Tetraoctylammonium bromide-oleic acid hydrophobic deep eutectic solvent for the efficient extraction of methyl orange from aqueous solution: Performance and mechanistic insights

Hydrophobic deep eutectic solvents (DESs) are potentially greener extraction phases for azo dyes, but their loading capacity, working pH range and matrix tolerance at the high dye concentrations typical of textile effluents are rarely reported. We examined a hydrophobic DES of tetraoctylammonium bromide (N₈₈₈₈Br) and oleic acid (OA) for liquid-liquid extraction of methyl orange (MO).

Among the initially screened HBA:HBD molar ratios (1.0:1.0-1.0:3.0), DES2 (1.0:1.5) was selected after refinement at 1.0:1.3-1.0:1.7, combining high extraction efficiency (EE, 99.84%) with a stable liquid state. With 10 min shaking, DES2 extracted more than 99% of MO over 50-2500 mg L^-1. EE also exceeded 99% over pH 2-11 (500 mg L^-1 MO); pH 3-9 is recommended as a conservative range that keeps OA largely protonated. Uptake was described empirically by pseudo-second-order kinetics (R² = 0.9999) and a Langmuir-type saturation model (q_max = 303.67 mg g^-1). FT-IR and ¹H NMR spectra indicated hydrogen bonding between N₈₈₈₈Br and OA and were consistent with MO extraction by hydrophobic partitioning and ion-pair association, with possible hydrogen-bonding and π-π/n-π contributions. Inorganic ions had little effect, except NO₃^- at 2000 mg L^-1 (EE 94.25%), and SDS did not interfere up to 20 mg L^-1. In spiked river water, lake water and textile-dyeing wastewater (1000-2000 mg L^-1), EE exceeded 98%.

To our knowledge, this is the first report of the N₈₈₈₈Br-OA DES and its application to azo dye extraction. DES2 is a high-capacity candidate extraction phase for removing MO from dye-contaminated water; its regeneration and environmental safety remain to be evaluated.