Enhancement of the solubility of organic dyes in aqueous ionic solvents doped with surfactants

Natural dyes can be poorly water-soluble, and improving their solubility with accurately designed solvents can benefit their application in most domains. Ionic liquids and deep eutectic solvents, systems with complex and diverse intermolecular interactions, are capable of dissolving compounds of different nature and polarity. A remarkable solubility increase of three natural dyes, indigo, alizarin and curcumin, is observed in the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate and in the eutectic solvent formed by tetrabutylammonium acetate with lactic acid. Further addition of water and surfactants to these ionic media can make them more suitable and cost-effective for industrial use. By means of molecular dynamics, we exposed striking differences in the solvation of dyes in the ionic liquid and the deep eutectic solvent, with important structural changes upon addition of water. The experimental solubility of the dyes, measured at different compositions and compared with water as solvent, point at the favourable impact of small additions of water and surfactants on the dissolution process. (c) 2022 Elsevier B.V. All rights reserved.