Using 29Si MAS-NMR we investigate the effect of pH on the final connectivity distribution of the silicon atoms in the Stöber particles. Our data suggest that the fraction of the silicon atoms that are ully-connected decreases as the ammonia concentration is increased. This suggests a more negative first shell substitution effect in the precipitated phase (liquid droplet) if condensation reactions are irreversible. A simple model is developed to describe the condensation kinetics in the precipitated phase and the results support the negative first shell substitution effect. These findings challenge the notion of a positive first shell substitution effect in alkaline conditions.
Bibliographical noteFunding Information:
We acknowledge the support of the National Science Foundation (CTS-89 10423) and the Center for Interfacial Engineering at the University of Minnesota. KL acknowledges the support of the Korea Research Foundation (KRF-2003-005-D00001) and Yonsei University Research Fund of 2001.
- Condensation kinetics
- First shell substitution effect
- Si MAS-NMR
- Stöber particles