Stability, colloidal silicas

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

The colloidal stability of silica Suspensions in the present work was assessed by sediment volumes and from the optical coagulation rate constant. In the first method, 50 mg of silica was dispersed in 5 cm3 polymer solution (concentration 10-2 g cm 3) in a narrow tube and the sediment height found at equilibrium. Coagulation rates of the same systems were found by plotting reciprocal optical densities (500nm, 1cm cell) against time. When unstable dispersions were handled, the coagulation was followed in... 

Colloid stability assessments of silica dispersions in CCI4 by sediment volume and coagulation rate are in general concordance and confirm the pattern previously reported by Barron and Howard... 

Many investigators of steric stabilization have measured colloidal stability without taking the effort to find out whether the stability actually resulted from electrostatic stabilization. In many published articles it has been concluded that steric stabilization had been attained and further study showed this was not the case. One such example is a recent paper on "steric" stabilization by an additive of the same type used in this work. (12) The published photograph shows the silica particles in oil stabilized at interparticle separations several times the distances provided by the adsorbed films no electrical measurements had been made, but it they had, this particular dispersant would have provided about -200 mV of zeta-potential and given excellent electrostatic repulsion. The reader should be wary of any claims of steric stabilization unless the electrostatic contribution has been measured. 

The concept of using colloids stabilized with chiral ligands was first applied by Bonnemann to hydrogenate ethyl pyruvate to ethyl lactate with Pt colloids. The nanoparticles were stabilized by the addition of dihydrocinchonidine salt (DHCin, HX) and were used in the liquid phase or adsorbed onto activated charcoal and silica [129, 130]. The molar ratio of platinum to dihydrocinchonidine, which ranged from 0.5 to 3.5 during the synthesis, determines the particle size from 1.5 to 4 nm and contributes to a slight decrease in activity (TOF = l s ). In an acetic acid/MeOH mixture and under a hydrogen pressure up to 100 bar, the (R)-ethyl lactate was obtained with optical yields of 75-80% (Scheme 9.11). 

Colloidal particles, foams used to collect and separate, 12 22 Colloidal powders, 23 55-56 Colloidal silica, 22 380, 382, 384 applications of, 22 394 modification of, 22 393-394 preparation of, 22 392-393 properties of, 22 391-392 purification of, 22 393 Colloidal silica gels, 23 60 Colloidal solids, 7 293-294 Colloidal stability, 7 286-291 10 116 22 55 Colloidal stabilizers, in polychloroprene latex compounding, 19 857 Colloid mills, 8 702 10 127 Colloids, 7 271-303 23 54. See also Polymer colloids analysis, 7 296 applications, 7 292-296 conducting, 7 524... 

Second, nucleation and growth of Stober silica particles is modeled by a controlled aggregation mechanism of subparticles, a few nanometers in size, as for example presented by Bogush and Zukoski (19). Colloidal stability, nuclei size, surface charge, and diffusion and aggregation characteristics are the important parameters in this model. 

In this current work, cerium-modified MCM-41 mesoporous molecular sieve was synthesized using heptahydrated cerium chloride, colloidal fumed-silica, sodium hydroxide, cethyltrimethylammonium bromide and water. The incorporation of cerium to MCM-41 improved the quality, stability and acid properties of the resulting ordered mesoporous material. Its surface and structural properties were extensively studied by nitrogen adsorption and high-resolution thermogravimetry. 

Colloid Stability as a Function of pH, Ct, and S. The effects of pertinent solution variables (pH, Al(III) dosage Ct, Al(III) dosage relative to surface area concentration of the dispersed phase S upon the collision efficiency, have been determined experimentally for silica dispersions and hydrolyzed Al(III). However, one cannot draw any conclusion from the experimental results with respect to the direct relationship between conditions in the solution phase and those on the colloid surface. It has been indicated by Sommerauer, Sussman, and Stumm (17) that large concentration gradients may exist at the solid solution interface which could lead to reactions that are not predictable from known solution parameters. 

Figure 5.9 Illustration of the effect of electrolyte on colloid stability. The photomicrographs A through D show how 1.1 tm size silica particles are progressively coagulated by increasing additions of alum (0, 10, 30, 40 ppm, respectively). The corresponding zeta potentials are -30 mV (A), -14 mV (B), -6 mV (C), and -0 mV (D). From Zeta-Meter [544], Courtesy L.A. Ravina, Zeta-Meter, Inc., Staunton, Va.

Recent experimental studies (1-3), on systems of sterically stabilized colloidal particles that are dispersed in polymer solutions, have highlighted the role played by the free polymer molecules. These experiments are particularly relevant because the systems chosen are model dispersions in which the particles can be well approximated as monodisperse hard spheres. This simplifies the interpretation of the data and leads to a better understanding of the intcrparticle forces. DeHek and Vrij (1, 2) have added polystyrene molecules to sterically stabilized silica particles dispersed in cyclohexane and observed the separation of the mixtures into two phases—a silica-rich phase and a polystyrene-rich phase—when the concentration of the free polymer exceeds a certain limiting value. These experimental results indicate that the limiting polymer concentration decreases with increasing molecular weight of... 

Fig. 1. Interaction potential between two colloidal particles as a function of the reduced centre-to-centre separation R = r/2a, where a is the radius of the particles. Curve 1, steric repulsion due to the adsorbed layer (Vs) curve 2, attraction due to the free polymer (Vd) curve 3, van dcr Waals attraction (X7.,) curve 4, sum of the contributions given by curves 1—3. System polvisobutene-stabilized silica particles and polystyrene (free polymer) in cyclohexane at 308 K. Molecular weight of the free polymer = 82,000, volume fraction of polystyrene, 0 = 0.02, a = 48 nm, thickness of the adsorbed layer 6 = 5 nm, x = 0.5 for polystyrene—cyclohexane, x, = 0.47 and xs = 0.10 for polyisobutene— cyclohexane, AjkT 4.54 and v = 0.10.

The various TEX-WETS are colloidal aqueous silica dispersions widely used in textile finishing to control yarn slippage, modify hand, and control luster. However applied, these products can be used to stabilize weave and to impart special finish effects to fabrics made of cotton, wool, synthetic fibers and filaments, and to mixtures. Treatment gives outstanding finishes on nylon and Dacron marquisettes, rayon fabrics and viscose rayon suitings. Application requires no special equipment nor curing. The dispersions are not cationic and, therefore, cannot be applied by exhaustion onto the fabric. 

The most striking difference between the group of silicas and most other oxides Is that over several pH units above pH° the oxide Is reluctant to dissociate protons, but beyond that charging becomes very easy. This observation does not stand on its own for a number of silicas the colloid stability Is inversely related to 0° In that uncharged sols are very resistant against coagulation by Indifferent electrolytes whereas they become less stable with increasing pH 2-3 4). This... 

Interlayer dielectric (ILD) CMP typically uses a fumed silica slurry dispersed in an aqueous medium at a pH near 11 Fumed silica is a widely adapted abrasive for ILD CMP because of its inexpensive price, high purity, and colloidal stability. However, fumed silica is difficult to disperse in an aqueous system, and it is difficult to control powder processing because of the large specific surface area of 90 15 mVg, making it very reactive. ILD CMP slurry was prepared at pH 11 to accelerate the chemical attack on the deposited PETEOS film on the wafer surface. But silica particles dispersed in aqueous media are partially dissolved at pH 11. Consequently the removal rate decreased and microscratches were generated on the wafer surface due to agglomeration of silica particles as surface potentials decreased. ... 

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