Emulsion stabilization with silica

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.)... 

Figure 5.7 Production of Pickering emulsions stabilized by silica nanoparticles of 12 nm diameter using the SEM valve. Comparison ofthe homogenization results (characteristic value X903 of droplet size distributions) obtained with the operational modes 1, 3, and 6.

Since the emulsions stabilities for this oil-phase only were not sufficiently high to achieve reliable results, asphaltenes precipitated from crude A were added to the model emulsions in the same amount as in the actual crude oU. Figures 12.7 and 12.8 show the viscosity ratio as a function of particle concentration for the model emulsions stabilized with very hydrophobic and mildly hydrophilic silica particles, respectively. Model oil and crude oil emulsions show many similarities. For all model emulsions the stability in the absence of particles is lower, because the absolute viscosity is much lower than the one encountered in crude oil emulsions. Consequently, the effect of the shear rate on the destabilization is more pronounced. However, as particles are added in larger amounts, crude oil and model oil emulsions behave in the same way. Nevertheless, in Figure 12.8 the viscosity ratios are shifted towards lower values since a strong flocculation phenomenon enhanced by the low viscosity of the samples brings coalescence. 

In emulsions, amine hydrochloride constitutes the aqueous phase and acrylic ester the organic phase. Cetyltrimethylanunonium bromide (CTAB) or span/twin (S/T)-type surfactants are used for emulsion polymerization. Solid dispersants such as talc and colloidal silica are often used to stabilize emulsions which are difficult to stabilize with usual surfactants. HydrophiUc colloidal silica (Aerosil 200) drastically increases the stability of some emulsions provided high amounts (up to 10%) of Aerosil are used. Random copolymers containing 10% hydroxyl groups can be used as polymeric dispersants for preparing w/o emulsions. 

For many of the applications the standard alkali stabilized colloidal silicas available were fine, but since the colloidal silica depended on the maintenance of fairly high pH in order to remain stable it was not compatible with lower pH wax emulsions. While the initial mixes might look quite stable the lower pH would eventually lead to slow coagulation of the silica resulting in poor shelf life for these products. The evolution of the aluminate modified materials which had much less dependence on pH for stability solved many of these problems and provided... 

Ma and Dai [121] reported the synthesis of polystyrene latexes armored with silica nanoparticles (10-15nm in diameter, PA-ST silica sol, Nissan Chemicals) via solids-stabilized emulsion polymerization. They used VA-086, 2,2 -azobis [2-methyl-lV-(2-hydroxyethyl)propionamide], as nonionic initiator. Whereas we found that Pickering emulsion polymerization of styrene using Ludox TM-40 and a low flux of radicals generated from potassium persulfate did not result in an armored latex, the hydroxyethyl groups probably enhance the wettability of the surface of the latex particles to promote silica adhesion. This was confirmed by a... 

The thickening and thixotropic effects in these classes of products attained with silica are too numerous to cite. Usually several effects are attained, namely, flatting or lower gloss, prevention of settling of pigment in storage, stabilization of emulsion, as well as dripless application. However, no information is available as to relative use of silicas versus other. thickeners in these products. Flatting seems to be the effect in paints and finishes most commonly mentioned in manufacturers bulletins. In inks, viscosity control is usually featured. 

Colloidal silica particles can be used as shell constituent for emulsion tern-plating [19]. By adding an organic solvent (isopentyl acetate) and 3-methacryl-oxypropyltrimethoxysilane into an aqueous dispersion of colloidal silica (7, 12, and 25 nm in diameter), a particle-stabilized emulsion, called Pickering emulsion [20], with small droplet sizes was formed due to the low interfacial tension between the colloidal silica particles and monomers. After polymerization of organotrialkoxysilane, the organic solvent in the core was evaporated to form hollow particles with a porous shell. These hollow particles have a hydrophobic interior, which is potentially useftil for adsorbing hydrophobic contaminants in water. 

Spherical silica particles containing retinol have been fabricated using O/ W/O multiple emulsion and the sol-gel method (Lee et al., 2001). O/W/O multiple emulsions were stabilized with hydroxypropyl cellulose (HPC) and surfactants such as Tween 20 and Span 80. In addition a polymeric stabilizer present in the intermediate aqueous phase was shown to improve the encapsulation efficiency. In the presence of polyvinyl alcohol the yield of encapsulation efficiency of retinol was 7%. With Pluronic P123 (a block copolymer of ethylene oxide propylene oxide) a yield of encapsulation of 31% could be reached. Figure 7.24 shows the retinol released profile from silica particles prepared in the O/W/O multiple emulsions and stabilized with different surfactant and different polymeric stabilizers in the intermediate aqueous phase. 

Hannisdal et al. [16] have studied the effects on the emulsion stability of the addition of silica particles, because their properties resemble those of natural inorganic colloids like clays. They coated some commercially available dry silica nanoparticles with heavy cmde oil components (asphaltenes and resins) to investigate the performance of these solids as stabilizers in water-in-model oil emulsions. 

Since all the emulsions presented very similar DSD, with a mean diameter centered at about 8 pm the water droplet size was not considered as an important factor affecting the stability behaviors of the emulsions stabilized by different silica nanoparticles. 

In some applications nonionic - surfactants (- fatty alcohol ethoxylates), which show the phenomenon of a cloud point, are used. Above their cloud point they act as defoamers, but below they may even act as foam stabilizers. Another special d. is silica that is made hydrophobic by treating it with - fatty amines. All these ingredients are used in solid or liquid (emulsion) form, with emulsifier to enhance dispersion. 

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. 

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... 

Preconcentration of As(III) diethyldithiocarbamate on silica chemically modified with hexadecyl groups was examined [1]. Emulsion liquid membrane (made up of L113A surfactant, liquid paraffin as stabilizer and kerosene as solvent with HCl and KOH as external and internal phases) separation of As(III) and As(V) was applied prior to detection with silver diethyldithiocarbamate ( =510 nm) [2]. The method was applied to Cu ore and slagged ash. 

Figure 2.4-3 (type A) shows an apparatus for studying emulsion formation and stability at pressures up to 345 bar and temperatures up to 80 °C. The emulsion is formed by introducing a liquid into a C02/surfactant solution with a six-port rotary valve (Valeo) and shearing the solution into small droplets by recirculation through a 100 pm i.d. silica capillary with an HPLC pump. The optical cell for DLS contains three windows at right angles and... 

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