Effect of co-adsorbed surfactant on the structure of self-assembled monolayer of thiol on polycrystalline gold

Abstract

The structure and formation kinetics of a self-assembled monolayer (SAM) of long-chain alkane thiol in the presence and absence of an anionic surfactant like sodium dodecyl sulfate (SDS) has been studied using impedance, cyclic voltammetic, and quartz crystal microbalance (QCM) measurements. The comparison of these results, especially the ionic permeability of the monolayer, suggests a composite monolayer where both the surfactant and thiol molecules are cooperatively adsorbed to form distinct surfactant patches. This is further substantiated by the voltammetric experiments with redox probe in solution. A gradual change in the double layer capacitance value for the thiol monolayer alone (1.4 µ F/cm²) to the composite monolayer (6.7 µ F/cm²) and to the surfactant alone (11 µ F/cm²) further supports the nature of the mixed monolayer. The kinetics of monolayer formation also shows interesting changes as revealed by the QCM studies, where a phase transition from simple to composite monolayer has been observed around an approximate coverage of 0.44. The same growth scheme is also observed for 1-pentanethiol, naphthalene disulfide, and diphenyl disulfide, suggesting that the mechanism appears to be general.