Deep Eutectic Solvent-Mediated Interfacial Structuring Controls Dye Adsorption and Photocatalytic Kinetics at TiO2-Water Interfaces.

Deep eutectic solvents (DES) are examined as supramolecular regulators of solid-liquid interfacial organization in TiO2-based photocatalytic systems. Choline chloride-derived DES, using ethylene glycol, glycerol, malonic acid, p-toluenesulfonic acid, or urea as hydrogen-bond donors, were investigated both as physical mixtures with TiO2 (TiO2 + DES) and as DES immobilized on TiO2 nanoparticles (TiO2-DES) to govern dye adsorption and degradation under visible light. Photocatalytic kinetics follow a Langmuir-Hinshelwood model, with apparent rate constants increasing by up to 17-fold relative to pristine TiO2. DES with HBDs as polyol- and organic acid-derived (ChCl:Ethyl, ChCl:Gly, and ChCl:Mal.Ac.) showed higher photocatalytic activity than ChCl:TsOH and ChCl:Urea. This enhancement may result from productive surface interactions that favor electron transfer and ROS generation, while the urea- and TsOH-based coatings likely form more strongly coordinating interfacial layers. The immobilized DES [TiO2-DES] generally reduces or maintains activity compared with TiO2 + DES. Surface plasmon resonance measurements reveal weak and reversible DES-TiO2 interactions, excluding permanent surface modification as the dominant mechanism. 1HH NMR spectroscopy demonstrates systematic upfield shifts of methylene blue resonances with increasing DES concentration, evidencing disruption of π-π stacking and stabilization of monomeric species in solution. The markedly stronger catalytic enhancement observed for methylene blue compared with methyl orange, which showed limited responsiveness to presence of DES (evidenced by 1H NMR) and is consistent with previously reported photocatalytic reports, highlights the decisive role of dye molecular structure in DES-mediated interfacial modulation.