(588c) Physical Basis for the Formation and Stability of Silica Nanoparticles in Basic Solutions of Monovalent Cations

Aqueous silica chemistry remains an important, yet not fully understood topic in the synthesis of mesostructured materials, catalysts and adsorbents, low-dielectric coatings, biomineralization, etc., to name a few. The classical view of silica polymerization is that it is a rather uncontrolled process leading to poorly defined structures with broad distributions of molecular weights and connectivity, as it is typically the case in sol-gel processes [1]. Recent experiments conducted in our laboratories reveal a rather different view of silica polymerization [2, 3]. We observe that at well-defined concentrations of silica in basic solutions of monovalent cations, further addition of silica leads suddenly to the formation of silica nanoparticles with a size that is fairly uniform, nearly independent of the cation identity (from Na+ to organic cations such as tetrabutylammonium, TBA+), and with a structure formed of a silica core and a cation shell. Moreover, the formation and dissolution of these nanoparticles appears to be a reversible process at room temperature over typical observation times [3].

Here we present studies of the colloidal stability and phase behavior of silica nanoparticles (<10 nm) using a combination of electrostatic and chemical equilibrium models. By accounting for the deprotonation and condensation of silica species we have been able to develop a model that predicts the phase diagram and critical aggregation concentration for silica nanoparticles in basic solutions [4]. Inclusion of a complexation model allows for the first determination of the equilibrium constant for nanoparticle silanol deprotonation along with the charge of the particle surface ? from which we are able to explain the colloidal stability of these small, 3-5 nm nanoparticles in solution. In addition, we show that the charging of the surface plays a significant role in the free energy of condensation and can explain why the formation of small particles is energetically favorable.

1. Iler, R. K. The chemistry of silica: solubility, polymerization, colloid and surface properties, and biochemistry. 1979, New York: Wiley.

2. Fedeyko, J. M.; Rimer, J. D.; Lobo, R. F.; Vlachos, D. G. J. Phys. Chem. B 2004, 108, 12271.

3. Fedeyko, J. M.; Vlachos, D. G.; Lobo, R. F. Langmuir 2005, 21, 5197.

4. Rimer, J. D.; Vlachos, D. G.; Lobo, R. F. J. Phys. Chem. B, Accepted 2005.