Deep eutectic solvents (DESs): new green solvents for the cleaning of nonpolar coatings

Deep eutectic solvents (DESs) are a mixture of two or more solid components leading to a strong depression on the melting point when compared to their individual counterparts [1]. They form eutectic mixtures of a hydrogen-bonding acceptor (HBA) and a hydrogen-bonding donor (HBD) able to self-associate via hydrogen bonds and Van der Waals interactions. The properties of the final DES can be adjusted through the selection of the individual components based on their chemical structure and molar ratio. The discovery of these non-toxic formulations generated a breakthrough in the world of green chemistry. As eco-friendly solvents, DESs are being used in many areas of science and technology due to their excellent physicochemical properties, such as low volatility, low cost, and non-toxicity [2]. However, only a few attempts have been made so far to use DESs in the field of Cultural Heritage Conservation [3,4]. In this framework, we choose to explore the potentialities of DESs for cleaning treatments on works of art, specifically for the removal of nonpolar coatings – such as waxes. The experimentation was performed using laboratory specimens consisting of one layer of wax applied on microscope glass slides. Four different types of waxes were chosen for the testing, namely two natural waxes (beeswax and carnauba wax) and two microcrystalline waxes, which are commonly used as coating layers on Cultural Heritage materials. The cleaning tests were performed using three different operational modes, i.e., swab cleaning, 1-minute-application using Japanese paper (Tengujo) as an intermediate layer, and 3-minute-application using the same intermediate layer. Multispectral imaging was carried out using visible (VIS) and ultraviolet (UV) light both before and after cleaning tests. The analysis was performed on samples, swabs, and Japanese paper to assess 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 the presence of solvents residues on the samples’ surface. The analysis was also carried out on the swabs and the Japanese paper sheets used for the cleaning treatments to detect the presence of wax and assess the effectiveness of the cleaning tests. 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.