Tuning the solvation of indigo in aqueous deep eutectics

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• Deep eutectic solvents (DESs) offer tunable properties for various applications.
• Understanding solute-solvent interactions is crucial for optimizing DES performance.
• Indigo dye serves as a model hydrophobic solute for solubility studies.

Purpose of the Study:

• To investigate the effect of systematic structural modifications in DESs on indigo dye solubility.
• To explore the relationship between DES composition, solute-DES interactions, and solubility.
• To validate the hypothesis that tailored DES structures can tune solubility.

Main Methods:

• Solubility measurements of synthetic indigo dye in three distinct DESs at room temperature.
• Systematic variation of DES components: choline chloride, tetrabutylammonium bromide, 1,4-butanediol, and p-cresol.
• Inclusion of varying water content (2.5–25 wt.%) to simulate atmospheric absorption.
• Molecular dynamics simulations for structural analysis, including radial distribution functions and coordination numbers.

Main Results:

• Indigo solubility varied significantly across the tested DESs, influenced by component hydrophobicity.
• Tetrabutylammonium bromide:1,4-butanediol DES showed highest solubility at low water content due to cation interactions.
• Choline chloride:p-cresol DES exhibited enhanced solubility at higher water content, attributed to strong solute-HBD interactions.
• Water addition generally decreased solubility by solvating DES components.

Conclusions:

• DES structure can be systematically altered to tune indigo dye solubility.
• Hydrophobic interactions between the solute and DES components play a critical role in solubility.
• DESs demonstrate versatility as solvents, capable of local-composition effects for specific solute solvation.