The Influence of Cations on Growth Kinetics of Silica Aggregates
- PDF / 251,685 Bytes
- 4 Pages / 420.48 x 639 pts Page_size
- 90 Downloads / 210 Views
THE INFLUENCE OF CATIONS ON GROWTH KINETICS OF SILICA AGGREGATES THEO P.M. BEELEN, PETER W.J.G. WIJNEN, KEES P.J. RUMMENS AND RUTGER A. VAN SANTEN Lab. Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 MB EINDHOVEN, The Netherlands. ABSTRACT Silica gels are prepared by acidification (pH = 2 - 4) of water glass. SAXS measurements show that the gel consists of fractal aggregates (D = 2.2). Although the fractal dimension is not influenced by addition of cations, Al 3 and Mg2+ retard the growth of the fractal aggregates, while TMA÷ has a promoting effect. INTRODUCTION Silica gels have been prepared by acidification of water glass, an aqueous solution of SiO in NaOH. Lowering of the pH induces polymerization (condensation) reactions: (HO)3SiO"
+
(HO)Si(OH)
3
==
(HO)3Si-O-Si(OH)
3
+ OH"
Consecutively, dimers are converted to trimers, tetramers, etc. The condensation reaction is catalysed by OH'. Due to the tetrahedral O-Si-O angle and the lower pKa values of the OHgroups on dimers or polymers, ring closure reactions cause a predominance of rings and interconnected polymers, resulting ultimately in almost spherical silica particles [1]. After collision of these primary particles a bond between a -SiOH and a -SiO group on the colliding particles may be formed, linking the particles together. In this way ramified aggregates are formed, forming ultimately a continuous network, the gel. Due to the participation of SiO" groups and OH" ions in the polymerization and aggregation reactions, apart from the influence of the pH also electrolytes are expected to play an important part in the preparation of silica gel as is indicated by numerous experimentel results [1]. 29 Recently much progress has been achieved by the application of Si NMR, especially because this technique discriminates between the numerous oligomers of silica [2,3]. With this information we were able to show that for example the accelerating properties of potassium in the dissolution of silica in water glass could be attributed to an K÷ induced enhanced activity of OH" on the Si-O-Si bond [4]. Also the formation of the fractal aggregates from the partially charged primary silica particles may be expected to be influenced by cations. Since 29Si NMR does not give relevant information on a colloidal scale, SAXS has been chosen to observe the growth of the aggregates. Due to the fractal properties of the aggregates, the rate of growing and the size of the aggregates can be extracted very efficiently from the spectra (5].
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
274
EXPERIMENTAL Water glass was obtained by dissolution of Aerosil 380 (Degussa) in MOH (M = Li, Na, K, Rb, Cs or tetramethylammonia (TMA)) with Si/M = 3.0 mol/mol and [Si] = 1.5 mol/liter. Silica gel was prepared by addition of calculated quantities of water glass and a cation solution to a calculated quantity of 1.0 N HCl. If necessary, the pH was adjusted by titration of the reaction mixture with a concentrated solution of MOH.
Data Loading...
