo rces and contact-area diameters, agreed with independent estimates fro m contact angle measurements. The monolayer surfaces were of a high de gree of smoothness: when undecane (C11H24) fluid was confined between the surfaces, oscillatory force-distance profiles were measured over a range of liquid thickness similar to that over which oscillatory forc es were measured between unmodified mica surfaces. The amount of water in the hydrolysis solutions greatly affected the ability to self-asse mble as a tightly-bound monolayer, and the optimal water concentration was determined. All monolayers were resistant to attack by nonpolar s olvents. The vinyl-terminated species could be chemically converted to the hydroxyl form, thereby creating a polar surface without apparent change in the organization of the monolayer.