Effect of hydrophilic polymers on the formation of size-controllable aqueous droplets in water-in-oil emulsion and the fabrication of porous micro-silica particles therefrom.

Size-controllable droplets were formed in a water in oil (W/O) emulsion using only hydrophilic polymers without a surfactant to fabricate porous micro-silica particles larger than 20 μm. Droplets of various size ranging from 1 to 30 μm were prepared by emulsifying aqueous solutions containing four types of polymers, namely polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and polypropylene glycol (PPG), in a pentanol oil phase. Following the addition of tetraethyl orthosilicate (TEOS) as a silica precursor, silica particles were grown via hydrolysis and condensation reactions. The silica particle size depends on the degree of hydrophilicity of the polymers, which determines the interfacial tension between the water droplets and oil. Micro-silica particles >20 μm were obtained from PEG-based emulsion droplets. Notably, the distribution and stability of silica particles can be optimized by controlling the molecular weight and concentration of the hydrophilic polymer. A porous silica structure was successfully obtained by decomposing the residual polymer via an appropriate calcination process. The most uniform and stable porous micro-silica particles with an average size of 20 μm were obtained from an emulsion containing 5 wt% PEG (molecular weight: 4000) after calcination at 500 °C. This novel process enables the eco-friendly synthesis of porous micro-silica particles using only hydrophilic polymer without a surfactant and control of pore size and particle size of >20 μm.