Thymol–Menthol-Based Deep Eutectic Solvents for Efficient Rhodamine B Remediation: Molecular Interactions, Economic Feasibility, and Environmental Assessment

Deep eutectic solvents (DESs), derived from natural and biodegradable resources, are promising alternatives to organic and ionic solvents. In this study, Thymol-Menthol-based deep eutectic solvents (T:M) were prepared, with Thymol serving as the hydrogen bond donor (HBD) and Menthol as the hydrogen bond acceptor (HBA) in various molar ratios (1:1, 1:2, 1:3, 1:4, 2:1, 3:1, and 4:1). Fourier Transform Infrared Spectroscopy (FTIR) confirmed the formation of intermolecular interactions, such as hydrogen bonds, between the precursors. ¹H and ¹³C Nuclear Magnetic Resonance (NMR) analyses validated the molecular structure of the solvents, while Thermogravimetric Analysis (TGA) demonstrated their thermal stability, with decomposition occurring at temperatures above 150 °C. For Rhodamine B (RB) remediation, the proposed DESs exhibited a shorter extraction time (10 min) and minimal solvent volume requirement (400 µL) compared to traditional organic solvents. COSMO-RS (Conductor-like Screening Model for Real Solvents) simulations revealed that the DESs possess a strong affinity for non-polar, hydrogen-bond-donating, and hydrogen-bond-accepting molecules, confirming their exceptional extraction efficiency for RB, primarily driven by hydrophobic interactions and hydrogen bonding. The studied DESs achieved high removal efficiencies (70–98%) for multiple textile dyes, including Methylene Blue (MB), Crystal Violet (CV), and Safranin (SR). Notably, phytotoxicity assessments indicated no adverse effects on Pisum sativum (pea) seed germination at low concentrations. The EcoScale score tool also verified that the solvent synthesis process is both environmentally benign and worker-safe. These findings underscore the potential of studied DESs as sustainable, high-performance extractants for the remediation of dye-contaminated wastewater.

Graphical Abstract