Tarnish protection of silver by octadecanethiol self-assembled monolayers prepared in aqueous micellar solution

Silver will tarnish when it is exposed to the sulfur polluted atmosphere. Self-assembled monolayers are an efficient, invisible and protective film against tarnishing. SAMs prepared by the adsorption of octadecanethiol onto silver in aqueous micellar solution are explored. The silver was first degreased by acetone in an ultra-sonicate bath, followed by activation in 10% sulphuric acid solution, then, immersed in aqueous micellar solution with 0.15 mol/L octadecanethiol and 40 g/L Cetane trimethyl ammonium bromide (CTAB), Triton X-100 (TX-100), Polyoxyethylene lauryl ether (PLE) or Pluronic PE6400 during 5 min at 60 °C. The film was characterized by contact angles, FT-IR, XPS and AFM. The film was totally composed of octadecanethiol and was influenced by the surfactant in the thickness and tilt angle. The anti-tarnishing ability of the film on the silver was evaluated by electrochemical measurements in 0.5 mol/L NaCl solution containing 10 mmol/L Na2S. The protection efficiency reached 91.9% The electro-oxidation reaction process was reduced substantially in 0.5 mol/L NaOH solution. EIS data proved that the inhibition of the monolayers was marked and the coverage of the film was more than 94.5%. The color, gloss and reflectance of silver with SAMs were almost similar to the bare silver. It was found that after 48 h accelerated tarnishing test, the protection of the thiol film remained efficient. The film controlled markedly the transfer of ion to the surface and the sulfuration rate of silver was reduced substantially.