Ionomer-like structure in mature oil paint binding media

Infrared spectra of samples from oil paintings often show metal carboxylate bands that are broader and shifted compared to those of crystalline metal soap standards (metal complexes of long-chain saturated fatty acids). Using quantitative attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), it is demonstrated that the broad metal carboxylate band is typically too intense to be explained by carboxylates adsorbed on the surface of pigment particles or disordered metal complexes of saturated fatty acids. The metal carboxylate species associated with the broad bands must therefore be an integral part of the polymerized binding medium. Small-angle X-ray scattering (SAXS) measurements on model ionomer systems based on linseed oil revealed that the medium contains ionic clusters similar to more classical ionomers. These structural similarities are very helpful in understanding the chemistry of mature oil paint binding media and the potential degradation mechanisms that affect oil paintings.


Composition of SAXS samples
Table 1: Overview of the composition of samples for SAXS measurements.LO is linseed oil, So is sorbic acid, MSo is either zinc or lead sorbate, So/LO refers to the molar ratio between sorbate molecules (either as free acid or metal complex) and linseed oil, and COOM/COOH refers to the proportion of total sorbate molecules that is bound to a metal ion (

Derivation of maximal COOM/COOR in case of pigmentbound metal carboxylates
With quantitative ATR-FTIR, it is possible to calculate the molar ratio between metal carboxylate bonds (COOM) and ester groups (COOR) in a (model) paint sample.We can calculate the maximum value of this ratio given the hypothesis that all COOM bonds contributing to the broad COOM band in the FTIR spectrum correspond to carboxylates bound to the surface of a pigment particle.
Making the assumption that all pigment particles are approximately spherical and monodisperse, the volume of each pigment particle is where r part is the radius of a particle.Since the total volume of pigment is simply V p = m p /ρ p (where m p and ρ p are the mass and density of the pigment, respectively), the number of pigment particles n part can be calculated as (2) The total available pigment surface area A p is then given by The pigment surface area needed for the adsorption of one carboxylate group is denoted by A COO .In our calculations, we derive a value for this parameter by assuming the dense packing of crystalline palmitic acid for carboxylates on the pigment surface (Moreno, E., et al. (2006).Acta Cryst., C62, o129-o131).This packing is probably an overestimate of the maximum surface coverage of disordered carboxylates on the pigment particles, but is useful as a limiting case.Using the dimensions of the unit cell in crystalline palmitic acid, we find that A COO = 23.3Å2 .The maximum number of COOM bonds n COOM that can form is The number of COOR groups n COOR in a given mass m LO of fresh linseed oil (LO) is where M w,LO is the average molecular mass of linseed oil (876 g mol −1 ), N A is the Avogadro constant, and the factor 3 represents the fact that each triglyceride in linseed oil contains three COOR moieties.We now make the additional assumption that every carboxylate group bound to a pigment surface is the result of hydrolysis of a linseed oil ester group, reducing the number of ester groups present in the system once a mixture of pigment and oil has been formed and cured, and further increasing the COOM/COOR ratio within the pigment surface hypothesis.This assumption is highly unlikely since many carboxylate groups will form through oxidation of the unsaturations in linseed oil triglycerides.However, since we do not have quantitative information on the relative concentration of these two types of carboxylate groups (formed through hydrolysis or oxidation), we favor the pigment surface hypothesis and assume all carboxylate groups derive from esters.In this case, the final maximum ratio between COOM and COOR groups in a system of pigment and oil is Finally, defining a weight ratio pigment to oil R wt = m p /m LO , replacing particle radii with diameters and some reorganization, the relation between the maximum COOM/COOR ratio and the pigment particle size and concentration as follows: This equation is plotted as function of particle size for a number of pigment concentrations in the main article, showing that for realistic particle sizes and pigment concentrations the maximum COOM/COOR ratio does not come close to experimental values obtained from model paints of ZnO in linseed oil.
4 ATR-FTIR spectra of zinc and lead ionomer samples  S1.

Figure 1 :
Figure 1: ATR-FTIR spectra of various zinc white paints, as reported by Osmond and co-workers in Osmond, et al. (2012).Appl.Spectrosc., 66(10), 1136-1144.(a) Windsor & Newton zinc white in safflower oil, film prepared in 1978, top of film; (b) Windsor & Newton zinc white in safflower oil, film prepared in 1978, underside of film; (c) custom ZnO paint with linseed oil, mixed with litharge, film prepared in 1990, underside of film; (d) custom ZnO paint with boiled linseed oil, film prepared in 1990, top of film; (e) custom ZnO paint with cold-pressed linseed oil, film prepared in 1990, underside of film.Dashed lines indicate the band integration limits used after ester CO band subtraction and spectral processing.See original paper by Osmond and co-workers for further details.

Figure 2 :
Figure2: ATR-FTIR spectra of (left) zinc and (right) lead ionomer samples used for the SAXS measurements discussed in the main article.Spectra were basline corrected and normalized to the CH 2 vibration band at 2925 cm −1 .Sample codes correspond to the compositions noted in TableS1.