Dissolution kinetics of octadecanethiolate monolayers electro-adsorbed on Au(1 1 1)

Monolayers of octadecanethiolate on Au(1 1 1) surface were formed under electrochemical control. The influence of the formation time on the reductive desorption process was studied by cyclic voltammetry and chronoamperometry. When the formation time is increased, the reductive desorption peak observed on the voltammograms is significantly shifted in the negative direction, while the cathodic charge is only slightly affected. This behaviour is attributed to a higher degree of organisation of the monolayers for longer formation times, highlighting the role of defect sites in promoting the dissolution. A good agreement was found between our experimental chronoamperograms and theoretical models describing the dissolution process by a shrinkage mechanism. It is demonstrated that a reorganisation process takes place, consisting in the merging of small condensed domains into larger ones. This annealing phenomenon is time and potential dependent, the largest condensed domains being obtained for the longest formation times and least negative potentials. © 2008 Elsevier B.V. All rights reserved.

Electrochemical and FTIR characterization of the self-assembled monolayer of 2-mercaptobenzimidazole on Au(1 1 1)

The behaviour of a self-assembled monolayer of 2-mercaptobenzimidazole (MBI) at the Au(111) electrode has been examined using cyclic voltammetry and in situ FTIR spectroscopy. The charge associated with the reductive desorption is pH independent while the oxidative partial redeposition charge increases when the pH is lowered. This is due to differences between the nature and the solubility of the MBI desorption product. In alkaline and neutral media MBI desorbs as the thiolate. In contrast, in acidic solutions the thiol is the desorbed product. Subtractively normalized interfacial reflection Fourier transform absorption spectroscopy (SNIFTIRS) has been applied to investigate the MBI monolayer in contact with aqueous solutions of different pH. The SNIFTIRS data are in agreement with the electrochemical results. Moreover, quantitative analysis of the IR data provided evidence that adsorbed MBI molecules assume a tilted orientation with an angle of 60±5° between the C2 axis of the molecule and the direction normal to the gold surface. © 2003 Elsevier B.V. All rights reserved.