Phase and dispersion stability effects in the synthesis of silica nanoparticles in a non-ionic reverse microemulsion

Silica nanoparticles synthesized by the controlled, base-catalyzed hydrolysis of tetraethoxysilane (TEOS) in cyclohexane-polyoxyethylene(5) nonylphenyl ether—water reverse microemulsions exhibit a bimodal size distribution when a relatively high water/surfactant molar ratio (R) is utilized. During the particle formation process there is a partial expulsion of water from the microemulsion phase and this creates a second phase of bulk water which apparently induces the formation of new silica nuclei. This reaction-promoted phase instability is attributable to the ability of ethanol, a product of TEOS hydrolysis, to shift the water solubility limit of the phase diagram to higher temperatures. At relatively low water/TEOS ratios, the resulting silica dispersions become unstable with the passage of time and eventually large flocs form. This behavior is rationalized in terms of a water-shell model according to which the presence of a water film around the silica particles facilitates the ionization of the surface silanol groups, which in turn permits the development of the necessary electrostatic stabilization. Investigation of the solubilization locale of ethanol with fluorescence techniques indicates that at low R there is a preferential partitioning of the alcohol to the continuous cyclohexane phase. Thus, under these conditions, ethanol molecules (which can also support silica ionization) are not available to substitute for water in the water shell.