Characterization of Self-Assembled Monolayers Formed from Sodium S-Alkyl Thiosulfates on Copper

We report the formation of self-assembled monolayers (SAMs) from sodium S-alkyl thiosulfates (CH3(CH2) n - 1S2O3 -Na+) onto copper in aqueous and organic solvents. These ionic compounds are considerably more soluble in water than alkanethiols, and they adsorb to the copper surface primarily as thiolates. Characterization by electrochemical impedance spectroscopy (EIS) reveals that SAMs formed from longer-chained thiosulfates (n = 12, 14) exhibit comparable barrier properties to those of thiol-based SAMs when formed in organic solvents but diminished effectiveness when formed in aqueous solution. However, the water-borne thiosulfate-based SAMs do provide an increase in corrosion resistance by ∼2−3 orders of magnitude compared to that of uncoated copper. Analysis by infrared (IR) spectroscopy indicates that the thiosulfate-based SAMs are less crystalline and less densely packed than thiol-based SAMs. Wetting measurements suggest that the surfaces of thiosulfate-based SAMs expose a greater number of methylene groups in comparison to that for the well-ordered methyl structure of alkanethiol-based SAMs. The combination of these analyses leads to a proposed structure for thiosulfate-based SAMs on copper in which the films contain molecular-scale defects, lower adsorbate density, and more disordered alkyl chains than SAMs formed from alkanethiols. A comparison of thiosulfate-based SAMs to those formed from benzotriazole (BTAH), a widely used corrosion inhibitor for copper, shows that longer-chained thiosulfates may be useful in inhibiting the corrosion of copper in aqueous environments.