Controlled droplet coalescence in miniemulsions to synthesize zinc oxide nanoparticles by precipitation

The precipitation of zinc oxide with a coalescence controlled emulsion technique is introduced. This technique involves the preparation of two stable water-in-oil submicron sized emulsions (miniemulsions). Each emulsion contains one water soluble precipitation precursor. In a second high pressure emulsification step droplet coalescence is induced leading to particle precipitation. In this contribution we present parameters influencing droplet coalescence rate such as emulsifier and emulsifier content as well as local flow conditions. Considering the precipitation of zinc oxide we furthermore investigate the influence of droplet coalescence rate on the size and composition of the particles synthesized. The results approve the importance of full coalescence of droplets prior to particle synthesis: Remaining uncoalesced droplets lead to a wide particle size distribution with a fraction of larger particles. If all droplets coalesce to some extent, we observe a decrease in particle size with increasing reactant concentration depicting a precipitation process dominated by homogeneous nucleation.

► We introduce a miniemulsion process to precipitate zinc oxide by droplet coalescence.
► We identified parameters to control coalescence in high pressure homogenization.
► The coalescence rate depends on the viscosity, pressure and valve geometry.
► Particle size depends on the coalescence rate and precursor concentration.