Silica gelation time

FIGURE 10J38 Gelation time versus Si02 concentratioa Two curves are shown (1) experimental observations are for Ludox SM silica particles (7 nm diameter) in 0.5 M NaCl solution, pH = 8.5, (2) predictions based upon equation 10.90 and [Pg.484]

Fig. 2. Loss tangent as a measure of gelation time for a silica sol (27) (a) loss (A) and storage (B) modulus as a function of aging time for H2O/TEOS mol ratio of 20 and HNO /TEOS mol ratio of 0.01 and (b) loss tangent as a function of aging time. To convert mPa to mm Hg, multiply by 7.50 x 10 . ...

References [3039,3040] are cited in [1] as a source of PZC of silica. Actually it is only speculated in [3040] that a maximum in gelation time may coincide with the point of zero charge, and in [3039] PZC is not mentioned at aU. 

These effects are demonstrated in Fig. 6.6 showing surface areas and gelation times for silica sols. From a practical point of view, it is necessary to compromise between large surface areas and reasonable gelation times. 

Silica gels, such as those in Fig. 6.6, are prepared by mixing solutions of water glass (sodium silicate with SiOj/NajO 3.22) and HCl. This is an example of condensation and produces (H0)3Si-0-Si(0H)j sols of about 1.5 nm at a pH of 6. Gelation times of 10 min result in gels so stiff they may be cm into cubes. The hydrogel has a pore volume of 2.0cm g and contains about 60%-70% H O. 

The formation of a hydrosol which sets to a true hydrogel in a short time is the basis for an important recent advance in the art of catalyst manufacture on a large scale. The gelation time of a silica-alumina hydrosol is a function of several variables (pH, concentration, degree of mixing of reagents, and temperature), the control of which permits a uniform dispersion of the hydrosol into an immiscible fluid, such as oil,... 

Effect of TFPS/TEOS molar ratio on the gelation time of silica sols (weight ratio of sol to NH4OH solution is 50 1)... 

Textural characteristics of silica and silica alumina hydrogels for each mixing mode (gelation time 150s)... 

Fig. 13 Changes in self-correlation function of silica gel (silicon dioxide) at sol-gel transition (gelation time is 406 min) [38].

The relative fluorescence intensity of pyrene excimer (E/T + M) as a function of gelation time for silica gel glasses from TMOS and TEOS (no added catalyst). Arrows indicate isostructural points of the two gels. O the change of the /[ /j ratio as function of gelation time [120]. 

The intermediate silica-organoclay nanocomposites clearly show the delamination of the layered silicates evidenced from the XRD patterns, showing the absence of the 001 rational order characteristics of the starting silicates. Figure 14.6 shows the loss of crystallinity of the layered silicates after reaction with TMOS in -butanol at 50°C in 1 1 clay/TMOS w/w ratio, with the gelation time ranging from 0.5 to 2 h. 

In general, acid hydrolysis and condensation results in linear or weakly branched chains and microporous structures in silica sols [48] and the resulting gelation times are generally long. In contrast, uniform particles are easily formed in base catalysis (i.e., mostly NH4OH based) and lead to a broader distribution of larger pores, which is less favorable for thermal insulation materials [49]. 

---