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 exploit these monolayers in the surface forces apparatus (SFA) are described. The length of the alkyl chain of the amphiphiles was 18 carbons (methyl functionality at the terminus) or 20 and 22 carbons (vinyl functionality at the terminus). The thickness of both monolayers, determined independently by ellipsometry and by in situ SFA measurement, was consistent with models in which the alkyl chains were directed away from the surface in all-trans conformations. The average tilt angle of the alkyl chains, determined by Fourier transform infrared spectroscopy, was 13 +/- 5 degrees from the surface normal. The surface energies of the monolayers, determined by in situ SFA measurement of the pull-off forces and contact-area diameters, agreed with independent estimates from contact angle measurements. The monolayer surfaces were of a high degree 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 forces were measured between unmodified mica surfaces. The amount of water in the hydrolysis solutions greatly affected the ability to self-assemble as a tightly-bound monolayer, and the optimal water concentration was determined. All monolayers were resistant to attack by nonpolar solvents. 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.