The nonvolatile nature of deep eutectic mixtures makes them ideal solvents for high-melting point compounds and those that decompose prior to melting. Padua et al. tested the hypothesis that the structure of deep eutectic solvents can be altered to induce specific solvation interactions, thereby tuning solubility.

Indigo, a hydrophobic dye used in the textile industry, is regarded as insoluble in water and most common solvents. Indigo’s structure is locked in a trans configuration, which results in the stability of its intramolecular H-bonds being solvent-dependent. This arrangement complicates the dyeing process, requiring the indigo to be reduced, then applied and reoxidized.

The authors systematically varied the hydrophobic character of the salt and the hydrogen bond donor of the deep eutectic, along with varying the water concentration, of three solvents: choline chloride salt in 1,4-butanediol, tetrabutylammonium bromide in 1,4-butanediol, and choline chloride in p-Cresol.

“This work provides the interesting concept of local composition versatility of eutectic solvents on which to base the design of new technological liquids,” said coauthor Agílio Padua.

The effects on solubility and microstructuring of the indigo were evaluated using molecular dynamics simulation. The result was an engineered, application-specific, and deep eutectic solvent that provided a favorable environment for dissolution of the indigo dye.

Finding effective and safe solvents for indigo could significantly reduce the environmental impact of the dyeing process. Padua noted the potential exists for broader applications in the design of more effective and environmentally sustainable technological fluids, such as lubricants and electrolytes for batteries.

Source: “Tuning the solvation of indigo in aqueous deep eutectics,” by Oliver S. Hammond, Guillaume Simon, Margarida Costa Gomes, and Agílio A. H. Padua, Journal of Chemical Physics (2021). The article can be accessed at