Evaporation-Induced Switching from Flocculated to Dispersed TiO2 Nanoparticles in Binary Solvents

Evaporation in mixed solutions containing both volatile and involatile solvents changes the properties of the binary solvent. We found that evaporating one solvent drastically increased the affinity of the mixed solvent to dispersant molecules and induced a switch from flocculated to dispersed TiO₂ nanoparticles (TiO₂NPs). We prepared dispersions of TiO₂NPs with mixtures of volatile cyclosiloxane (Dx), involatile polar oil, and a dispersant, namely polyhydroxystearic acid (PHSA). Dx is a nonsolvent but the polar oil is a good solvent for PHSA. The dispersions were applied on a quartz substrate, and Dx was evaporated. The original applied films were turbid, in which flocculated TiO₂NPs formed network structures. However, as the evaporation of Dx progressed, the drying films became transparent and the network structures of TiO₂NPs loosened and disappeared. After the evaporation of Dx, the applied films were transparent to visible light but blocked the transmission of UV light. The flow characteristics of the dispersions also changed. The original dispersions showed shear-thinning but became more Newtonian-like as the fraction of Dx decreased. Generally, particles are concentrated in drying dispersions and become packed or flocculated as evaporation progresses. Our findings show that initially flocculated nanoparticles can be redispersed after evaporating a specific solvent. The effects of different Dxs and compositions on the switch from flocculated to dispersed TiO₂NPs and their applicability as sunscreens are discussed.