Stability of silica sols

A higher electrokinetic potential leads to a higher stability of silica sol. In general, the stability of acidic silica sol is better than that of basic silica sol. 

A particularly interesting example on the interplay between the proton exchange for the surface hydroxyls and the adsorption or desorption of solution species is provided by silica. As mentioned above, it is known that the hydrolysis of silica species is minimal at 6 < pH < 7. It is therefore expected that the number of >SiOH groups (Equation 8.102a) increases when the pH scale descends and the number of >SiO groups (Equation 8.102b) increases when the pH scale ascends. Moreover, it is expected that the stability of silica sols would be the least at 6 < pH < 7. As shown in Figure 8.31 this is exactly what is observed. - ... 

What Derjaguin considers the central issue of colloidal solutions remains largely unresolved for silica sols. This book mentions the ideas of the proponents of both the kinetic and the thermodynamic approach to the problem of stability of silica sols and is intended to stimulate the continuation of the healthy controversy started at the R. K. Iler Memorial Symposium. In this manner a consensus should eventually be reached that will allow the establishment of common quantitative parameters in the treatment of stability of silica sols and other disperse phase materials composed of polyvalent atoms linked by strong covalent bonds and the explanation of their experimentally observed behavior. 

Furthermore, the particle core has a density consistent with amorphous silica. The adsorption of oligomers around the core of the particles has implications that may explain the high charge density and exceptional stability of silica sols (9). Enhanced stability may also arise from solvation forces. Such an interfacial structure has important consequences in both determining the mechanism and enhancing the capacity for sorption of other ionic species from solution. 

Consideration of the stability of silica sols, that is, the so-called anomalous stability, will, in later sections of this chapter, focus on 1 1 electrolyte concentrations of 0.1 M and greater. At this salt concentration, the range of the repulsive electrostatic forces between particles is small, and any subtle differences in the electrostatics between silica and other oxide sols in itself cannot provide the necessary repulsion to stabilize silica sols at such high electrolyte levels. 

At least under some conditions the stability of silica sols is in complete contradiction to the DLVO theory, as pointed out by Kitchener (50). Thus at pH 2, where the charge on silica particles, is zero, the particles aggregate least rapidly and the sol has highest temporary stability. However, it is only in alkaline solution where the particles are highly charged that sols are permanently stable. Here the double layer theory is more logically applicable. 

The stabilization of silica sols that was carried out through eliminating numerous hydroxyl groups formed by hydrolysis was expected to lead to a substantial intramolecular condensation with a very little involvement (if any) of intermolecular reactions. Was it really the case for the actual situation ... 

There is some evidence to suggest that monosilicic acid is the principal form of silica in the soil solution (Alexander et al. [1954]), and McKeague and Cline [1963] argue that since the amounts of soluble silica recorded in the soil solution are considerably less than the solubihty of amorphous silica, the stability of silica sol in soils is highly suspect. The heat of... 

In the absence of a suitable solid phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous silica to quartz at high temperatures. Particle size influences the stability of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkali (1,33). 

The following fundamental aspects of the colloid chemistry of silica are briefly reviewed in this chapter nucleation, polymerization, and preparation stability of sols surface structure characterization methods sol-gel science gels and powders and uses of silica sols and powders. Silica in biology is not within the scope of this book. Scientists working in this area should soon put together a protocol covering progress done since the publication of Iler s book. 

B. A. Keiser s contribution to this book (the introduction to the section Preparation and Stability of Sols ) constitutes an excellent introduction to silica nucleation, polymerization, and growth in both aqueous and alcoholic systems for the preparation of silica sols. Yoshida s chapter (Chapter 2) focuses on industrial development in the preparation of monodisperse sols from sodium silicate and predicts further progress in the development of silica sols that have shapes other than spherical, such as elongated, fibrous, and platelet. Colloidal silica particles with these shapes show novel properties and open the possibility of new industrial applications. 

The advent of concentrated monodisperse silica sols in the 1950s appeared to offer an ideal model to test the DLVO theory a stable system of solid spheres with a particle diameter that could be varied in a broad range from about 5 to 100 or 300 nm. However, it soon became quite evident to many researchers, both in Iler s laboratories and elsewhere, that silica sols do not conform to the DLVO theory as originally formulated (27-32). As an example, Figure 9 illustrates the problem, showing an area in the stability-pH curve of experimentally proven relative stability (metastability) of silica sols at around the zero point of charge where the theory predicts minimum stability. In addition, the plot of experimental... 

Healy (Chapter 7) and Dumont also prefer the first approach. Healy sets down a model based on the control of coagulation by surface steric barriers of polysilicate plus bound cations. Healy s electrosteric barrier model is designed to stimulate new experimental initiatives in the study of silica sol particles and their surface structure. Dumont believes that many particular aspects of the stability of silica hydrosols could be explained not only by the low value of the Hamaker constant but also by the relative importance of the static term of the Hamaker equation. 

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

Figure 6. Flow chart of production of silica sols stabilized with ammonia, amine, or quaternary ammonium hydroxide.

To construct a model for anomalous stability, observations of silica sols at pH 11 are used. Allen and Matijevic (19) observed a critical coagulation concentration of approximately 0.15 M for Na+ at pH 11.0. If they take this pH 11 sol to, for example, pH 8.0, it remains stable in salt concentrations much greater than 0.15 M, and the critical coagulation concentration increases even further as the pH is reduced from pH 8 to pH 6. 

In their pioneering studies of silica sols, Alexander and Iler (4) employed low-temperature nitrogen adsorption to determine the surface areas of the colloidal particles after removal of the aqueous medium. The Brunauer-Emmett-Teller (BET) areas were found to be only slightly larger than the values obtained from the particle size distributions as determined by light scattering and electron microscopy. These remarkable measurements indicated little change in the particle size or shape after the stabilized silica sols were carefully dried. 

In this chapter we wUI describe some recent investigations of the formation and interfacial structure of a series of commercially produced sUica sols (Nalco, Dupont, and Ludox) with different diameters in the range 1-30 nm. This type of sol, which has wide industrial applications, has been used as a model system in numerous studies of coUoidal silica, although in general, the nature of the silica-water interface has received little attention. The interfacial structure may well explain some of the unusual properties of silica sols, such as the high surface charge and exceptional colloidal stability together with the enhanced capacity for sorption and com-plexation of ionic species in solution. Indeed in the past these features have been ascribed to a surface gel layer ... 

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