2D-IR spectroscopy for oil paint conservation: Elucidating the water-sensitive structure of zinc carboxylate clusters in ionomers

Abstract

The molecular structure around metal ions in polymer materials has puzzled researchers for decades. This question has acquired new relevance with the discovery that aged oil paint binders can adopt an ionomer structure when metal ions leached from pigments bind to carboxylate groups on the polymerized oil network. The characteristics of the metal-polymer structure are expected to have important consequences for the rate of oil paint degradation reactions such as metal soap formation and oil hydrolysis. Here, we use two-dimensional infrared (2D-IR) spectroscopy to demonstrate that zinc carboxylates formed in paint films containing zinc white pigment adopt either a coordination chain– or an oxo-type cluster structure. Moreover, it was found that the presence of water governs the relative concentration of these two types of zinc carboxylate coordination. The results pave the way for a molecular approach to paintings conservation and the application of 2D-IR spectroscopy to the study of polymer structure.

Quantifying solvent action in oil paint using portable laser speckle imaging

“…At the same time, hydrolysis of the esters in linseed oil, the liberation of free fatty acids (FAs) and, eventually, the formation of zinc soaps took place. Figure 3a shows a collection of ATR-FTIR spectra, clearly showing the increasing concentration of amorphous zinc carboxylates 38,46,47 (COOZn, broad band at 1585 cm −1 in Fig. 3a) and ultimately, the formation of crystalline zinc soaps after 22 days of ageing (sharp peak at 1540 cm −1 in Fig.…”

The concentration and origins of carboxylic acid groups in oil paint

“…However, thorough understanding of the underlying chemical and physical mechanisms causing paint degradation started only recently. [2][3][4][5][6] The chemical composition of a mature oil paint binder has proved to play an essential role in several of these degradation phenomena. Understanding the driving forces behind degradation processes will ultimately provide useful knowledge for the preservation and conservation of oil paintings.…”

“…2,3 The ionomeric binding medium, like commercial ionomers, 7 contains clusters of metal carboxylates (COOM), oen identied by a broad asymmetric n a COO À infrared (IR) absorption band. 3,6 These ionomeric metal carboxylate complexes were discovered to represent an intermediate stage in paint ageing that can ultimately lead to the appearance of crystalline metal soaps (metal complexes of long chain saturated fatty acids). 5 Metal soaps play an important role in many types of oil paint degradation.…”

“…28 A practical solution is the complexation of COOH groups with Zn 2+ ions (released by ZnO), leading to the appearance of a broad, free-lying IR absorption band for zinc carboxylates (COOZn) centered around 1585 cm À1 . 2,6 We have recently unequivocally assigned the structure and coordination in these COOZn complexes: they adopt either a coordination chain-or an oxo-type cluster structure. 6 The COOZn IR absorption band can be used as a method to accurately estimate the concentration of COOH in polymerised LO that is accessible for reaction with zinc ions.…”

“…2,6 We have recently unequivocally assigned the structure and coordination in these COOZn complexes: they adopt either a coordination chain-or an oxo-type cluster structure. 6 The COOZn IR absorption band can be used as a method to accurately estimate the concentration of COOH in polymerised LO that is accessible for reaction with zinc ions. This concentration is most relevant for oil paint degradation since amorphous COOZn is the intermediate in crystalline metal soap formation.…”