Synthesis of Nanosize Silica in Aerosol OT Reverse Microemulsions

Abstract

Silica nanoparticles were synthesized in sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/decane/ammonium hydroxide reverse microemulsions via the base-catalyzed hydrolysis of tetraethoxysilane (TEOS). The fluorescence spectra of 1,3,6,8-pyrenetetrasulfonic acid (PTS) trapped in the water pools indicate that free water molecules do not become available until the water-to-surfactant molar ratio ( R) exceeds about 10. At R values below about 4, no particles are produced. Under these conditions, the surfactant molecules are closely packed at the interface and the water molecules are strongly bound to the surfactant polar groups and the sodium counterions; the net result is to inhibit TEOS hydrolysis. As R increases from 5 to 9.5, the diameter of the resulting particles increases, and the size distribution decreases. These trends are rationalized in terms of changes in the local concentration of hydrolyzed TEOS species in the reversed micellar pseudophase, coupled with the protective action of the surfactant film. The fact that the final particle sizes exceed the diameters of the corresponding water pools indicates a role for intermicellar matter exchange in the particle growth process. The dispersions are not stable, and gel-like precipitates eventually form. The low dispersion stability is attributed to the presence of sodium ions and to the possible decrease in pH caused by the hydrolysis of the AOT surfactant molecules.