Kinetics of Formation for n-Alkanethiolate Self-Assembled Monolayers onto Gold in Aqueous Micellar Solutions of C12E6 and C12E7

We have investigated the kinetics of formation for n-alkanethiolate self-assembled monolayers (SAMs) onto gold from aqueous micellar solutions of hexaethylene glycol monododecyl ether (C12E6) and heptaethylene glycol monododecyl ether (C12E7). Micelles provide hydrophobic domains to solubilize the alkanethiols in aqueous solution and facilitate their delivery to the gold surface. The kinetics of SAM formation depends on the micellar size, the concentration of solubilized thiol, and the alkanethiol chain length. The kinetics data for SAM formation in aqueous micellar solutions of C12E6 and C12E7 are best fit by a diffusion-limited, second-order Langmuir adsorption model that accounts for diffusion of the thiol-laden micelles and release of the alkanethiols from the micelles. The observed rate constant for SAM formation decreases exponentially with alkanethiol chain length, consistent with an activated diffusion process for the release of the alkanethiol from the micelle. The activation energy for release of the thiols from the C12E6 and C12E7 micelles to the metal surface is ∼0.22 kcal/mol per CH2 group in the alkanethiol. This relatively small barrier is consistent with a mechanism in which alkanethiols release from micelles to admicelles that are present on the gold surface through a collision-induced process.