Evolution of Zinc Carboxylate Species in Oil Paint Ionomers

Abstract

Oil paint binding media are complex polymer networks that harbor populations of metal ions. Understanding of the reactivity of these metal ions, often closely linked to paint degradation, is crucial to support paintings conservation efforts. By developing a spectrum fitting approach for the analysis of Fourier transform infrared spectra, we have studied in detail how the molecular structures of zinc carboxylate species in oil paint ionomers change in the lifetime of a painting. It was found that high ZnO pigment content, humidity, and low paint viscosity all stimulate the formation of ionomeric zinc carboxylate species, while the structures (chain or oxo) adopted by those species depend on carboxylate concentration and humidity. Moreover, we found evidence for a difference in reactivity between the two structures for ionomeric zinc carboxylates toward the formation of crystalline zinc soaps. The results have yielded an abundance of information about the internal chemistry of oil paint layers and metal-containing polymers in general.

A Critical Review on the Analysis of Metal Soaps in Oil Paintings

“…[ 9 , 13 , 33 , 50 , 59 , 69 , 74 , 89 , 94 , 95 , 96 ] Many studies have demonstrated how IR spectroscopy can provide essential information both regarding the behaviour of metal ions in oil paint and their surrounding molecular environment. [91] …”

“…After the crystallisation and zinc soaps formation, two different structures may be observed (one that optimizes zinc carboxylate coordination ‐type A‐ and one that optimizes fatty acid chain packing ‐type B). [ 55 , 91 , 100 ] Depending on the nature of fatty acids present in the system, geometry A or B are preferred. The former geometry formed when short‐chain, dicarboxylic and unsaturated fatty acids are present while the latter when long‐chain saturated often all four types are simultaneously present in a sample, an overlap of the signals can occur fatty acids are present.…”

“…The former geometry formed when short‐chain, dicarboxylic and unsaturated fatty acids are present while the latter when long‐chain saturated often all four types are simultaneously present in a sample, an overlap of the signals can occur fatty acids are present. [91] Beerse and co‐workers used ATR‐FTIR to study oil paint model systems and the influence of parameters (i. e. pigment concentration, carboxylic acid concentration and oil viscosity) in the relative concentration of chain and oxo zinc carboxylate species in oil paint binding media. Moreover, thanks to a computational modelling, they were able to simulate the evolution of these zinc carboxylates during paint drying, aging and under high humidity conditions.…”

The stability of paintings and the molecular structure of the oil paint polymeric network

“…As a result, the organic molecular composition of an oil paint layer evolves from polyunsaturated triglycerides to a significantly more complex system, whose composition evolves over years, even centuries, entailing the simultaneous presence of free fatty acids, free dicarboxylic acids, mono-, di- and triglycerides, and cross-linked fractions 2 , 3 , 5 , 7 , 10 , 11 , 13 – 17 . In addition, when metal soaps are formed, carboxyl moieties are bound to metal cations, in variable proportions depending on the age of the paint, the environmental conditions 18 , 19 , and the nature of the pigment 20 , leading to the formation of free metal soaps and ionomer-like networks 21 – 23 .…”

“…Free fatty and dicarboxylic acids can be determined qualitatively and quantitatively by GC-MS based techniques 11 , 13 , 31 , 32 , HPLC-MS 14 and flow injection analysis coupled with ESI-MS 33 . Metal soaps have been extensively studied with spectroscopic techniques which are helping to distinguish between free metal soaps and those belonging to the ionomeric network 12 , 20 , 21 , 23 , 30 , 34 – 40 . Free metal soaps may also be determined by using mass spectrometry 32 , 41 .…”