Formation of a self-assembled monolayer via the electrospreading of physisorbed micelles of thiolates

We present a spectroelectrochemical study of the reductive desorption/oxidative redeposition of a monolayer of hexadecanethiols observed in an aqueous alkaline electrolyte solution. We found that the solvent/thiol interactions play an important role in the electrodeposition of self-assembled monolayers of insoluble thiols such as hexadecanethiols. Ex situ vibrational spectroscopy shows that the same monolayer can go from a chemisorbed state to a physisorbed state and back to a chemisorbed state without noticeable change in the orientation of the alkane chains. In situ spectroelectrochemical measurements reveal that the reductive desorption and the oxidative redeposition proceed through the same two-step mechanism. The first step of the reductive desorption is assigned to the reduction of the chemisorbed thiols. The thiolates formed in this step remain physisorbed, and only a small change in the orientation of the alkane chains is observed. The second step of the reduction occurs at more negative potentials and is assigned to the formation of physisorbed micelles of thiolates. This gives rise to a capacitive current and an important reorientation of the thiolates. The physisorbed micelles of thiolates are oxidatively redeposited on the positive-going potential scan of a cyclic voltammogram by the same two steps but in the reverse order.