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Kinetics of adsorption of alkanethiols on gold

The field of self-assembled monolayers has grown rapidly since the discovery of these structures (1983) and their ability to modify physical and chemical properties of surfaces. The process of self-assembly consists of two distinct steps: (i) a fast adsorption of head group of an organic molecule on a specific substrate, and (ii) a slow organization of alkyl chains by intermolecular van der Waals interactions. These layers enable to tailor and to optimize surface properties for a variety of technological applications (chemical and biochemical sensors, biocompatibilization of metal surfaces, DNA immobilization, corrosion protection) as well as for fundamental studies of surface phenomena. In this study, the adsorption kinetics of 1-dodecanethiol on the gold substrate was studied in-situ using electrochemical impedance spectroscopy. The reason for the thiol adsorption on gold is based on two considerations: (i) gold is a relatively inert metal and does not form stable oxides on its surface, and (ii) there is a strong specific interaction with sulfur which allows the formation of a stable monolayer. Dependence of the surface coverage on the concentration was described by the Langmuir isotherm at all investigated concentrations of dodecanethiol, ranging from 3×10^(-6) to 1×10^(-3) M, in 0.1 M ethanolic solution of LiClO4. Parameters characterizing the adsorption thermodynamics, the adsorption coefficient, and the free energy of adsorption were determined. The thiol monolayers were characterized using contact angle measurements. The contact angle value (≈ 100°) has shown a high hydrophobicity. The electrochemical properties have been studied in a 0.1 M NaClO4 using cyclic voltammetry and electrochemical impedance spectroscopy. The thiol monolayer behaves as a barrier film, almost an ideal dielectric which effectively prevent electrolyte from contacting the gold substrate.

gold; alkanethiol; self-assembled monolayers; Langmuir adsorption isotherm; electrochemical impedance spectroscopy; cyclic voltammetry; contact angle measurements.

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