Emulsion templating of poly (acrylic acid) by ammonium hydroxide/sodium hydroxide aqueous mixture for high-dispersed hollow silica nanoparticles

A water-in-oil (W/O) emulsion, in which the water phase including poly (acrylic acid) (PAA), ammonium hydroxide (NH3 aq.), and sodium hydroxide aqueous solutions (NaOH aq.) dispersed in ethanol (EtOH), was proposed for producing hollow silica nanoparticles. The PAA molecules with the NH3-NaOH mixture are insoluble in EtOH and can be a template. When tetraethoxysilane (TEOS) is added as a silica source, a silica shell is formed on the PAA-NH3-NaOH template to form a core-shell particle and the addition of water removes the template to form hollow silica nanoparticles. The optimum volume ratio of 25% NH3:1.0 M NaOH was 2:1 to obtain high-dispersed hollow silica nanoparticles. A thermogravimetric analysis indicated that the condensation force of the PAA molecules in the PAA-NH3-NaOH template is similar to that in the PAA-NH3 template. The small angle X-ray scattering (SAXS) analysis also showed that the electron densities in the PAA-NH3-NaOH and PAA-NH3 templates are similar. In the proposed PAA-NH3-NaOH system, NH3 aq. mainly contributes to form PAA condensation by an ionic interaction and NaOH aq. preferentially work as a sol-gel catalyst for the fast silica coating on the template surface.