In this paper, we demonstrate that organic thiosulfates (Bunte salts) with the general formula R−SSO3M, where R is either an aliphatic or aromatic group and M a monovalent cation, constitute a novel class of surface-active compounds with a sulfur-containing headgroup. Bunte salts form self-assembled monolayers (SAMs) on gold under anaerobic conditions and chemisorb forming a Au−S bond, in which the chemical nature of sulfur is indistinguishable by X-ray photoelectron spectroscopy (XPS) from gold thiolate formed upon chemisorption of thiols and disulfides. The S−SO3 bond in the thiosulfate is cleaved during adsorption on the gold surface and the sulfite moiety is released. We have prepared one alkyl thiosulfate (sodium S-dodecylthiosulfate, C12SSO3Na) and two aromatic redox-active thiosulfates (potassium S-(2,5-dihydroxyphenyl)thiosulfate, QSSO3K, and dipotassium S,S ‘-(3,6-dihydroxy-1,2-phenylene)bisthiosulfate, Q(SSO3K)2) and compared the formation and properties of the SAMs prepared from these Bunte salts and the corresponding thiols (1-dodecylmercaptan, C12SH, and 1,4-dihydroxy-2-mercaptobenzene, QSH) using XPS, cyclic voltammetry, and ac impedance spectroscopy. The chemisorption of Bunte salts takes place 1−2 orders of magnitude slower than the adsorption of thiols. The SAMs formed from aromatic Bunte salts QSSO3K or Q(SSO3K)2 have lower surface coverage than those prepared using QSH. With aliphatic compounds, the films prepared from Bunte salts are either slightly or relatively well-blocking, although they do not reach the quality achieved with thiol-based SAMs. The differences in the adsorption time scale and surface coverage are attributed to the bulky thiosulfate headgroup. A major advantage of using Bunte salts derives from the general synthetic pathway to organic thiosulfates, generally involving a one-pot synthesis starting from the corresponding halides and yielding the products as odorless crystalline compounds. This offers a synthetically feasible way of introducing a sulfur-containing surface-active headgroup into several redox-active or other functional molecules, allowing their incorporation in SAMs. This facilitates the preparation of functional monolayers for applications in sensor technology and molecular electronics.

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