Self-assembled monolayers of alkanethiolates on thin gold films as substrates for surface force measurements. Long-range hydrophobic interactions and electrostatic double-layer interactions

Surfaces prepared by self-assembly of alkanethiolates onto thin (10 nm) gold films supported on glass have been used as substrates for surface force measurements between macroscopic surfaces. Surface roughness, the order in the monomolecular film, wetting properties, and their stability in aqueous electrolyte solutions have been investigated using atomic force microscopy, infrared absorption spectroscopy, and contact angle measurements. Direct force measurements have been performed with a noninterferometric bimorph surface force apparatus, using surfaces with differently functionalized thiolates; measurements were made in air, water, and aqueous electrolyte solutions. Results from force measurements between hydrophobic and neutral hydrophilic surfaces in air and water are presented and discussed, as well as some results from measurements with surfaces exposing carboxylic groups in various aqueous electrolyte solutions. It is demonstrated that alkanethiol monolayers self-assembled onto thin gold films on macroscopic surfaces are well-suited as substrates for direct measurements of long-range surface forces. The utility of these surfaces as substrates for investigations of short-range phenomena is limited until the effect of the roughness on the adhesion between the surfaces has been properly quantified and the contribution of the van der Waals force to the total interaction has been calculated. The latter is difficult to assess due to the large number of layers in the system and the presence of conducting surfaces.