Substrate and solution effects on the long-range "hydrophobic" interactions between hydrophobized gold surfaces

Hydrophobic surfaces prepared by adsorption of hexadecanethiol, 1,10-dithiodecane, octadecanoic acid, and hexadecanol onto gold have been used to study long-range "hydrophobic" interactions, and direct-force measurements were performed in water, aqueous NaCl electrolytes, and water/ethanol mixtures, using a bimorph surface force instrument. Results confirm that for very stable hydrophobic surfaces the contact angle is sufficient to predict the presence of an attraction in excess of van der Waals forces, in which case the attraction is caused by the coalesence of microscopic bubbles on the surfaces. For the less-stable hydrophobic films, the properties of the adsorbed layer are important for the qualitative nature of the interaction. For such surfaces different-and as yet unknown-mechanisms cause the attraction.