Formation of n-Alkanethiolate Self-Assembled Monolayers onto Gold in Aqueous Micellar Solutions of n-Alkyltrimethylammonium Bromides

We have investigated the kinetics of formation for n-alkanethiolate self-assembled monolayers (SAMs) onto gold in aqueous micellar solutions of n-alkyltrimethylammonium bromides (CmTAB; m = 12, 14, 16, and 18). The cationic micelles provide hydrophobic domains to solubilize the alkanethiols and facilitate their delivery to the gold surface. The kinetics results for SAM formation in aqueous micellar solutions of CmTAB are well described by a first-order Langmuir adsorption model. The measured rate constant decreases exponentially with increasing hydrophobicity (chain length) of the alkanethiol adsorbate and the CmTAB surfactant. The measured rate also decreases with increasingly positive potentials of the gold electrode. A mechanism to describe SAM formation in CmTAB(aq) that is consistent with reported results of solute exchange between ionic micelles in solution consists of (1) fragmentation of thiol-laden micelles to produce submicelles, (2) incorporation of thiol-laden submicelles into adsorbed micelles (admicelles) at the gold surface, (3) displacement of surfactants and counterions (rate-limiting step) by the alkanethiols, and (4) chemisorption of the alkanethiol.