Evaporation-Induced\nSwitching from Flocculated to\nDispersed TiO<sub>2</sub> Nanoparticles in Binary Solvents

Evaporation\nin mixed solutions containing both volatile\nand involatile\nsolvents changes the properties of the binary solvent. We found that\nevaporating one solvent drastically increased the affinity of the\nmixed solvent to dispersant molecules and induced a switch from flocculated\nto dispersed TiO<sub>2</sub> nanoparticles (TiO<sub>2</sub>NPs). We\nprepared dispersions of TiO<sub>2</sub>NPs with mixtures of volatile\ncyclosiloxane (D<i>x</i>), involatile polar oil, and a dispersant,\nnamely polyhydroxystearic acid (PHSA). D<i>x</i> is a nonsolvent\nbut the polar oil is a good solvent for PHSA. The dispersions were\napplied on a quartz substrate, and D<i>x</i> was evaporated.\nThe original applied films were turbid, in which flocculated TiO<sub>2</sub>NPs formed network structures. However, as the evaporation\nof D<i>x</i> progressed, the drying films became transparent\nand the network structures of TiO<sub>2</sub>NPs loosened and disappeared.\nAfter the evaporation of D<i>x</i>, the applied films were\ntransparent to visible light but blocked the transmission of UV light.\nThe flow characteristics of the dispersions also changed. The original\ndispersions showed shear-thinning but became more Newtonian-like as\nthe fraction of D<i>x</i> decreased. Generally, particles\nare concentrated in drying dispersions and become packed or flocculated\nas evaporation progresses. Our findings show that initially flocculated\nnanoparticles can be redispersed after evaporating a specific solvent.\nThe effects of different D<i>x</i>s and compositions on\nthe switch from flocculated to dispersed TiO<sub>2</sub>NPs and their\napplicability as sunscreens are discussed.