Interactions between hydrophobic self-assembled monolayers. Effect of salt and the chemical potential of water on adhesion

The focus of this paper was to obtain an understanding of the forces acting between hydrophobic surfaces submerged in aqueous solutions. An atomic force microscope was used to measure hydrophobic forces between self-assembled monolayers of hexadecanethiol. Electrolytes do not alter the solvent/surface interactions as indicated by the lack of electrostatic repulsive forces at any concentration and the insensitivity of contact angles to salt type or concentration. The jump in distances were systematically larger than can be predicted for gold/gold and hexadecane/hexadecane surfaces interacting across water. Adhesive forces increase with electrolyte concentration and are sensitive to electrolyte type at a given concentration. However, the adhesive forces fall onto a master curve when plot ted as a function of the water chemical potential. Our studies suggest that lowering the solvent chemical potential produces an increase in the pull-off force.