ABSTRACT
Silicon-29 NMR Spectroscopy . . . . . . . . . . . . . . . . 598
Oligomerization of Aqueous Monomeric
Silicic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . 598
Influence of Alkali Metal Hydroxides
on Dissolution Rate . . . . . . . . . . . . . . . . . . . . . 599
Small-Angle X-Ray Scattering . . . . . . . . . . . . . . . . 600
Fractal Behavior in Silica Gel Chemistry . . . . . . 600
Influence of Total Silica Concentration
on Aggregation Kinetics. . . . . . . . . . . . . . . . . . 601
Influence of Polyvalent Cations on
Aggregation Kinetics . . . . . . . . . . . . . . . . . . . . 602
Aging of Aqueous Silica Gels . . . . . . . . . . . . . . . . 603
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604
The use of modern spectroscopic techniques in the study of the formation of aqueous silica gels
is described. The oligomerization process of monomeric silicic acid was studied by silicon-29 NMR
spectroscopy; at high pH values cyclic trimeric silicate species were favored compared to the linear
structure. Aggregation of primary silica particles of molecular size (,1 nm) was studied by analysis of small-angle x-ray scattering patterns. All systems studies (pH 4.0) indicate reaction-limited aggre-
gation. Polyvalent cations influence the rate of aggregate formation in a negative way: aluminum at
low concentrations (1 mol%) significantly inhibits aggregation. A new model for the aging process
that proposes that monomeric silicic acid is transported (via solution) from the periphery of the aggre-
gate into the core of the solution is given.
