Characterization methods, colloidal silica

The characterization of colloidal silica has been the subject of numerous studies involving both physical and chemical methods. Iler (I) summarized many of the available methods, with particular emphasis on chemical approaches. Other more recent reviews (2-4) featured instrumental methods that are useful for characterizing silica sols and other colloids. This chapter describes some of the relatively new separations... 

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. 

This characterization method (SEC) is effective and relatively simple for silica sols and other colloids. This technique is commonly used for... 

The finely divided colloidal silica powders useful in the composition of the invention are characterized by having specific surface areas as determined by nitrogen adsorption according to the BET method, of from 5 to 800 m /g and preferably 50 to 250 m /g, and being further characterized by the fact that the aggregates of ultimate silica particles are generally less than 10 p. in diameter. 

This characterization method SEC is effective and relatively simple for silica sols and other colloids. This technique is commonly used for determining the molecular-weight distribution of a wide range of synthetic and natural macromolecules [23]. However, its application for characterizing colloids has not been widely practiced. SEC has been utilized to determine the particle-size distribution of various polymer lattices, but few applications of the method for characterizing silica sols have been reported [24,25]. 

At a relatively early stage in the polymerization it is possible to characterize the polymeric silica, or silica particles in terms of the specific area of the silica-water interface. This is done by measuring the adsorption of hydroxyl ions in the pH range 4.00-9.00 (Beckman Type E electrode) in a nearly saturated salt solution which permits the surface charge denstly to approach a maximum. This method was developed by Sears (85) to determine the specific surface areas of colloidal particles and gels. Then it was found that if carried out rapidly it could give reproducible... 

Monodisperse spherical colloids and most of the applications derived from these materials are still in an early stage of technical development. Many issues still need to be addressed before these materials can reach their potential in industrial applications. For example, the diversity of materials must be greatly expanded to include every major class of functional materials. At the moment, only silica and a few organic polymers (e.g., polystyrene and polymethylmethacrylate) can be prepared as truly monodispersed spherical colloids. These materials, unfortunately, do not exhibit any particularly interesting optical, nonlinear optical or electro-optical functionality. In this regard, it is necessary to develop new methods to either dope currently existing spherical colloids with functional components or to directly deal with the synthesis of other functional materials. Second, formation of complex crystal structures other than closely packed lattices has been met with limited success. As a major limitation to the self-assembly procedures described in this chapter, all of them seem to lack the ability to form 3D lattices with arbitrary structures. Recent demonstrations based on optical trapping method may provide a potential solution to this problem, albeit this approach seems to be too slow to be useful in practice.181-184 Third, the density of defects in the crystalline lattices of spherical colloids must be well-characterized and kept below... 

This is an effective and relatively simple method for characterizing silica sols and other colloids [75]. It has also been used to determine the particle size distributions of polymer lattices [76,77]. Separations are performed in a column packed with particles having pores substantially of the same size. A carrier liquid is passed through the column as a mixture of colloidal particles passes through the bed, the larger ones exit first since they are too large to sample the pore volume. Intermediate sized colloids enter the pores and are retained according to the volume that can be... 

One of the most important developments in the 1980s in the search for novel concepts to characterize colloidal systems such as silica sols and gels was the advent of the fractal approach. Another very important development in the past 10 years was the application of 29Si CP MAS NMR methods to the study of the silica surface. This technique made it possible, for example, to identify without ambiguity the presence of silanediol groups on the silica surface (55). 

To determine the size of unknown silica sols with relatively narrow distributions, the retention time of the band for an unknown sample is compared to a peak-position calibration plot such as that shown in Figure 9. Just as for the determination of molecular weight for polymers, broad distributions of silica sols can be measured with appropriate software by using known calibration methods involving peak positions or by using known standards with broad distributions (26). However, commercial software for specifically characterizing silica sols and other colloids is apparently not yet available. 

This separation method HDC was developed within the Dow Chemical Company in the early 1970s [27]. This method has been utilized for determining the particle size of many polymer lattices [27,28], but it also can be used for characterizing a wide range of silica sols. Separation by the hydrodynamic effect is illustrated in Figure 40.12 [29]. Colloids flowing between particles in a packed bed or within a capillary are subjected to different velocities... 

Information about the bound water fraction in some colloid systems, silica gels, and biological systems is usually inferred on die basis of the frequency- and time-domain DS measurements from the analysis of the dielectric decrements or die relaxation times (64, 150-152). However, the nonionic microemulsions are characterized by a broad relaxation specfrum as can be seen from the Cole-Cole plot (Fig. 33). Thus, these dielectric methods fail because of the difficulties of deconvoluting die relaxation processes associated widi the relaxations of bound water and surfactant occurring in the same frequency window. 

FIGURE 1.254 Pore size distributions of unmodified and modified nanosilica A-380 (Table 1. 3) calculated on the basis of the nitrogen adsorption/desorption isotherms using DFT method with the model of pores as voids between spherical particles and NMR cryoporometry with IGT equation. (Adapted from/ Colloid Interface ScL, 308, Gun ko, V.M., Turov, V.V., Zarko, V.I. et al., Comparative characterization of polymethylsiloxane hydrogel and silylated fumed silica and silica gel, 142-156,2007h. Copyright 2007, with permission from Elsevier.)... 

---