An emulsion is formed by forcing one immiscible liquid into another through a glass membrane which contains a uniform pore size (Figure 6.10). The size of the droplets formed
FIGURE 6.10 Batch version of a membrane homogenizer.
depends on the diameter of the pores in the membrane and the interfacial tensions between the oil and water phases (Kandori 1995). Membranes can be manufactured with different pore diameters so that emulsions with different droplet sizes can be produced (Kandori 1995). The membrane must be sufficiently strong so that it is not broken by the pressures applied to the fluids during homogenization. The membrane technique can be used as either a batch or a continuous process. In the batch process, droplets are formed by forcing the dispersed phase through a cylindrical membrane which is dipped into a vessel containing the continuous phase (Figure 6.10). In the continuous process, the homogenizer consists of a cylindrical membrane through which the continuous phase flows, which is located within a tube through which the dispersed phase flows. The dispersed phase is pressurized so that it is forced through the membrane, where it forms small droplets in the continuous phase.
An increasing number of applications for the membrane homogenizer are being identified, and the technique can now be purchased commercially for preparing emulsions in the laboratory (Kandori 1995). These instruments can be used to produce oil-in-water, water-in-oil, and multiple emulsions. The major advantages of membrane homogenizers are their ability to produce emulsions with very narrow droplet size distributions and the fact that they are highly energy efficient because much less energy is lost due to viscous dissipation. Emulsions with mean droplet sizes between about 0.3 and 10 |im can be produced using this technique.
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