SYNTHESIS OF NANOSIZE SILICA IN AEROSOL OT REVERSE MICROEMULSIONS

Silica nanoparticles were synthesized in sodium bis(2-ethylhexyl) sulf osuccinate (AOT)/decane/ammonium hydroxide reverse microemulsions via the base-catalyzed hydrolysis of tetraethoxysilane (TEOS). The fluores cence spectra of 1,3,6,8-pyrenetetrasulfonic acid (PTS) trapped in the water pools indicate that free water molecules do not become availabl e until the water-to-surfactant molar ratio (R) exceeds about 10. At R values below about 4, no particles are produced. Under these conditio ns, the surfactant molecules are closely packed at the interface and t he water molecules are strongly bound to the surfactant polar groups a nd the sodium counterions; the net result is to inhibit TEOS hydrolysi s. As R increases from 5 to 9.5, the diameter of the resulting particl es increases, and the size distribution decreases. These trends are ra tionalized in terms of changes in the local concentration of hydrolyze d TEOS species in the reversed micellar pseudophase, coupled with the protective action of the surfactant film. The fact that the final part icle sizes exceed the diameters of the corresponding water pools indic ates a role for intermicellar matter exchange in the particle growth p rocess. The dispersions are not stable, and gel-like precipitates even tually form. The low dispersion stability is attributed to the presenc e of sodium ions and to the possible decrease in pH caused by the hydr olysis of the AOT surfactant molecules. (C) 1995 Academic Press, Inc.