Author: Souty Beskhyroun
Figure. 1. Author cleaning a painting.
The use of ionic liquids in the treatment of cleaning paintings attracted the attention of conservators and restorers as a replacement for the conventional organic solvents. Ionic liquids are often coupled with green chemistry, and are sometimes described as ‘green solvents’. These claims have been made due to their negligible vapor pressure. Therefore, they are non- flammable and cannot be inhaled. These aspects present ionic liquids as safer and more environmentally benign solvents than the conventional volatile organic solvents. As an oil painting conservator interested in studying green solvents, I have a question: if these properties are adequate to consider ionic liquids as green solvents? The answer is no. During the synthesis of some ionic liquids, large amounts of volatile organic solvents are sometimes used which intrinsically makes the ionic liquids not green. But they are continuously developing the technology of designed benign synthesis of ionic liquids which comply with the principles of green chemistry.
1.1 What are the ionic liquids?
Ionic liquids (ILS) are defined as organic salts that are liquids at room temperature. Alike to any salt, ILS are made up of separate cationic and anionic species. Changing the structures of cations and anions of ionic liquids offers the opportunity to optimize their physicochemical properties for specific processes. In general, they have good properties as they are relatively non-volatile, so they do not regenerate atmospheric organic volatile compounds thus reducing the risk to health and the environment. They also show good thermal stability and they do not decompose over the large range of temperatures. Ionic liquids have relatively high viscosity compared to the conventional solvents that have been exploited to inhibit the penetration into the historical layers. Ionic liquids cover a range of polarities. They are capable to interact via hydrogen bonding, dispersive, dipolar, and hydrophobic interactions. They are also miscible with water and low toxicity solvents. Growing attention towards the synthesis and the purification of ionic liquids became essential. Sometimes the synthesis and purification of ionic liquids needs a larger amount of volatile organic solvents, which could be omitted or reduced in other ionic liquids manufacturing. Overall, the view of ionic liquid’s greenness has been somewhat restricted. Bio-separation processes of ionic liquids are part of modern biotechnology in which efficient and biocompatible methods for separation and purification of ILs have been developed.
1.2 Ionic liquids for cleaning paintings
In the field of paintings cleaning, the properties of ionic liquids have been exploited to remove natural and synthetic resins. They are also combined with enzymes to remove the proteinaceous materials from painted surfaces. ILs are also used to remove the white calciferous deposits from painted wood. Pernak et al tested Didecyldimethylammonium nitrate [DDA][NO3] on the removal of white calciferous deposits on painted pine boards from St. Michael’s church in Gsawa, Kujawy-Pomeranian voivodship, Poland. After the application of [DDA][NO3], the board showed accentuation of the wood color. The non-volatile characteristics of the ionic liquid spontaneously gave [DDA][NO3] further advantages in penetrating 3.8mm of the board, manifesting the fungicidal activity of the board. Pereira combined 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] and 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4] with two different enzymes; one acid (Pepsin) and one alkaline (from Aspergillus Sojae), to remove four proteinaceous varnishes (egg white, isinglass, animal glue, and casein) from oil and tempera paintings. The enzyme combination with [BMIM][BF4] was overall more successful in the removal of the proteinaceous natural varnish. Pacheco et al tested thirteen ionic liquids in the removal of four of naturally aged varnishes; Dammar varnish, polycyclohexanone based varnish, PVAc based varnish, and acrylic varnish on stimulated old and modern paintings. The thirteen selected ILs included methylimidazolium, tetralkylammonium, and tetralkylphosphonium as cations combined with chloride, tetrafluoroborate, triflate, trifluoroacetate, acetate, dicyanamide and alkylsulphate as anions. The experimental procedure of ILs was conducted by application onto the painted surface for ten minutes, followed by removal with a dry cotton swab and the residues were removed with another cotton swab embedded with a compatible solvent to ILs miscibility.
The analysis of the mock-ups after ILs application showed that the morphology of the painted surfaces is similar to the unvarnished one with a clearance of ILs residues on the surface proving their effectiveness for all vanishes except the acrylic varnish.
From the studies in the field of paintings cleaning, Ionic liquids show advantages related to their properties comparing to the conventional solvents. However, even though there are safe properties of ionic liquids than the volatile organic solvents, there are not adequate to consider ionic liquids as green solvents. The biosynthesis of ionic liquids is essential to develop biocompatible methods for the separation and purification of green ionic liquids. Greener synthesis of ionic liquids would be appropriate for green chemistry. Hence, Ionic liquids could be conducted for greener paintings cleaning.
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