a,w-alcanedithiol self assembled monolayers (SAMs) prepared by the immersion method in millimolar dithiol solutions have reductive desorption charges which are higher than the charges of the corresponding monothiols [1, 2]. We have attributed the extra reduction charge of the dithiol SAMs to the reduction of intralayer disulfide bonds at the SAM/electrolyte interface [2]. The breakage of S-S bonds and S-Au bonds seem to occur at the same electrode potential. The presence of surface disulfide bonds are expected to disorder the monolayer structure and this was confirmed by STM measurements which showed no structure on the nanoscale [3]. Based on density functional theory calculations we predicted that the surface concentration of disulfide bonds should decrease with the increase in the alkyl chain length [2]. The key role of the chain length and the preparation procedure in the surface structure and chemistry of SAMs of dithiols on Au(111) were reported recently [4,5].
In this work we employed reductive desorption experiments and electrochemical impedance spectroscopy to characterize the amount of surface disulfides and the integrity of SAMs of dithiols prepared under different experimental conditions.
The monolayer formation was investigated in polar and apolar solvents, in the presence and absence of O2 and using different disulfide reducing agents (TCEP, DTT and bME) in the forming solution. All investigations were performed for alkanedithiols of different chain lengths. Reducing agents were employed during and after the formation of the SAM. Optimal experimental conditions were found for the formation of dithiol SAMs with no disulfide bonds for both short and long alkyl chains.
