Figure. 9. Photomicrograph of W/O emulsion (Courtesy of Nakashima and


fairly constant in the vicinity of 90 and 28 kPa over the range of concentration of PE-64 studied. Taking into consideration the difference in the Dp values between Membranes 1 and 2, this result seems to be reasonable.

Nakashima et al. prepared a W/O emulsion from an edible oil by membrane emulsification technique using hydrophobic SPG membrane. A photomicrograph of this emulsion is shown in Figure 9 (18). It should be mentioned here that the surface of the SPG membrane was treated, to render it hydrophobic, by octadecyltrichlorosilane (ODS) and triethylchlorosilane (TMS). Single emulsifier (Span 80) is added to the continuous oil phase (soybean oil) prior to permeation of water phase. Particle size of the emulsion thus obtained had a quite uniform distribution. The size of the water droplets was bigger compared to the W/O emulsions produced by the author as described before. This would be due to the difference in the surface hydrophobicity of the SPG membranes and the interfacial tension.

3.2.3. Factors Determining the Size of the Droplet

Effects of the interfacial tension on the size of the droplets produced in W/O emulsion by the SPG membrane emulsification technique were verified. The author produced the W/O (water-in-toluene) emulsions with Membranes 1, 2 and 3, at various interfacial tensions using PE-series surfactants as an emulsifier. The Dw, U and Pc values for the resulting W/O emulsions are summarized in Table 7 and will be discussed in a next section.

(a) Interfacial Tension

Figure 10 shows a plot of interfacial tension versus the Dw of water droplets produced with Membrane 3. Here, the Dw values for the PE-64 system are plotted at Dw=0 because of their extremely low interfacial tension. It is interesting to note that the curve can be roughly classified into three regions as A, B and C as follows: (A) y<l dyne/cm and Dw<2 (.im, (B) l<y<5 dyne/cm and Dw ~2 fim and (C) y>5 dyne/cm and Dw┬╗2 ^m. The dispersions of the monodispersed W/O emulsions formed in A, B and C regions were very stable, metastable (completely separated into two phases after 1 day) and unstable, respectively. The PE-series surfactants adsorb onto the water droplets, thereby lowering the interfacial tension and forming rigid and thin adsorbed layers (20-22). The water droplets are stabilized by steric repulsion of the elastic, adsorbed layers of PE-series surfactants. Therefore, not only the interfacial tension but also the conformation of the PE-series surfactant molecules is resposible for producing stable and monodispersed W/O emulsions by the SPG membrane emulsification technique. It is clear from above results that the interfacial tension is one of the factors for determining the water droplet size. However, the Dw is less than Dp in all cases and the mechanisms of the formation of such fine monodispersed water droplets cannot be ascertained on the basis of these results alone.

Table 7

Results of W/0 emulsions prepared by SPG emulsification method

Table 7

Results of W/0 emulsions prepared by SPG emulsification method







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