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Water droplet (liquid 1)

Water droplet (liquid 1)

p SPG filter^

Toluene (liquid 2)

Drop volume method

SPG emulsification method

Figure 12. Comparison of schematic diagrams of drop volume and SPG membrane emulsification methods.

measured by a mercury porosimetry.

It can be recognized in Figure 2 that the outlets of Membrane 1 are uniformly controlled in shape and size, but the Membranes 2 and 3 possess irregular surfaces (specifically in the case of Membrane 3) and the outlet diameter of the pores can be estimated to be approximately from 10 to 15 nm which is closer to the theoretical values of the effective pore diameters of 8.0 and 13.8 nm for Membranes 2 and 3, respectively. The consistency in the effective pore diameters estimated by the calculation and by a scanning electron microscope observation confirms that not only the interfacial tension but also the size and shape of the outlets of the pores are the important factors governing the size of the water droplets. The results obtained lead us to an important conclusion that the SPG membrane with the pore outlets uniformly controlled in size and shape is preferable for producing a monodispersed W/O emulsion using the SPG membrane emulsification technique.

3.3. Applications of Membrane Emulsification

3.3.1. Preparation of Uniform Silica Hydrogel Particles

Preparation of many kinds of monodispersed metal oxide particles by the hydrolysis of alkoxide has been widely investigated (25-27). Especially, sol-gel processes with silicon alkoxides, e.g. tetraethoxysilane [TEOS] and tetramethoxysilane [TMOS], have been used extensively to produce silica particles in the submicron range (25-27). Silica has applications in a food industry. Silica gel has not only been used as an adsorbent but also may have applications as flavor, aroma and nutrient delivery system. Furthermore, silica gel has been used as a chill-proofing agent, because this kind of gel has a selective adsorption capacity to the haze-forming protein of beer. In recent years, Si02 nanoparticles were prepared by utilizing W/O microemulsions; specifically, by controlled hydrolysis of TEOS in oil soluble surfactant/ammonium hydroxide solution/cyclohexane or isooctane reversed micellar systems (28-30). However, since the alkoxide compounds react strongly with water, it is difficult to control the hydrolysis conditions during the synthesis.

Since the uniform water droplets were simply formed by pressing water into the oil phase through the pores of the SPG membrane using a copolymer type surfactant as an emulsifier. This method would be applied to precipitate the monodispersed colloidal particles in droplets of water in a W/O emulsion.

Based on this concept, the author prepared a W/O emulsion of sodium silicate solution (5 cm3)-in-toluene (30 cm^) at room temperature by a batch method using PE-64 as an emulsifier (15). The pH of the sodium silicate solution (Si02/Na20 of 3.12 ratio) was first adjusted to 2.0 by the addition of a 2.3 mol/dm^ sulfonic acid solution, to attain slow polymerization of silicic acids at room temperature. To avoid polymerization, the acidic sodium silicate solution was prepared just before its use. The sodium silicate emulsion so prepared was further polymerized in a 100 ml Teflon-sealed and screw-capped vessel by gently mixing with a magnetic stirrer for 7 days at room temperature. In a preliminary experiment, the acidic sodium silicate solution was solidified and converted to a hydrogel under the same conditions. For the sake of comparison, the control W/O emulsion [water (5 cm^) dispersed in toluene (30 cm^)] was prepared under the same conditions.

The mean diameters of freshly prepared sodium silicate droplets (Ds0) and that of the silica hydrogel particles formed after the polymerization for 7 days (Ds7), the monodispersity ratio (U) and Pc values are tabulated in Table 8 along with those of water droplets in a control W/O emulsion (Dw). The U values of Ds0 and Ds7 in the micron range are close to unity, indicating that the sodium silicate emulsions and silica hydrogel particles are of fairly narrow size distribution. Moreover, Dso is comparable to Dw, especially for the Membrane 2, while Pc for sodium silicate emulsions is slightly higher than that for the water droplets in the control W/O emulsion, implying that the mechanism of formation of sodium silicate emulsion is essentially identical to that of the W/O emulsions described in a section 3.2.3. However, Dso of the sodium silicate emulsions does not depend on the concentration of PE-64. Similar result can be seen for Dw, which can be interpreted on the basis of the low interfacial tension (<0.1 dyne/cm) between aqueous solution and toluene phase over the whole concentration region as described before.

The discovery that Ds7 is smaller than Dso suggests that the silicic acid polymerized slowly and released water molecules in the sodium silicate droplets and that the hydrogel particles shrunk as the polymerization progressed. The gradual shrinkage of hydrogels can be due to the slow gelling rate of acidic silicic acids at pH 2 and room temperature (31-33). The net polymerization reaction yields three dimensional gel networks of polysilicic acid which gradually converts into a rigid silica hydrogel particle releasing water (34).

The silica hydrogel thus prepared can be converted to silica gel particle by removing the liquid medium. The structure of the silica gel is compressed and the porosity reduced to at least some degree. Because of its high specific surface area, the silica gel in monodispersed sphere, granular and/or pellet forms appears to be a catalyst, carrier, an adsorbent and a desiccant. Some miscellaneous observations in food processing are outlined below. According to Kennedy et al. (35) polysaccharides are preferentially adsorbed on silica surfaces coated with polyaromatic molecules. It is surprising that a hydrocarbon surface should have any special affinity for such highly hydrophilic molecules. Antioxidants of the polyhydroxyl aromatic type can be adsorbed on fine silica gel particles. It proved to be as good an antioxidant as carbon black yet the film is clear (36). Recently, Matsuzawa and Nagashima developed a high performance new hydrated silica gel, which gives chill-stable beer at low dosage rates in a short period of contact with beer (37).

Nakashima et al. prepared monodispersed microspheres of silica which are indispensable for HPLC by combining a two step membrane emulsification technique with interfacial reaction used for the preparation of inorganic microspheres. The preparation conditions and scanning electron micrograph of the resulting silica microsphere are displayed in Figure 13.

Table 8

Formation of sodium silicate and W/O emulsions by the SPG membrane emulsification method sodium silicate emulsion control W/O emulsion

Table 8

Formation of sodium silicate and W/O emulsions by the SPG membrane emulsification method sodium silicate emulsion control W/O emulsion

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