In a cultural heritage context, fatty acids are usually found as breakdown products of lipid-containing organic remains in archaeological findings, binders in aged oil paintings, and additives in modern art-related materials. They may further interact with the ionic environment transforming into metal soaps, a process that has been recognized as a threat in aged paintings but has received less attention in archaeological objects. The investigation of the above related categories of materials with infrared spectroscopy can provide an overall picture of the organic components’ identity and demonstrate their condition and prehistory. The capability of investigating and distinguishing fatty acids and their metal soaps through their rich infrared features, such as the acidic carbonyl, the carboxylate shifts, the variable splits of alkyl chain stretching, bending, twisting, wagging, and rocking vibrations, as well as the hydroxyl peak envelopes and acid dimer bands, allows for their direct detailed characterization. This paper reviews the infrared spectra of selected saturated fatty monoacids and diacids, and their corresponding sodium, calcium, and zinc salts and, supported by newly recorded data, highlights the significance of their spectroscopic features.
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.
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