Stöber method is widely employed to obtained silica particles of different size [1]. Originally, micrometer-sized particles have been obtained, but nanosized ones can be synthesized using microheterogeneous medium; microemulsions seem to be particularly useful for such purpose [2]. In the case of water-in-oil microemulsion, spatially constrained droplet of water act as template, determining size of nanoparticles formed. Usually, the reaction medium is composed of cyclohexane, alkylphenol ethoxylate, water,  and tetraethoxysilane (TEOS) and the sol-gel process is induced by the addition of an aqueous ammonium hydroxide [3]. In some cases, medium-chain alkyl alcohol (pentanol or hexanol) was added as co-surfactant. During the process, partial expulsion of water from the microemulsion phase may occur, resulting in formation of second phase of bulk water which, causing bimodal size distribution [4].

Present contribution describes modification of standard synthetic process. Oil-in-water microemulsion was formed using heptane, 2-ethylhexanol, TWEEN®85 and TEOS. After some specified incubation time, ammonium hydroxide was added and thereaction mixture was stirred for 24 hours at room temperature. Prior to synthesis, pseudo-ternary diagram was created for oil-rich area and Winsor IV region was identified. These microemulsions were used for synthesis of silica particles. Resulting SiO₂ particles were characterized by dynamic light scattering, electrokinetic measurements, specific surface area measurements and scanning electron microscopy. Particles' diameter was ranging between 150 and 600 nm, usually monodisperse distribution was obtained. The specific surface area of nanoparticles was about 250–300 m²/g. Notably, productivity per unit volume of solution was  3 to 5 times higher than for previously reported procedures.

Our method can be extended, because polymeric or monomeric materials can be added to dispersed aqueous phase. In our studies, β-cyclodextrin and hydroxyethylcelluse have been used,, giving particles between 40 and 160 nm, wht the surface area larger than 300 m²/g.

The work was supported by Wroclaw Research Centre EIT+ within the project "The Application of Nanotechnology in Advanced Materials” - NanoMat (POIG.01.01.02-02-002/08) co-financed by the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2).

[1] W. Stöber, A. Fink: Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 1968, 26(1), 62–69.

[2] B. Niemann, F. Rauscher, D. Adityawarman, A. Voigt, K. Sundmacher: Microemulsion-assisted precipitation of particles: Experimental and model-based process analysis, Chem. Eng. Process. 2006, 45(10), 917–935.

[3] F.J. Arriagada, K. Osseo-Asare: Synthesis of nanosize silica in a nonionic water-in-oil microemulsion: effects of the water/Surfactant molar ratio and ammonia concentration, J. Colloid Interface Sci. 1999, 211(2), 210–220.

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