Silica particles stabilization

Binks BP, Whitby CP (2004) Silica particle-stabilized emulsions of silicone oil and water aspects of emulsification. Langmuir 20(4) 1130-1137... 

Little work has been published on depletion stabilization in nonaqueous dispersion media. Clarke and Vincent (1981a) have noted that it is possible to prevent silica particles stabilized by polystyrene in ethylbenzene from undergoing depletion flocculation by adding a high concentration of polystyrene (v2 =0 015 for a free polystyrene molecular weight ofca 2 x 10 ). This was the first reported observation of depletion stabilization in nonaqueous dispersion media. 

Data on catalytic activity of adsorbed proteins give the information of great importance about conformational state in adsorbed states. Proteolytic enzymes show the ability to autholysis, that can proceed in the solution and in adsorption layers. We observed inhibition of autholysis of protein molecules in adsorption layers [22]. Sometimes the addition of suspensions with hydrophibic or hydrophilic particles leads to change in the rate of autholysis [101]. For savinase the half-life of enzymatic activity in solution is 3.5 hours, and this period is strongly reduced in the presence of particles with hydrophobic surfaces. On the contrary, hydrophilic silica particles stabilize the adsorbed enzyme against autholysis. 

Smith J, Webber GB, Warr GG, Atkin R (2014) Silica particle stability and settling in protic ionic liquids. Langmuir 30(6) 1506-1513. doi 10.1021/la403978b... 

Figure 7.7 Influence of nanoparticles on the phase behavior of bina7 blends, (a) Chlorinated polyethylene (CE)/poly(ethylene vinyl acetate) (EVA) blend shows an upward shift in the cloud point curve upon addition of fumed silica [79] here, silica particles stabilize homogeneous...

Limited silica fines stabilization data indicated that increasing copolymer molecular weight from 100,000 to 1,000,000 daltons had, if anything, a negative effect on silica fines stabilization. At a molecular weight of 1,000,000 daltons, this copolymer appeared to be more effective in stabilizing silica fines than silica/kaolinite, calcite, or hematite fines. However, the results may be due in part to the larger particle size and lower surface area of the silica fines (see Table II). 

When the DMAEMA content of NVP - DMAEMA copolymers was reduced from 20% to 8%, the silica fines stabilization effectiveness appeared to improve slightly. When the 80/20 NVP - DMAEMA copolymer was converted to a terpolymer containing 8% DMAEMA (CH SO, silica fines stabilization was substantially unaffected. However, stabilization of silica/kaolinite fines was greatly improved. This suggested that the interaction of polymer quaternary nitrogen atoms with anionic sites on mineral surfaces was important for the stabilization of migrating clays but a different interaction was important for the stabilization of silica fines. Calcite fines stabilization improved while hematite fines stabilization effectiveness decreased. This also indicated the nature of the adsorbed polymer - fine particle complex varied for different minerals. 

Dissolve SPDP in dimethylformamide (DMF) at a concentration of 6.2 mg/ml (makes a 20 mM stock solution). Add 50 pi of the SPDP solution to the 1 ml particle suspension and mix to dissolve. Note The small quantity of DMF in a polymeric particle suspension should not affect particle stability, even if the polymer type is susceptible to swelling in pure DMF. Other particle types, such as metallic or silica based, usually are not affected by organic solvent addition, unless their surfaces are non-covalently coated with a dissolvable polymer. 

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. 

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. 

Figure 3.2 Modifying silica particles to include ethylene bridges gives the particles exceptional chemical stability. Si, grey O, red H, black C, orange. The images highlight the difference between silica (top) and ethylene-bridged silica (bottom) particles.

Rgure 5.15. Variation of the coalescence frequency a> with at constant r for OAV emulsions stabilized by silica particles. The lines are only visual guides. (Reproduced from [46,47], with permission.)... 

Philipse, A.P. van Bruggen, M.P.B. Path-mannanharon, C. (1994) Magnetite silica dispersions. Preparation and stability of surface modified silica particles with a magnetic core. Langmuir 10 92—99... 

Silica particles synthesized in nonionic w/o microemulsions (e.g., poly-oxythylene alkyl phenyl ether/alkane/water) typically have a narrow size distribution with the average value between 25 and 75 nm [54,55]. Both water and surfactant are necessary components for the formation of stable silica suspensions in microemulsions. The amounts of each phase present in the micro emulsion system has an influence on the resulting size of the silica nanoparticle. The role of residual water (that is the water that is present in the interface between the silica particle and the surfactant) is considered important in providing stability to the silica nanoparticle in the oil... 

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. 

The formation of ordered sphere-packing structures was observed in certain rheological experiments as just described. Due to the extremely uniform size of the particles, an ordered dense packing structure will develop during sedimentation of the Stober silica particles (see Fig. 2.1.12) when the dispersion is either sterically or electrostatically stabilized. The gemstone opal is essentially based on this principle (80-88). A transmission election replica picture is shown in Figure 2.1.13. The uniform... 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous silica particles in a liquid, almost always water. Commercial products contain silica particles having diameters of about 3—100 nm, specific surface areas of 50—270 m2/g, and silica contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

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.

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