Characterization and reaction kinetics of zinc carboxylates (soaps) relavant to oil paint degradation
Date
2022
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Publisher
University of Delaware
Abstract
Chapter 1: Nuclear magnetic resonance spectroscopy has advanced our understanding of cultural heritage objects. Solution NMR, solid state NMR, unilateral NMR, magnetic resonance imaging (MRI) and other MR techniques have been used on a wide variety of materials such as stone, ceramics, paintings, biological remains, paper, wood, textiles, resins, gums, and synthetic materials. This review highlights NMR studies that provide structural and chemical identification, moisture content and distribution, uncovers artistic techniques, determines geographical origins, identifies constituent materials of an object and helps to determine the best cleaning or treatment method for conservation. In addition, physical and chemical transformations and structural modifications due to deterioration of an object can be monitored by NMR methods, and this information provides conservators with clues as to the most appropriate methods of preservation of a unique artifact. With the continued development of NMR pulse sequences, probes and sensors, the sensitivity and utility of NMR spectroscopy in cultural heritage continues to grow. ☐ Chapter 2: A series of medium- and long-chain zinc carboxylates (zinc octanoate, zinc nonanoate, zinc decanoate, zinc undecanoate, zinc dodecanoate, zinc pivalate, zinc stearate, zinc palmitate, zinc oleate, and zinc azelate) was analyzed by ultra-high-field 67Zn NMR spectroscopy up to 35.2 T, as well as 13C NMR and FTIR spectroscopy. We also report the single-crystal X-ray diffraction structures of zinc nonanoate, zinc decanoate, and zincxixoleate—the first long-chain carboxylate single-crystals to be reported for zinc. The NMR and X-ray results suggest that the carboxylates exist in three distinct geometric groups, based on structural and spectroscopic parameters. The ssNMR results presented here present a pathway for future DNP-NMR-based, minimally invasive method for testing artwork for the presence of zinc carboxylates. ☐ Chapter 3: The formation of heavy-metal carboxylates, colloquially known as soaps, in masterworks of art has become a concern of art conservators around the world. The reaction of free fatty acids from the oil binder with heavy-metal pigment particles to produce inclusions of soaps, such as lead or zinc palmitate, is essentially inherent in many oil paint-based works. The exact underlying chemical process is not known; however, we hypothesize a multi-step process summarized as 1. Free fatty acid (FFAs) formation; 2. Initial saponification; 3. Nucleation of soap; 4. Irreversible damage becomes visible. Monitoring the chemical reaction of zinc soap formation under forced circumstances (excess 13C-labeled FFAs) via NMR spectroscopy indicated a highly coincident nature of steps 2 and 3 affected by both the presence (or absence) of aluminum stearate and the relative humidity. The experimental data is well fit to the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model for diffusion-limited nucleation.
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Keywords
Carboxylates, Cultural heritage, NMR, Paints, Soaps, Zinc