Aggregation and dispersion behavior of nanometer and submicrometer scale TiO 2 particles in aqueous suspension were investigated using three kinds of mechanical dispersion methods: ultrasonic irradiation, milling with 5-mm-diameter balls, and milling with 50 lm beads. Polyacrylic acids with molecular weights ranging from 1200 to 30 000 g/mol were used as a dispersant, and the molecular weight for each dispersion condition was optimized. Viscosities and aggregate sizes of the submicrometer powder suspensions were not appreciably changed in the ultrasonic irradiation and 5-mm-ball milling trials. In contrast, in the trials in which nanoparticle suspension was used, ultrasonic irradiation produced better results than 5-mm-ball milling. Use of ultrasonication enabled dispersion of aggregates to primary particle sizes, which was determined based on the specific surface area of the starting TiO 2 powders, even for relatively high solid content suspensions of up to 15 vol%. Fiftymicrometer-bead milling was also able to disperse aggregates to the same sizes as the ultrasonic irradiation method, but 50-lmbead milling can be used only in relatively low solid content suspensions. It was concluded that the ultrasonic dispersion method was a useful way to prepare concentrated and highly dispersed nanoparticle suspensions.
The effects of silica particle diameter on dispersion and aggregation behavior in water were analyzed, using alkoxidederived silica powders with particle diameters of 8 -260 nm. The present study focused on the relationships between the surface silanol structure and the interaction forces between solid surfaces in water. The surface silanol structure and interaction between particles were determined using Fourier transform infrared spectroscopy, Fourier transform nearinfrared spectroscopy, and atomic force microscopy. For relatively large particles (>30 nm in diameter), the surface silanols primarily were hydrogen-bonded silanols, and isolated silanols disappeared. The hydrogen-bonded silanols formed a hydrogen-bonded water layer on the particle surface; therefore, the additional hydration force was strong between these relatively large particles. In contrast, the surface density of isolated silanols increased as the particle diameter decreased to <30 nm, and the additional hydration force between ultrafine powders disappeared. The aggregation behaviors of alkoxidederived silica powders were dependent on the hydration force, which was changed by the surface silanol structure.
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