Self-Assembled Monolayers of aromatic thiols on gold and copper in aqueous environment: an electrochemical and spectroscopic study

Thiolate Self-Assembled Monolayers (SAMs) offer the unique opportunity to tailor surface properties of noble and coinage metal in extremely fine way through a simple bottom-up approach, making them very attractive in a wide range of technological applications. On copper surfaces, n-alkanethiolate monolayers have been extensively studied as corrosion inhibitors in aqueous solutions revealing excellent performances but relatively poor stability, a problem affecting SAMs in many of them applications. On the other hand, very few studies have been performed on SAMs constituted by simple aromatic thiols and none of them assessed their long term stability, although the relatively strong ring interaction should in principle positively affect the layer durability. In the present work the protective properties and the long term stability in strongly acidic solution (H2SO4 0.5 M) of two aromatic SAMs, namely Benzenethiol (BT) and 2- Naphthalenethiol (2-NT) was compared to that of one long-chain n-alkylic SAM, the 1- Undecanethiol (1-UT) by means of XPS, Raman spectroscopy, DCA and electrochemical techniques. The results not only demonstrated the aromatic SAMs to be remarkably stable, but also highlighted a favourable influence of the aqueous environments on the layers structures, resulting in a durable enhancement of their protective properties. Subsequently, the effect of different p-substituent groups (-F, -CH3, -OH, -COOH, - NHCOCH3) on the performance and the stability of the aromatic SAMs was investigated. All such molecules, similarly to BT, showed a steep increase of their inhibition efficiency upon ageing. The rate and the durability of the enhancement resulted mainly dependent from the polar effect of the tail group; moreover, the hydrophobicity seems to influence the protective properties of the layers as well. Finally SAMs of BT and 1-UT on gold, both freshly prepared and aged in ultrapure water, were characterized electrochemically. Again BT layers showed an improvement of their structural order following the exposure to aqueous environments, indirectly supporting the hypothesis of a direct involvement of water molecules in this process. In addition, a new method to prepare high quality aromatic SAMs on gold has been proposed.