Monodispersed silica sol

Monodisperse silica sols of particle size up to about 2-3 fini in diameter were first obtained by Stober in an alcohol-ammonia system with enough water to hydrolyze a silane precursor (22). 

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... 

Monodisperse silica sols are important in the previously mentioned Talalay process for making foam rubber pillows. If a 4-nm silica sol is normally used, then a substitution of a 3.5-nm particle at the same silica dosage would theoretically increase the strength of the rubber by 14.3%. If, however, the particle size is changed from 4.5 to 5.0 nm, the strength of the rubber would decrease by 11 to 20% at the same silica dosage. Under these conditions, the crumbling effect of the foam rubber would return. 

Kazantseva et al. (65) synthesized monodisperse silica sols by hydrolysis of a mixture of tetraethoxysilane and C2H5OH-H2O-NH3 at room temperatures. The degree of monodispersion of the silica sols increased with time and corresponded to an increase in the diameter of the spherical particles. When hydrolysis had proceeded for 3.5-4 h, the same uniform size of the particles reached —300 nm. 

SILICA NUCLEATION, POLYMERIZATION, AND GROWTH PREPARATION OF MONODISPERSE SILICA SOLS... 

Si02 gel DDAB/toluene/water (48.7% DDAB, 19.5% decane, 31.8% aqueous silica sol) TMOS (partially hydroiyzed)/H20 (+ 0.4-10 wt%HF) Bicontinuous p,Es used as templates for microporous silica gels monodisperse pores (2 nm pore radius) large specific surface area (—103 m2/g) (51)... 

The work of Mallouk et al. (39) offers an interesting extension of the microemulsion sol-gel technique. In this case, microemulsion-derived silica nanoparticles were used as templates for preparing ordered mesoporous polymers with tailored pore sizes. Utilizing the Triton N-101/cyclohexane/hexanol/water/ammonia microemulsion, monodisperse silica nanoparticles were first synthesized. The silica product... 

Monodispersed polystyrene sols are used as calibration standards for electron microscopes, light scattering photometers, Coulter counters, particle sieves, etc. Monodispersed silica is used for antireflection lens coatings. Monodispersity (even at a modest level) can usefully be exploited in photographic film, magnetic devices, pharmaceutical preparations and catalysis. 

Silica sol which is a stable sol of monodispersed particles is made by the decomposition of sodium silicate at low concentration under controlled pH and surface treatment and then evaporation of water. A stable floccular aggregates of silica particles are made by the peptization of dilute hydrogels under controlled pH. The former material is used as fiber and paper sizing, binder and the latter is used for water treatment. 

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 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. 

Various raw materials can be used in the manufacturing of monodis-persed sol. Examples of these materials include silicon metals (6), silicon tetrachloride (7), ethyl silicate (8), water glass (2), and silica powder (9). In this chapter, I focus attention on the preparation of monodispersed sols from water glass, a raw material that is presently used in large amounts industrially for the inexpensive production of silica sols. 

It is difficult to define precisely the term aqueous silica sols and thereby contrast them with other forms of silica (colloidal silica, colloidal quartz, pyrogenic silica, and so forth). Bulk chemical distinctions are not very useful. The definition chosen here follows Iler s terminology (I). Aqueous silica sols are characteristically composed of spherical particles nucleated and grown by alkaline hydrolysis of sodium silicate solutions. They are often monodisperse systems and have particle diameters in the range 1-100 nm (density, —2.2 g/cm3) that lead to sols that vary from optically transparent to opalescent. 

The Future, Two types of silica sol products are needed in the 1990s specialty products and organosols. Specialty products are used in high-technology areas. Price is generally unimportant if they work. Typical examples of specialty products are monodisperse sols [i.e., one particle size, low sodium, and low metal (aluminum, iron, etc.) concentrations, and no aggregation]. 

As for the preparation of colloidal crystals using polymer spheres, the monodispersity of the cells strongly influences the order of the material. Hence yeast cells were carefully grown to form spherical cells of similar diameter. These cells were dip coated with a silica sol on a microscope slide [39]. A mono-layer of the cells arranged in a hexagonal close packing form on the microscope slide. The interstitial sites between the cells contained silica, an SEM image of the film is shown in Fig. 4. The cells remain alive and such films have potential applications in catalysis and as sensors. 

J. Joseph, C.-Y. Tseng, C.-J. Pan, H.-M. Chen, C.-W. Lin, K.C. PiUai, B.-J. Hwang, Growing well-defined monodispersed silica in polyimide host membranes nsing a surfactant assisted sol-gel process. Polymer 51 (2010) 5663-5668. 

PDMS is relatively weak and stands to benefit most from reinforcement. PDMS is easily absorbs the precursor materials generally used in the sol-gel process. Nearly monodisperse silica microparticles can be obtained using siloxane elastomer mixtures. In some cases, the PDMS has been part of a copolymer, with ureas, imides, amideimides, ° and diani-lines. In other approaches, the particle surface is modified, for example, with a polysiloxane. Siloxane/silica nanocomposites have also been used as culture-stone-protective materials. ... 

Sol-gel growth techniques. The most common example is the production of synthetic opal. The first step consists to produce a suspension of monodisperse silica nanospheres in ethanol obtained by the direct hydrolysis of tetraethyl ester of orthosilicic acid, SifOC H l with ethanol using ammonia as a catalyst according to the following reaction ... 

Figure 11 Sol-gel condensation of tetramethoxysilane inside a self-assembled organopaUadium sphrae produces monodisperse silica nanoparticles. (Reproduced with permission from Ref. 44. Nature Publishing Group, 2010.)...

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