Infrared spectroscopy reveals the reactivity of fatty acids on copper surfaces and its implications for cultural heritage objects

Abstract

The reactivities of various fatty monoacids and diacids on copper metal-containing surfaces were investigated through reflection-absorption infrared spectroscopy. The formation of copper soaps is detected on pure copper surfaces, while copper and zinc soaps are simultaneously formed as mixtures on brass surfaces. Following the changes of acidic carbonyl and carboxylate infrared bands, it is shown that fatty monoacids C8, and C10 react with clean/polished copper and its zinc alloy within 2–4 hours, while those with chains > C12 react within 15–40 days. An explanation for the above results is offered on a molecular mobility basis, where liquid monoacids at room temperature favour higher reaction rates. Furthermore, it is argued that longer-chain FMAs allow for favourable orientation resulting in self-assembled monolayer (SAM)-type molecular packing on Cu surface, may explain the slower reaction. Fatty diacids, on the other hand, do not form any carboxylate products under these conditions. In light of this interesting result, it is argued that dicarboxylic acids may also pack as self-assembled layers on Cu and ultimately protect it. The implications for fatty organic remains in copper containers are discussed, with emphasis on the stability of archaeological copper metal and carboxylates serving as molecular markers for the presence of fatty monoacids as residues in archaeological containers. Besides, the utilization of fatty acids, based on the above behaviour in copper-based dosimeters, such as those used in the Oddy test, is suggested.