The Silica-Water System

As will be seen shortly, an analogous result is obtained with the silica-water system, where the BET monolayer capacity of water calculated from the water isotherm is roughly equal to the hydroxyl content of the silica surface. 

Solubility. An important aspect of silica chemistry concerns the silica— water system. The interaction of the various forms of silica with water has geological significance and is applied in steam-power engineering where the volatilization of silica and its deposition on turbine blades may occur (see Power GENERATION), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble silicates, colloidal silica, and silica gel. 

The unusual nature of the silica-water system has been noted by J. A. Kitchener (7), who pointed out that the endless confusion in the literature concerning the silica-water interface has arisen because the hydration and solubility characteristics have not been understood. For example, there is the question as to why silica sols are extraordinarily stable at pH 2 where the zeta potential is zero and become increasingly sensitive to electrolytes at higher pH. where the potential is highest—in contradiction to the generally accepted electrical double layer theory. Another mystery is that crystalline quartz becomes coated with a film of amorphous silica even though the solution is undersaturated with soluble silica with respect to a surface of amorphous silica. 

Few if any of these various disilicic acids, which are highly polymerized in two dimensions, are identical. The relation of these to the previously discussed hydrated crystalline silicas obtained from hydrated sodium polysilicates is not known. It is evident that a large number of crystalline hydrated silicas may exist, some more stable than others, but all obtained from crystalline silicates by ion exchange. No crystalline hydrate silica is likely to be formed directly in the silica-water system in the absence of cations to bring about the organization of a regular polysilicate structure. 

The significance of the iep of silica in the silica-water system involving SifOH) and polymerized or solid silica surfaces is still not clear, but the preponderance of evidence suggests that for monomeric Si(OH)4 the iep may be between pH 2 and 3, and for polymeric forms between 1.5 and 2. 

A major difference between the silica-water system and other aqueous solutions of inorganic compounds is that in the case of silica a catalyzed making and breaking of siloxane bonds occurs, whereas no such requirement for a catalyst seems to be... 

Silicas seems to behave, to some degree, differently than the other oxides. The outstanding characteristic of the silica-water system is the tendency to form colloidal solutions or hydrated... 

Figure 9. Effect of pH on the stability (gel time) of the colloidal silica-water system. Thick solid lines represent experimental results (3). Shaded areas and white area in between are approximate zones corresponding to behavior predicted by the DLVO theory (1, 26), some in contrast with experimental results minimum stability predicted at pH around 2-3, increasing stability predicted at pH between 3 and 6-8, and maximum stability predicted at pH higher than 8. (Reproduced with permission from reference 3. Copyright 1979 John Wiley ir Sons,...

Water, however, modifies the patterns of self-organization, and this is why the structural outcome of such reactions depends greatly on the water content. The peculiar self-aggregation behavior of the IL/water system is seen by comparing two sol-gel derived IL-silica hybrid materials (using lu), prepared with varying amounts of water (Fig. 6.3-4), but the same ratio of IL to silica. 

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 ... 

Another relevant system involves oleic acid (OA) adsorption at the silica-water interface. This method was first demonstrated by Ding et al. [45] and was next used by Mahdavian and coworkers to encapsulate very small silica nanoparticles [46]. In the latter case, a core-shell structure with a core composed of aggregated silica particles and a shell made of MMA, styrene and acrylic acid (AA), was formed. The authors suggest that the polymerization proceeds through oligoradical entry into the OA admicelles. 

FIGU RE 1.263 Free surface energy as a function of PMS concentration in the PMS/water systems. (Adapted from J. Colloid Interface Sci., 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,... 

Here we will summarize the reaction steps that lead to sihca gel formation, and focus on the physical chemistry of the aging processes. NMR proton spin—latice relaxation measurements can be used to follow the morphological changes that occm in a silica gel as a function of time. Owing to the restricted movements and, hence, increased correlation times of the water molecules near to the silica-water interface, protons of water included in a porous solid have longer relaxation times than water in the bulk. A decrease in the spin-spin relaxation time is to be interpreted as an increase in the number of water molecules restricted in their movements due to the proximity of the sihca structure. Figure 9.20 shows the spin-spin relaxation for a silica system as a function of relaxation time. The decrease in Tg in the first 20 min of the reaction indicates the formation of increasing silica surfaces in contact with water. 

Figure 4.13. Effects of pH in the colloidal silica-water system.

In principle, information regarding the condition of adsorbed and interlayer water, as well as about its magnetic environment, can be obtained from nuclear magnetic resonance studies of clay-water systems. Such studies have been presented by Pickett and Lemcoe [1959], Blaine [1961], Ducros and Dupoot [1962], Graham et al [1964], Wu [1964], and Touillaux et al. [1968]. Studies on silica-water systems were made by Zimmerman et al. [1956], Zimmerman and Lasater [1958], and Fraissard et al [1963]. 

Silica SiOj Results in the formation of scale in boilers and cooling water systems, can produce insoluble scale on turbine blades due to silica vapori2ation in high pressure boilers (usuallu over 600 psi). 

A second consideration is that RO tends to be sensitive to incoming suspended matter. Comprehensive and sometimes expensive pre-treatment technologies are generally needed with RO, whereas ion exchange is less sensitive to the suspended matter. Further, RO systems are sensitive to hardness, so that softening is usually required as a pre-treatment. As a rule, RO membranes cannot handle high silica waters. 

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