Several forces drive the assembly of alkyl thiols on a gold surface. First, the sulfur-gold interactions are quite strong, ~45 kcal/mol, which allows for a relatively strong bonding of the film-forming molecules to the surface. Furthermore, hydrophobic interactions between carbon and hydrogen atoms in the alkyl thiol molecules can significantly lower the overall surface energy thus promoting the formation of a self-assembled monolayer, especially if the alkyl chain contains at least ten carbon atoms. 1,2
References: 1. M.D. Porter et al. J. Am. Chem. Soc., 109, 3559 (1987). 2. C.D. Bain et al. J. Am. Chem. Soc., 111, 7155 (1989).