The Nature of Silicate Solutions

It should be pointed out that such a composition of 50 vol. % solids dried to an extremely brittle state before the last of the water is removed. In considering the behavior of fillers in plastics, for example, more than about 40% by volume gives a hard, relatively brittle mass. Likewise, less than 40% by volume of colloid in silicate should give a less brittle film as the last of the water is removed. In this case, by similar calculations it is found that this corresponds to a SiO-iNajO fatio of 3.5. Perhaps by coincidence this is near the common ratio used in adhesives. 

It seems, therefore, that at least some of the behavior of soluble silicates with ratios greater than 2 is consistent with the theory that the solutions consist of NaHSiOj in solution along with very small particles of colloidal silica with silanol surfaces. 

The fundamental equilibria involved have in the past been listed as follows  

For the above equations at 25 C, taking the ionization constant of water as 10 [HKHSiO,-] 

Taking this value of 2.5 mM for the concentration of Si(OH) , and Roller s and Ervin s equilibrium constants, it is possible to calculate the concentration of the various species at equilibrium. 

---

Many minerals of known structure have been studied by solid state Si NMR [13] and have thus provided a partial basis for identification of silicate species in solution. After pioneering Si NMR work on silicate solutions at low fields [14] had shown that the Si atoms with different connectivities could be easily identified, rapid progress followed and the introduction of sophisticated NMR techniques revealed more detailed information on the nature of silicate solutions. Figure 2 illustrates the power of high-resolution Si NMR very sharp lines allow in principle the distinction of many species. The achievements of this technique or the combination of NMR spectroscopy with chemical trapping have been impressive and include the following ... 

Modern analytical instrumentation has been used in the last 23 years for determining commercially important characteristics of soluble silicates, and the nature of silicate species in silicate glasses and solutions. The classical wet methods for assay of silicate solutions are alkali titration and gravimetric determination of silica, which can also be determined, with lesser precision, by the alkali fluosilicate method. 

In 1968 Wilson published an account of his early search for alternatives to orthophosphoric acid as a cement-former with aluminosilicate glasses. Aluminosilicate glasses of the type used in dental silicate cements were used in the study and were reacted with concentrated solutions of various organic and inorganic adds. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses. 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

The amount of charges on particle surfaces depends on the mineralogy of the solid and the nature of the aqueous solution in which it occurs. Several important kinds of surfaces are common in the environment (Table 11.3). Here we especially consider (1) oxides or oxyhydroxides, (2) alumino-silicates or clay minerals, and (3) natural organic matter and other solids like carbonates. 

Under our experimental conditions, no significant influence of the CTAB polar head group on the nature of the silicate oligomers is observed. Indeed, the spectra of experiments 3 and 11 are similar to those of the corresponding silicate solutions free of surfactant (spectra not reported) [10,16],... 

Silicic Adds. The behavior of silicate ions in solution, the dependence of various properties on pn, the nature of silica sols and gels, and the study of hydrated silicas constitute chapters in inorganic and colloid chemistry that go far beyond the scope of this review. Germane to the present subject, however, are certain observations on the formation of monosilicic acid and its stepwise polymerization. 

The hydrolysis of silicate minerals involves chemical reactions with weakly acidic or alkaline aqueous solutions. With natural waters that contain dissolved C02-that is, C02(aq) which can be written as carbonic acid (H2C03)-the hydrolysis of magnesium silicate (olivine) proceeds as... 

If sufficient silica was present, the ratio of silicate to cation had little effect on the nature of the substances obtained. If the solution was deficient in silica, additional silica was obtained from the walls of the flask. In some instances, the cations combined with the glass to form clays on the walls of the flask. Following this lead, they were able to place basalt in contact with a magnesium solution and form montmoril-lonite. The silica content of products formed is larger in a neutral or alkaline medium than in an acid one. At low pH values, the silica sheets are incomplete and the... 

NMR spectroscopy is a powerful technique for identifying the structure and concentration of silicate and aluminosilicate anions in gels and solutions used for zeolite synthesis. A review is presented of the types species that have been observed and the dependence of the distribution of these species on pH and the nature of the cations present. 

High resolution Si NMR spectroscopy can provide considerable insights into the structure and distribution of silicate and aluminosilicate anions present in solutions and gels from which zeolites are synthesized. The narrowness of individual lines and the sensitivity of the chemical shift to details of the local chemical environment make it possible in many instances to identify exact chemical structures. Studies using Si NMR have shown that the distribution of anionic structures is sensitive to pH and the nature of the cations in solution. Alkali metal cation NMR has demonstrated the formation of cation-anion pairs the formation of which is postulated to affect the dynamics of silicate and aluminosilicate formation and the equilibium distribution of these species. NMR has proven useful in identifying the connectivity of A1 to Si,... 

There is a = 4% difference between our value of the solution enthalpy of the highly siliceous calcined si] -MFI sample and the value published recently by Johnson et al. (29) for the dissolution in 24. i% HF of a sample of silicalite I prepared in alkaline medium in the absence of fluoride (-144.93X0.18 kJ/Si02 for a Si02/HF ratio equivalent to the ratio used here). Such a difference could be related to the nature of the samples and to morphology differences. Our calcined sample contained traces of fluorine whereas the silica-... 

---