EFFECT OF GEOMETRIC CONSTRAINTS ON THE SELF-ASSEMBLED MONOLAYER FORMATION OF AROMATIC DISULFIDES ON POLYCRYSTALLINE GOLD

The self-assembled monolayer (SAM) formation tendency of two comparati vely small aromatic disulfide molecules, namely naphthalene disulfide (NDS) and diphenyl disulfide (DDS), has been investigated using quartz crystal microgravimetry (QCM), cyclic voltammetric, and impedance tec hniques. The typical time period for monolayer formation on polycrysta lline 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 mu F/cm(2) for bare gold to similar to 14 and similar to 8 mu F/cm(2) for NDS and DDS, respectively) upon monolayer formation for b oth the cases has been observed, which correlates well with the QCM ar ea per molecule values (similar to 49 and 36 Angstrom(2) for NDS and D DS, respectively). The difference in the permeability of the two monol ayers to simple ionic species was also investigated using K7Fe(CN)(6) as redox probes in aqueous solution. A mixed linear/radial mode of dif fusion is observed at the DDS-modified electrode in contrast to a pred ominant 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 x 10(-5) and 4.1 x 10(-5) cm/s for th e Fe(CN)(6)(3-/4-) couple in the case of NDS and DDS, respectively. Th ese results strongly demonstrate the effect of geometrical constraints in controlling the microscopic structure and the packing density of t he SAMs and highlight the importance of intramolecular conformational changes in controlling the monolayer packing density.