Geral

Journal of Non-Crystalline Solids 315 (2003) 166–179 www.elsevier.com/locate/jnoncrysol Chemical reaction kinetics leading to the first Stober silica nanoparticles – NMR and SAXS investigation
D.L. Green a, S. Jayasundara b, Yui-Fai Lam c, M.T. Harris a a,*

Department of Chemical Engineering and Institute of Physical Science and Technology, University of Maryland,
College Park, MD 20742, USA b Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742, USA c NMR Lab, Department of Chemistry, University of Maryland, College Park, MD 20742, USA
Received 20 November 2001

Abstract
29
Si-NMR and 13 C-NMR were used in methanol and ethanol to monitor the intermediates or hydrolyzed monomers that lead to the formation of the first primary particles as detected by small angle X-ray scattering. This identification was facilitated by using initial NH3 and H2 O levels at the lower end of those experienced in Stober synthesis to slow the reaction kinetics. We found that [NH3 ] and [H2 O] control the balance between hydrolysis of tetraethylorthosilicate
(TEOS) and the condensation of its hydrolyzed monomers. Transesterification between methanol and TEOS did occur; however, it was negligible compared to the production of hydrolyzed intermediates. The first nanostructures appear at a hydrolyzed monomer concentration around 0.1 M, indicating that formation of the primary structures is thermodynamically controlled by a supersaturation of the intermediate species. Differences in particle size between methanol and ethanol are attributed to thermodynamic interactions between the solvent and the hydrolyzed intermediates.
Ó 2003 Elsevier Science B.V. All rights reserved.

1. Introduction
Stober synthesis [1], the ammonia-catalyzed reactions of tetraethylorthosilicate [Si(OR)4 or
TEOS; R ¼ C3 H5 ] with water in low molecular weight (MW) alcohols, produce highly monodisperse, spherical silica nanoparticles that range in