Uniform-Sized Silica Nanocapsules Produced by Addition of Salts to a Water-In-Oil Emulsion Template.

Size control of silica hollow particles was achieved using a water-in-oil (W/O) emulsion system, where interfacial condensation of organosilanes (octyltrichlorosilane and methyltrichlorosilane) took place around the aqueous droplets. Good emulsion stability was obtained using soybean oil as the oil phase because of its high viscosity. This high stability led to bimodal distributions in the sizes of the aqueous droplets and final hollow particles, with particle sizes observed of a few tens of nanometers and a few micrometers. The presence of NaCl was found to be required in the water phase to afford uniform-sized silica hollow spheres. Hydrolysis of the organosilanes caused a supersaturation of the aqueous NaCl solution dispersing in the oil continuous phase, followed by crystallization from droplets. Nanosized aqueous droplets acted as a template to form uniform-sized nanospherical hollow silica particles as a result of the diminishing number of larger aqueous droplets.