Self-assembly mechanism of thiol, dithiol, dithiocarboxylic acid, disulfide and diselenide on gold: An electrochemical impedance study

The self-assembly mechanism of normal aliphatic thiol (RSH), disulfide (RSSR), diselenide (RSeSeR), dithiol (R(SH)₂) and dithiocarboxylic acid (RS₂H) onto a gold surface was studied in real time by electrochemical impedance spectroscopy (EIS). The different stages of adsorption could be clearly followed from the interfacial capacitance variation. An initial very fast adsorption, varying from a few seconds to several minutes depending on concentration, is the major adsorption step. This fast step is followed by long-term additional adsorption and self-assembled monolayer (SAM) consolidation. However, an intermediate step, probably due to transformation from the initial physisorbed state to the self-assembled state, could be identified with RSH and R(SH)₂. An intermediate rearrangement of RS₂H molecules after their initial diffusion controlled Langmuir (DCL) adsorption through the thiol functional group was also recognized. Initial adsorption of RSH and R(SH)₂ followed either purely diffusion controlled or DCL kinetics for a very short time. Their continuing fast adsorption followed DCL kinetics. The fast adsorption step of RSSR and RSeSeR also followed the same mechanism. The findings made with EIS on the SAM organization were analyzed by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The R(SH)₂ based SAMs had comparatively poor organization.