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The adsorption of aromatic thiols on Cu and the SAM film stability in acidic solutions have been studied by XPS, contact angle and electrochemical techniques. Three short molecules, benzenethiol (BT), 2-naphthalenethiol (2-NT) and 4-acetamidothiophenol (4-AA), were selected as representatives of aromatic thiols to highlight the effect of aromatic rings and hydrophilic terminal groups on the copper protection. All the three molecules form stable S-Cu bonds as a consequence of their adsorption process on polycrystalline copper. Although none of them provides a full copper passivation, the adsorbed films persist without major degradation on Cu electrodes even after 12 h immersion in 0.5 M sulfuric acid. Comparing the freshly prepared adsorbed films, the larger 2-NT molecule provides a better Cu passivation, but the shorter BT molecule favours a higher surface coverage. The terminal groups of 4-AA are responsible for a higher Cu surface wettability in water, compared to that with SAMs of the other molecules, and allow for an easier charge-transfer to the electrolyte and for a higher electrochemical capacitance. After long enough ageing, however, the 4-AA-based molecular films are able to self-organize and to provide a steadily improving copper passivation. Adlayers of the BT and 2-NT molecules, on the contrary, over a long time tend to protect less and less the Cu substrate, probably because of progressive electrolyte infiltration.

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