Emulsion shear mixing

Add (A) to (B) slowly, with high shear mixing. Continue mixing until emulsion reaches 70F. Package. 

Low internal phase emulsions typically result when high shear conditions are used for emulsification, while low shear mixing can lead to high internal phase, or concentrated, emulsions [435]. There are several conditions needed to form a concentrated emulsion. Low shear mixing is required while the internal phase is slowly added to the continuous phase, and the surfactants used to create the emulsion need to be able to form elastic films [435—438]. The formation of concentrated emulsions has also been linked to surfactant-oil phase interactions [436] and therefore the oil-water interfacial tension and the potential for surfactant-surfactant interactions [439]. 

The process employed Eor the preparation of a primary emulsion, high-speed mixers (e.g., Elado (Ystral), UltraTurrax or Silverson) may be used. For the secondary emulsion preparation, a low-shear mixing regime is required, in which case paddle stirrers are probably the most convenient. The mixing times, speed, and order of addition need to be optimised. 

The conductivity and high-frequency dielectric constant of an emulsion are both indicators of which phase (water or CO2) is the continuous one. If CO2 is the continuous phase in an emulsion, the emulsion conductivity will be extremely low. In contrast, if water is the continuous phase, substantial conductivity will be observed, provided that ionic solutes are present. Likewise, the dielectric of a CO2-continuous emulsion should be similar to that of CO2 (ca. 1.2), whereas the dielectric of a water-continuous emulsion should approach that of water (ca. 80). Emulsions were created by using a high-pressure emulsifier (Emulsiflex, Avestin) to provide high-shear mixing for systems containing CO2, water, and Mn(PFPE)2 surfactant as shown in Figure 2.4-8. 

Figure 21.2 Schematic illustration of the two-step homogenization for producing W/O/W emulsions, (a) Step 1 high-shear mixing to produce a fine W/O emulsion, (b) Step 2 low-shear mixing to produce a W/O/W emulsion.

Prepolymer ionomers with isocyanate end-groups and of sufficiently low molecular weight can be mixed with water in the presence of hydrophobic organic solvents, such as methylene chloride or toluene, or, in the case of sufficiently low viscosities, even without solvent. The prepolymers form emulsions, which in turn form polymer dispersion after chain extension by reaction of the isocyanate. Such a process requires powerful high-shear mixing equipment in the presence or absence of solvents. Solutions of ionomers in hydrophilic solvents form emulsions spontaneously in the presence of water. The formation of the emulsion must be... 

First, the drug is dispersed or dissolved in the lipid melt and the molten lipid is then dispersed in the hot aqueous phase by high-shear mixing (e.g. Ultra-Turrax vortexing) to obtain the crude pre-emulsion. Depending on their lipophilicities, the stabilizers can be dispersed or dissolved in the water phase or in the lipid melt. The quality of the pre-emulsion strongly influences the quality of the final dispersion whereas the size of the droplets in the pre-emulsion should preferably be in the lower m-range. 

On the other hand, the slow shear mixing of high internal phase ratio emulsions located in the shaded zones of Fig. 3c has been found to be very efficient in producing exttemely small droplets, irrespective of the surfactant concentration and stirring energy. There is thus another minimum drop size (shaded) strip located in each of the A regions, near and parallel to the vertical branch of the inversion line [3,50,51]. 

Preparation of Emulsions. An emulsion is a system ia which one Hquid is coUoidaHy dispersed ia another (see Emulsions). The general method for preparing an oil-ia-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosia acid, and separately mix a proportionate quantity of an alkah, such as potassium hydroxide, with the water. The alkah solution should then be rapidly stirred to develop as much shear as possible while the oil phase is added. Use of a homogenizer to force the resulting emulsion through a fine orifice under pressure further reduces its oil particle size. Liquid oleic acid is a convenient fatty acid to use ia emulsions, as it is readily miscible with most oils. 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

High Shear High shear impellers take a variety of proprietary forms and are used primarily for producing emulsions. Their design maximizes the portion of the mixing energy dissipation which is classified as shear. High shear impellers are available for both tank and inline applications. 

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