Characterization of silicic acids

Fig. 5. Plot of logm B and Na normalized release rates vs. the activity of silicic acid for both phase-separated and physically homogeneous glass specimens. All rates were plotted at steady-state conditions. MAGNOX and HLP-31 represent phase-separated whereas LAWA33 and HLP-9 represent homogeneous glass specimens. The behaviour of homogeneous glass includes an inverse relationship between rates and silicic acid activity and a difference between B and Na rates as silicic acid activity increase. The magnitude of the difference between B and Na rates is related the amount of excess Na (see Fig. 3). Relatively faster element release rates ( lOx), identical release rales of Na and B, and independence from activity of silicic acid appears to characterize phase-separated glass.

Amorphous silica exists also in a variety of forms that are composed of small particles, possibly aggregated. Commonly encountered products include silica sols, silica gels, precipitated silica, and pyrogenic silica (9,73). These products differ in their modes of manufacture and the way in which the primary particles aggregate (Fig. 8). Amorphous silicas are characterized by small ultimate particle size and high specific surface area. Their surfaces may be substantially anhydrous or may contain silanol, —SiOH, groups. These silicas are frequendy viewed as condensation polymers of silicic acid, Si(OH)4. 

Taking into account that non porous aerosil particles have a spherical structure, this theoretical treatment is applicable also in this case. A silica represents a heterochain inorganic polymer of silicic acid. However, most of its modifications are difficult to characterize in such terms as molecular mass and amount of functional groups per monomer unit and this question is still under discussion. However, such estimates can be made for aerosil, a highly disperse silica produced by combustion of silicon tetrachloride in the oxygen hydrogen flame. 

Aging of silicic acid sols To characterize the silicate species in sodium silicate solutions, the silicic acid liberated therefrom must be examined promptly. At a silica concentration of lOOmM Si02 at pH 1.7, 25 C, measurable change begins in about 30 minutes. The sol must be examined within this time. 

K.G. Proctor and D.E. Leyden, Surface Acidity Characterization of Siliceous Materials by Variable Temperature Diffuse Reflectance FTIR, Chemically Modified Oxide Surfaces, Vol. 3, D.E. Leyden and W.T. Collins Eds., Gordon and Breach Science Publishers, New York, pp. 137-149, 1990. 

MTX is readily separated from CTX through the use of silicic acid colunm and a stepwise elution with chloroform and methanol (Tachibana 1980). MTX elutes with chloroform/methanol (1 1) and ciguatoxin with (9 1). A MTX-like toxin is produced by G. toxicus and possibly by R concavum. The toxin has been produced in abundance in dinoflagellate cultures however, it is not well characterized. 

The depolymerization of a particular species of silicic acid is a first-order reaction so that the species can be characterized by a specific reaction rate constant. 

The theory of the formation and structure of silica gels formed by the polymeriza tion of silicic acids hu been dbcussed in Chapter 3 and similar aspects of gels made from sols of discrete colloidal particles dealt with in Chapter 4. Characterization of gels has been described in the foregoing section. Described here are methods of mak ing gels and the factors and process variables that affect and control gel properties and uses., ... 

Because it is one ofthe oldest systems investigated, the PDMS-Si02 systems is probably one of the most well characterized hybrid system. The compatibility between the silanol-terminated PDMS and TEOS has also made the mixture particularly attractive to many researchers (Wilkes, 1985 Huang, 1987 Guo, 1999 Teowee, 1996). This compatibility, as shown in the reaction below between a silanol-terminated PDMS chain and a molecule of silicic acid, is typical ofthe kind of reaction found in so-called type II hybrids where cross-linking between organic and inorganic components can take place ... 

Di Renzo, F, Chiche, B., Fajula, R, Viale, S., and Garrone, E. Mesoporous MCM-41 alumino-silicates as model silica-alumina catalysts spectroscopic characterization of the acidity. Stud. Surf. Sci. Catal 1996,101, 851-860. 

Bonelli, B., Onida, B., Chen, J.D., Galarneau, A., DiRenzo, R, Fajula, R, and Garrone, E. Spectroscopic characterization of the acidic sites of Al-rich microporous micelle-templated silicates. Micropor. Mesopor. Mater. 2004, 67,... 

GT-3 was separated from GT-1 and GT-2 on silicic acid using 100% methanol. The separation of this fraction is the same as that used by Tachibana (22) and Yasumoto (13) for maitotoxin. Unlike GT-1 and GT-2, this fraction was found to be an irreversible inhibitor of the guinea pig ileum preparation. The time studies of GT-3 are important in that they characterize GT-3 as a very toxic fraction but one which is very slow in its action. 

In this review, we focus on cluster models of Br0nsted acid sites, bridging hydroxyl groups that result from the incorporation of trivalent aluminum atoms into the siliceous framework during synthesis. These sites are by no means the only active sites within zeolites, but they are among the best characterized. 

When a mixture of methylsilane (or dimethylsilane) deposited with ozone in an argon matrix at 17 K was photolyzed, the corresponding silanone, MeHSi=0 (or Me2Si=0), was generated and characterized by their infrared spectra. In the case of the parent silane, H3SiH, a similar photolytic reaction with ozone in an Ar matrix led to the identification of SiO, H2Si=0 (2), (HO)HSi=0 (silanoic acid) and (HO)2Si=0 (silicic acid)18. 

---