On the stability of surfactant-free water-in-oil emulsions and synthesis of hollow SiO2 nanospheres

This article examines the relative stability of five surfactant-free water-in-oil (w/o) emulsions produced by 1-min irradiation of the mixture with 42-kHz ultrasounds; it also reports on the synthesis of hollow silica nanospheres in such emulsions. The oily phase in these w/o emulsions consisted of cyclohexane, dodecane, benzene, octane, and hexane, whereas the dispersed phase consisted of aqueous ammonia at pH 11. Light scattering experiments revealed that the size of the dispersed phase in the cyclohexane emulsion was fairly constant with time (ca. 0.97 μm) and was the most stable emulsion, whereas for the least stable dodecane w/o emulsion the dispersed phase particles increased in size from ca. 1.5 μm to 7.3 μm after 48 min. The stability of the w/o emulsions decreased in the order cyclohexane > octane > benzene > hexane > dodecane. Hollow silica nanospheres were synthesized by a soft template method involving the five surfactant-free water-in-oil emulsions by hydrolysis of tetraethoxysilane (TEOS) at the water/oil interface (pH 11). Formation of these hollow SiO2 nanospheres is described in terms of the stability of the various emulsions and characterized by transmission electron microscopy, which showed well-defined hollow silica nanospheres of relatively uniform sizes (100 ± 20 nm) in the cyclohexane emulsion. The latter w/o emulsion was further examined for the pH dependence of the formation of the silica nanospheres. At pHs 6 and 7, respectively, highly aggregated hollow silica nanoparticles of ill-defined structure formed with relatively thin skin, whereas well-structured silica nanospheres were obtained at pHs 8 and 9 although the nanospheres also tended to aggregate. By contrast, at pH 10 well-defined hollow silica nanospheres were produced with a silica skin 3 times thinner (5 nm) than that of nanospheres produced at pH 11 (ca. 14 nm). Various stages in the overall process are described.