Colloid stability silica

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. 

We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

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. 

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.

The hydration force is important for distances between plates less than a few nanometers. Since the DLVO potential barrier between spherical particles or droplets is located at separation distances of the order of the Debye length, it is clear that at least at high electrolyte concentrations the hydration force becomes relevant. The qualitative effect of hydration was earlier recognized, regarding the stability of silica colloids at high electrolyte concentrations1 or the stability of amphoteric latex particles at high concentrations of some electrolytes.12... 

Stability of mixed iron-silica colloidal solutions... 

In an experimental study of mixed iron-silica colloids we found the opposite phenomenon, stabilization of colloids of iron hydroxide by silica colloids, which is manifested very clearly in undialyzed iron hydroxide sols of average concentration with a ratio of Fe203 SiOj = 1 3. Colloidal iron in mixed solutions proved to be more resistant to the action of electrolytes than in isolated sols of iron hydroxide. Only colloidal silica shows any stabilizing effect on sols of iron hydroxide dilute solutions undersaturated with Si(OH)4 are not stabilizers. It is characteristic that colloidal silica is capable of stabilizing colloidal iron in the same pH ranges in which pure silica sols are stable in acid (pH < 4) and alkaline (pH > 8) environments. In slightly acid environments (pH = 5-6) iron-silica sols are unstable and decompose to form mixed sediments, which sometimes are not uniform due to different rates of coagulation and deposition. 

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. ... 

Yasrebi, M. et al.. Role of particle dissolution in the stability of binary yttria-silica colloidal suspensions, 7. Am. Ceram. Soc., 79, 1223, 1996. 

Pham, K.N., Fullston D., and Sagoe-Crentsil, K., Surface modification for stability of nano-sized silica colloids, J. Colloid Interf. Sci., 315, 123, 2007. 

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