Self-assembled monolayers (SAMs) of organothiols on metal surfaces have attracted enormous attention in recent years owing to their ease-of-preparation, long-term stability, and controllable surface chemical functionality. The understanding of the molecular packing of SAMs is important for the interpretation of fundamental studies and for the development of practical applications. SAMs of aromatic compounds with two ligands (like mercaptopyridine, bipyridine and pyridine-carboxylic acids) on gold are receiving a strong interest in bioelectrochemistry due to their functionality in order to facilitate direct electron transfer of metalloproteins such as cytochrome c [1] as well as the possibility of interacting via the ð-system [2].
In this paper, potentiodynamic and potentiostatic methods combined with electrochemical impedance spectroscopy (EIS) measurements as well as the ellipsometric characterisation are used in order to compare the adsorption of two aromatic molecules in a wide range of conditions. Preparation of monolayers of mercaptopyridine (N or S-bonded) and isonicotinic acid (N or COO-bonded) on monocrystalline gold surfaces is reported. The influence of temperature and time of dipping in the electrical and optical properties of the adsorbed layers is analysed. The impedance characteristics of these systems are discussed in terms of ideal and non-ideal circuit networks and associated with electrical analogs of the corresponding physical and chemical processes taking place at the interface.
