Hydrophobic Deep Eutectic Solvents (DESs): new green solvents for the removal of nonpolar coatings from Cultural Heritage artifacts

The role of Green Chemistry in improving sustainability in the Cultural Heritage Conservation field is increasingly growing. The general aim is to replace traditional hazardous methods and products that are still widely used in the field, especially for the removal of aged coatings from artistic surfaces, thus ensuring the safety of both the artworks and the operators. Even though a green effective approach to cleaning treatments is now recognized as an urgent need, novel environmentally friendly solutions and protocols are yet to be investigated. Deep Eutectic Solvents (DESs) are homogeneous solvents composed of eutectic mixtures of hydrogen-bond donors (HBD) and hydrogen-bond acceptors (HBA) that have a melting point much lower than those of the individual components [1]. They are considered promising alternatives to conventional organic solvents due to their excellent physical-chemical properties (i.e., low volatility, high dissolution power, biodegradability, and low toxicity), together with their easy synthesis, accessibility of their natural compounds, and low cost. Although few attempts have been made so far to use hydrophilic DESs in the Cultural Heritage field, hydrophobic DESs have not been yet investigated [2,3]. For this reason, we exploited the potential of hydrophobic DESs as new eco-friendly solvents for the removal of nonpolar coatings from artistic surfaces. Four different types of waxes were selected for the investigation, namely two natural waxes and two microcrystalline waxes, as historically used protective coatings for stone and metal sculptures [4]. Firstly, known amounts of the four waxes and the tested DESs were mixed in cuvettes for an initial assessment of the DESs’ dissolution power. Then, laboratory specimens were used for testing, applying the waxes – both pure and mixed with a fluorescent marker – on microscope glass slides. Cleaning tests were performed using well-established operational modes, such as swab cleaning, 1-minute and 3-minutes applications with an intermediate layer. Multispectral imaging was performed on the samples and the swabs both before and after cleaning tests, using visible (VIS) and ultraviolet (UV) light. The analysis, together with the presence of a fluorescent marker, allowed assessing the effectiveness of the tested DESs in solubilizing the four waxes. Fourier Transform Infrared spectroscopy (FT-IR) in Attenuated Total Reflectance (ATR) mode was carried out both before and after cleaning tests as well, to evaluate both the presence of solvents residues on the samples’ surface and the presence of dissolved wax on the swabs. The experimental process provided the first valuable results, proving DESs’ potential of being used as sustainable solvents for cleaning treatments on Cultural Heritage materials and, therefore, giving reason to further research.