Reaction of Pb(II) and Zn(II) with Ethyl Linoleate To Form Structured Hybrid Inorganic–Organic Complexes: A Model for Degradation in Historic Paint Films

Abstract

To investigate soap formation in drying oils in historic paints, the reaction between metal acetates (K + , Zn 2+ , Pb 2+ ) and ethyl linoleate (EL) was studied using optical microscopy, X-ray powder diffraction, and electron microscopy. Pb(II) and Zn(II) react rapidly with EL to form highly structured, spherulitic, luminescent crystallites that aggregate. Evidence from Fourier transform infrared (FTIR) and scanning electron microscopy/energy dispersive X-ray analysis and high-resolution synchrotron powder X-ray diffraction indicates that these are organic–inorganic hybrid complexes or coordination polymers. FTIR absorbance peaks at ca. 1540 cm –1 for Pb(II) and ca. 1580 cm –1 for Zn(II) are consistent with the formation of carboxylate complexes. The complexes formed offer insight into the degradation processes observed in oil paint films, suggesting that soap formation is rapid when metal ions are solubilized and can occur with unsaturated fatty acids that are present in fresh oils. These complexes may account for the atypical luminescence observed in lead-containing cured oil paint films.

Electron Microscopy Imaging of Zinc Soaps Nucleation in Oil Paint

“…The lack of any crystalline metal soap features could mean that the zinc soap accumulations are still too disordered to exhibit sharp IR bands and are actually contributing to the broadband in Figure 3d. This interpretation is supported by the work of MacDonald et al (2016), who found very similar domains of (semi-) crystalline metal carboxylate in ethyl linoleate reacted with lead or zinc acetate. In that system, XRD of an isolated metal carboxylate accumulation showed some weak crystalline features, while an FTIR spectrum of the same sample exhibited only a broad amorphous metal carboxylate band.…”

Investigating the Photocatalytic Degradation of Oil Paint using ATR-IR and AFM-IR

“…Furthermore, in artists paint the complexity of this process is compounded by interaction with the inorganic pigments, which can affect both the drying process and the degradation kinetics. For example, pigments behaving as Lewis acids or bases locally induce metal salt or soap formation and may accelerate hydrolysis, 8,9,10,11,12,13,14 whereas photocatalytic pigments, such as titanium white, can induce additional radical formation. 15,16 Titanium white pigments were introduced in the beginning of the 20 th century, and paints containing these pigments have been used in many renowned artworks produced since (e.g., paintings by Picasso, Mondrian, and Pollock).…”

Revealing the Distribution of Metal Carboxylates in Oil Paint from the Micro‐ to Nanoscale

“…[3f] A broad carboxylate absorption also develops in model complexes obtained by reacting zinc acetate and ethyl linoleate, suggesting that soap formation in oil paint involves polyunsaturated fatty acids which, however, oxidize rapidly and are not present in historical paints. [7] Hereafter, we refer to this feature in the IR spectra as "zinc carboxylate". In the paint layer, peaks at 1540 cm -1 (νas (COO -), sharp), 1464 cm -1 (δ (CH2)) and 1398 cm -1 (νs (COO -)) can be attributed to zinc stearate.…”