Ludox®

Fig. 5. X-ray diffraction pattern of commercial samples (a) coUoidal silica, DuPont Ludox TM50 (b) silica gel, Sylodent 700 (c) precipitated silica, PPG...

Fig. 7. Transmission electron micrograph of a commercial coUoidal siUca, DuPont Ludox TM50. Magnification of 225, OOOx. ...

It can be advantageous to heat the chromatogram to 160 °C for 15 min before treating with nitrous fumes and to place it in the reagent chamber while still hot [1]. Heating to 260 °C has even been recommended for the purpose of reducing the fluorescent background [14], whereby the layer is previously immersed in 1 percent Ludox solution (silidc acid sol) to increase its stability [2]. The fluorescence of the substances detected usually remains stable for at least 2 weeks [2]. 

Figures 6 and 7 show the absorption spectra of colloidal CdS and ZnS at various times of illumination. The two colloids were prepared by adding an NaSH solution to solutions of Cd(C10j2 or Zn(C104)2, respectively, colloidal silicon dioxide (commercially available from Dupont Ludox HS30) being present at 6 x 10 M as stabilizer in both cases. The absorption starts in both cases close to the wavelengths that correspond to the photon energies (515 nm or 2.4 eV for CdS 340 nm or 3.7 eV for ZnS) at which the absorptions begin in the macrocrystalline materials. It is seen that illumination causes not only a decrease in the intensity of the absorption spectrum but also a change in the shape of the spectrum. The onset of light absorption is shifted towards...

Fig. 6. Absorption spectrum of 2 lO"" M CdS at various times of illumination > 490 nm) in the presence of air >) Stabilizer 6 10" M colloidal SiOj (Ludox HS30). Initial mean diameter of particles 3 nm...

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method (mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkali-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method (dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal SiC>2 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al3+ usually present in colloidal silica (33). 

Dissolved titanium method. Hydrothermal synthesis using tetrapropylammonium peroxytitanate (prepared from TEOT, distilled water, 30% aqueous H2O2, and 25% aqueous TPAOH) as the source of Ti and colloidal silica (Ludox AS-40) as the source of Si and TPAOH as template. All additions done at 278 K Preparation using TiCl2, 14% aqueous TPAOH, 30% colloidal silica, and demineralized water Preparation at low pH using fluoride ions as mineralizing agent... 

For SiC>2, we have only considered sources for silica suspensions which were non-porous, such as Ludox (39), pyrogenic silica (40), heat-treated BDH silica (22), or ground quartz (41). The data from these sources at 0.1M concentration has been collected in Figure 7. The data of the various researchers is quite consistent, in spite of the differences in origin of the suspensions, and the different electrolytes used. The slope of the points above pH 7 shows that the adsorption capacitance for cations is very large for both sodium and potassium ions, around 200 pF/cm2. Such a capacitance corresponds to a distance of 0.25.X, when using the dielectric constant of immobilized water molecules. The equilibrium constant for adsorption is low, however, since both KNa+ and Kk+ lie between 0.1 and 0.01 dms/mol. A possible interpretation of these results is as follows there is little specific attraction between SiC>2 and alkali cations,... 

When deconvolution is required, the time profile of the exciting pulse is recorded under the same conditions by replacing the sample with a scattering solution (Ludox (colloidal silica) or glycogen). 

Fig. 6.12. Data obtained by the single-photon wavelength 340 nm observation wavelength timing technique using a mode-locked ion- 390 nm. Reference scattering solution argon laser that synchronously pumps a cavity- (Ludox). Number of channels 900 channel dumped dye laser. Sample solution of POPOP width 4.68 ps. Result t = 1.05 + 0.01 ns in cyclohexane (undegassed). Excitation x = 1.055.

Bolt 191), however, found in determinations of the charge density of Ludox silica sols a value of 1.8 negative charges per 100 A at pH 10. 

Instrumentation. The steady-state fluorescence spectra were measured with Perkin-Elmer MPF-44B fluorescence spectrophotometer. The single-photon counting instrument for fluorescence lifetime measurements was assembled in-house from components obtained from EG G ORTEC. A PRA-510B light pulser filled with gas was used as the excitation source. Instrument response function was obtained with DuPont Ludox scatter solution at the excitation wavelength. 

The turbidity of Ludox (a colloidal silica manufactured by DuPont) has been studied as a function of concentration with the following results ... 

Evaluate the molecular weight of the Ludox particles, using a value of H = 4.08 x 10 7 for the system. Calculate the characteristic diameter for these particles, assuming the particles to be uniform spheres of density 2.2 g cm-3. 

Zeolite rho was prepared from aluminosilicate hydrogels containing sodium and cesium cations. The procedure is entirely comparable with the synthesis of faujasite except for substitution of CsOH for about 10% of the NaOH in the faujasite synthesis gel. Alumina trihydrate (Alcoa C-33 grade) was dissolved in 50% NaOH solution at 100°. After cooling to ambient temperature, the required amount of CsOH solution was added, and the resulting liquor was blended into 30% silica sol (duPont Ludox LS-30) with vigorous mixing. After 3-7 days incubation at 25°, the synthesis gel was held at constant temperature, 80, 90, or 100°, until crystals formed maximum crystallinity was usually achieved in 2-4 days. 

Mesostructured molecular sieves such as Si-MCM-41 and Al-MCM-41 were synthesized using gels prepared by reacting colloidal silica (Ludox AS) with an Al(OH)3, solution in the presence of a surfactant. Al was incorporated in tetrahedral coordination inside the pristine crystals. However, as expected, dealumination occured upon calcination at 600°C/12h yielding materials having both tetrahedral and octahedral Al-species. 

KIT-1 was prepared using the recipe of Ryoo et al[l] using Ludox HS-40 as a silica source. HMS, MCM-41 and MCM-48 were prepared by hydrothermal procedure following... 

Perikinetic Coagulation. The concentration of small silica colloids (Ludox LS d = 15 m/ ) used in these studies was between 0.18 gram/liter and 0.43 gram/liter (36.5 — 86.0 meter2/liter or 5.0-11.5 X 1013 par-ticles/ml.). The dispersion, contained in a spectrophometric cell, was... 

B) Relative colloid stability as a function of the total concentration of coagulant added, Ct, at constant pH—obtained from the measurement of perikinetic coagulation rates according to Equation 3. (Ludox LS = 0.3 gram/liter)... 

It can be shown with Equation 8 that both transport mechanisms contribute equally to the coagulation of a Ludox LS dispersion, containing... 

Cummins, P.G., Penfold, J., and Staples, E. (1992). Nature ofthe adsorption of the nonionic surfactant pentaethylene glycol monododecyl ether on a ludox silica sol. J. Phys. Chem., 96, 8092-8094. 

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