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-a lkyl thiosulfates (CH3(CH2)(n-1)S2O3-Na+) onto copper in aqueous and organi c solvents. These ionic compounds are considerably more soluble in water th an alkanethiols, and they adsorb to the copper surface primarily as thiolat es. Characterization by electrochemical impedance spectroscopy (EIS) reveal s that SAMs formed from longer-chained thiosulfates (n = 12, 14) exhibit co mparable barrier properties to those of thiol-based SAMs when formed in org anic solvents but diminished effectiveness when formed in aqueous solution. However, the water-borne thiosulfate-based SAMs do provide an increase in corrosion resistance by similar to2-3 orders of magnitude compared to that of uncoated copper. Analysis by infrared (IR) spectroscopy indicates that t he thiosulfate-based SAMs are less crystalline and less densely packed than thiol-based SAMs. Wetting measurements suggest that the surfaces of thiosu lfate-based SAMs expose a greater number of methylene groups in comparison to that for the well-ordered methyl structure of alkanethiol-based SAMs. Th e combination of these analyses leads to a proposed structure for thiosulfa te-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, sh ows that longer-chained thiosulfates may be useful in inhibiting the corros ion of copper in aqueous environments.