J. Phys. Chem. 1994,98, 1311-1316
1311
Molecular Simulations of the Polymerization of Silicic Acid Molecules and Network Formation Stephen H. Garofalini' and Glen Martin Department of Ceramics and Interfacial Molecular Science Laboratory, Institute for Engineered Materials, Rutgers University, P.O. Box 909, Piscataway, New Jersey 08855 Received: July 16, 1993; In Final Form: November 1 . 1993"
Polymerization of 2 16 silicic acid molecules, H4Si04, has been studied using the molecular dynamics computer simulation technique. Multibody potentials which reproduce bulk and surface structures of silica and silicate glasses, as well as molecular configurations containing Si, 0,and H ions, were used in the simulations. Results of the simulations are consistent with experimental data of sol-gel systems. Chains form at the early stages of polymerization, followed by ring formation, consistent with interpretations of N M R data and semiempirical molecular orbital quantum calculations. The activation energy for formation of branching Qn species is 12 kcal/mol, consistent with the experimental data of 12 kcal/mol for gelation. The relative time evolution of the various Qn species is consistent with experimental NMR data.
Introduction
In sol-gel processing of silica, hydrolysis of the Si(OR)4 molecules (where R represents some organic group such as an alkyl) to form Si(OR)4Y(OH)ymolecules is the initial step prior to the polymerization reaction. A water-producing condensation reaction between the hydroxyl groups attached to Si is the basis of the polymerization of the Si(OR)+y(OH)v tetrahedra to form oligomers. In the case of two completely hydrolyzed molecules, as can occur in acid-catalyzed reactions, the polymerization reaction can be shown as
-
2H4Si04
H6Si,0,
+ H,O
which actually describes the polymerization of two silicic acid molecules (two monomers) to form a pyrosilicic acid molecule (dimer) and a water molecule. Although this reaction has been presented in many papers discussing sol-gel processing of silica, the actual atomic level mechanisms of the reaction can only be inferred. Experimental techniques, by their very nature, provide data which are either averaged over very large numbers of atoms or long times, or both. Experimental techniques such as NMRl-5 can give us data which indicate that the concentration of QOspecies decrease in favor of the formation of QI species (where the subscript n in Qn indicates the number of bridging oxygen attached to the silicon), but the actual reaction mechanism is not observable and must be inferred from our knowledge of the chemistry of the possible reaction paths. Iler6 proposed that reaction 1.1 occurs after ionization of one of the silicic acid molecules via removal of a hydrogen ion, indicating that the reaction occurs as H,SiO;
+ H+ + H4Si0,
-
H6Si,0,
+ H,O
(1.2)
Continued reactions allow for the loss of the lower n Qnspecies and the eventual formation of Q4 species in a fully polymerized network. Naturally, there are always some Qn
