But aren’t all soaps metal soaps? A review of applications, physico-chemical properties of metal soaps and their occurrence in cultural heritage studies

Abstract

Metal soaps, the organic salts resulting from the interaction of fatty acids and metal cations, arouse interest in the scientific field because of their versatility in a great range of chemical applications as well as because of the mechanism of their formation during degradation processes. This article presents a review of the synthetic pathways used to produce metal soaps, their relevant physico-chemical properties, and how these reflect in their applications. Common industrial uses of metal soaps are reported, with a particular focus on those applications, such as cosmetics, paints, and coatings, that have an impact on the cultural heritage field. In addition, the occurrence of metal soaps in cultural heritage studies is presented, ranging from archaeological and ethnographic artefacts to fine art objects, and discussed per class of materials. An overview of the presence or absence of metal soaps in historical artefacts due to the interaction of metal parts or mineral pigments with fatty acids is given herein. This collection shows a variety of situations in which metal soaps—particularly lead, zinc and copper soaps—can form on composite objects made of different materials such as wood, leather and fatty-acid-containing materials (e.g., waxes), in the presence of metal, metal alloys or pigments.

A Comprehensive Study for Determination of Free Fatty Acids in Selected Biological Materials: A Review

“…The copper soap method is a calorimetric method. In the methodology, oil (~8 g) and standard alkali (~30 mL) are titrated to determine the acid value, which is called “saponification” [ 53 , 54 ]. Cu(NO 3 ) 2 .3H 2 O and cupric acetate–pyridine are likely to be employed as the copper reagent [ 55 ].…”

“…When the % FFA increases, the % transmittance becomes lower owing to the intense color change [ 53 ]. These copper soaps are highly soluble in organic solvents, such as benzene, chlorobenzene, xylene, 1-propanol, 1-butanol, and 1-pentanol, and they may have a corrosive impact in the engines, which necessitates a protective layer [ 54 , 56 ]. Nevertheless, the copper soaps can be used in cosmetic and pharmaceutical applications due to their antibacterial properties [ 54 ].…”

“…These copper soaps are highly soluble in organic solvents, such as benzene, chlorobenzene, xylene, 1-propanol, 1-butanol, and 1-pentanol, and they may have a corrosive impact in the engines, which necessitates a protective layer [ 54 , 56 ]. Nevertheless, the copper soaps can be used in cosmetic and pharmaceutical applications due to their antibacterial properties [ 54 ]. In a similar perspective, Bhutra et al [ 57 ] investigated biomedical use of copper soaps, which was derived from the FFA analysis of VO and UCO.…”

Interpretation of the Infrared Spectra of Metal-Stearate Salts

“…Fatty acid metal salts are widely used in multiple industries and final products. 1 Typically, they are produced from a reaction of a fatty acid such as stearic acid with a metal oxide resulting in a long-chain aliphatic compound with a terminal carboxylate (RCO 2 -). They are a major soap component and are used in plastics, rubber, paints and finishes, cosmetics, greases, and oils.…”

“…The differing physicochemical properties of these metal soaps result in their use in a wide variety of applications. 1 Consequently, a practicing vibrational spectroscopist is highly likely to encounter stearic acid metal salts on multiple occasions and should be knowledgeable of their characteristic infrared spectral bands.…”