An Ultrasound-Assisted Minireactor System for Continuous Production of TiO2 Nanoparticles in a Water-in-Oil Emulsion

In this work, ultrasound-assisted synthesis of TiO2 nanoparticles in a water-in-oil emulsion in a minireactor has been introduced. For this, a lab-scale ultrasound-assisted minireactor system was developed. Experiments were carried out at different precursor solution flow rates, precursor and surfactant concentrations, and minireactor geometries. Lowering the precursor concentration and flow rate significantly decreases the average particle size of TiO2 nanoparticles. However, increasing the surfactant concentration decreases the average particle size up to a certain extent after which it increases due to an increase in viscosity of the microemulsion. Also, the use of a spiral tube and straight tube minireactor leads to a lesser average particle size due to efficient mixing characteristics. Results show that these parameters influence the particle size of TiO2 nanoparticles. Moreover, ultrasound-assisted synthesis (at an ultrasound frequency of 22 kHz) is able to produce TiO2 nanoparticles of significantly reduced size compared to those produced without using ultrasound. This is achieved by intensified dispersion of aqueous droplets in the oil phase and prevention of droplet coalescence, nucleate grouping, and nanoparticle agglomeration due to the cavitational effect of ultrasonication. The lowest average particle size of 20.9 nm of TiO2 nanoparticles was obtained at a precursor concentration of 4 vol %, a precursor solution flow rate of 150 mL/h, and a surfactant concentration of 0.006 g/mL using the minireactor of spiral geometry.