TiO2 colloids in a natural hydrophobic deep eutectic solvent (DES) with water-programmable rheology and high UV-shielding performance

Hydrophobic deep eutectic solvents (DESs) have recently emerged as sustainable and versatile liquid media, yet their fundamental behavior as colloidal dispersants remains poorly understood. Here, we examine a natural hydrophobic DES composed of thymol and coumarin as a medium for dispersing TiO 2 nanoparticles. Spectroscopic analyses confirm the formation of a hydrogen-bonded eutectic structure and reveal favorable adsorption of DES molecules onto TiO 2 surfaces. Time-domain nuclear magnetic resonance (NMR) measurements further show that this DES provides markedly stronger solvation of TiO 2 compared with conventional hydrophobic liquids. Although the resulting suspensions possess limited intrinsic stability, we demonstrate that the introduction of a minute amount of water—immiscible with the DES yet strongly wetting toward TiO 2 —significantly enhances the rheological rigidity of the suspensions, likely through water-mediated capillary bridging, thereby effectively suppressing sedimentation-driven phase separation. Beyond stability control, the DES-based suspensions exhibit exceptional UV-shielding performance. The neat DES shows inherent UV absorbance, and the incorporation of TiO 2 elevates the sun protection factor (SPF) to values exceeding 100, outperforming comparable aqueous and oil-based systems. The addition of water improves coating uniformity while maintaining high SPF performance. These findings establish hydrophobic DESs as a promising platform for designing environmentally benign colloidal formulations with tunable mechanical and optical functionalities.