Effect of geometric constraints on the self-assembled monolayer formation of aromatic disulfides on polycrystalline gold

Abstract

The self-assembled monolayer (SAM) formation tendency of two comparatively small aromatic disulfide molecules, namely naphthalene disulfide (NDS) and diphenyl disulfide (DDS), has been investigated using quartz crystal microgravimetry (QCM), cyclic voltammetric, and impedance techniques. The typical time period for monolayer formation on polycrystalline gold has been found to be about 2 h for both the molecules from the QCM data. A significant change in double-layer capacitance values (from 21 μF/cm² for bare gold to ~14 and ~8 μF/cm² for NDS and DDS, respectively) upon monolayer formation for both the cases has been observed, which correlates well with the QCM area per molecule values (~49 and 36 Ų for NDS and DDS, respectively). The difference in the permeability of the two monolayers to simple ionic species was also investigated using K₃Fe(CN)₆ as redox probes in aqueous solution. A mixed linear/radial mode of diffusion is observed at the DDS-modified electrode in contrast to a predominant linear one at the electrode derivatized with a NDS monolayer. Impedance measurements indicate apparent surface coverages of 99.6 and 99.8% and rate constants of 9.4 × 10⁻⁵ and 4.1 × 10⁻⁵ cm/s for the Fe(CN)₆^3−/4− couple in the case of NDS and DDS, respectively. These results strongly demonstrate the effect of geometrical constraints in controlling the microscopic structure and the packing density of the SAMs and highlight the importance of intramolecular conformational changes in controlling the monolayer packing density.