Deviations from 1 : 1 compositions in self-assembled monolayers formed from adsorption of asymmetric dialkyl disulfides on gold

Alkanethiolate monolayers on polycrystalline gold, formed by typical soluti on self-assembly conditions with immersion in dilute ethanolic solution of asymmetric dialkyl disulfides CH3(CH2)(n-1)SS(CH2)(15)CH3 (n = 4, 8, 12), w ere found to consist predominately of CH3(CH2)(15)S- moieties. This observa tion shows that large deviations from expected 1:1 compositions can occur. The data support an exchange mechanism involving dissociative chemisorption of the S-S moiety and a subsequent replacement of the shol ter chains by t he longer ones via short chain disulfide or thiolate desorption and readsor ption of mixed chain length disulfide. The efficiency of the substitution p rocess increases with increasing length difference of the hydrocarbon chain s in the respective disulfides.