162 JOURNAL OF COSMETIC SCIENCE under all conditions. As discussed above, for systems where water is the continuous medium, polyfructose (inulin) is the most suitable A chain. This polymer chain is highly soluble in water and remains solvated up to high temperatures. It can also tolerate reasonable amounts of electrolyte. For dispersions, where the continuous medium is a hydrocarbon oil (e.g., W/0 emulsions) poly(hydroxystearic acid) is the most suitable A chain(s). The last criterion for effective stearic stabilization is to have a sufficiently thick adsorbed layer to avoid any weak flocculation. This is particularly important for con- centrated emulsions. A value of o of the order of 5-10 nm is usually sufficient. O/W EMULSION STABILIZATION USING HYDROPHOBICALL Y MODIFIED INULIN (HMI) BASED POLYMERIC SURFACTANT As mentioned above, the graft copolymer based on inulin (hydrophilic polyfructose chain) on which several alkyl groups have been grafted (INUTEC® SPl) was recently evaluated as an effective stabilizer for 0/W emulsions. This polymeric surfactant was prepared using inulin (INUTEC® N25) with a degree of polymerization greater than 23. The inulin chains were hydrophobically modified by grafting several alkyl chains on the inulin backbone. Two oils, Isopar M (supplied by Exxon) and cyclomethicone (supplied by Dow Corning) were used to prepare the 0/W emulsions (10). Most emulsions consisted of 50/50 (v/v) ratio oil in water, and the polymeric surfactant concentration was changed from 0.25 (w/v)% to 2 (w/v)%. The emulsions were prepared using a high- speed stirrer, an Ultra-Turrax (CAT X620). The emulsion quality was assessed by optical microscopy. Samples of the emulsions were stored at room temperature and at 50°C, and the droplet size was qualitatively assessed by taking optical micrographs at various intervals of time. Figure 9 shows typical micrographs of diluted 50/50 Isopar M/water emulsions con- taining 2 (w/v)% INUTEC® surfactant that were stored for periods of 1.5 and 14 weeks at 50°C. As can be seen, there is no apparent increase in droplet size during this storage time, and this was taken as an index of stability against coalescence. Similar results were A B Figure 9. Optical micrographs of diluted 50/50 Ispoar M/water emulsions containing 2(w/v)% INUTEC® SPl that were stored at 50°C for 1.5 (A) and 14 (B) weeks.

EMULSION STABILIZATION 163 also obtained for emulsions stored at room temperature. No oil separation was detected after this period at RT and 50°C. In order to evaluate the minimum polymeric surfactant that is required to prepare a stable emulsion, systems (50/50 O/W) were prepared at 0.25, 0.3, 0.4, 0.5, 1, 1.5, and 2%. All the samples were assessed for stability using the procedure described above. All emulsions containing 0.5 (w/v)% polymeric surfactant remained stable both at room temperature and 50°C. These samples did not show any apparent oil separation even after storage for 10 months at 50°C. Based on these results, it was shown that for 50/50 (v/v) O/W emulsions, an emulsifier concentration in the region of 0.5 (w/v)% is suffi- cient for stabilization. This is about an order of magnitude lower than the concentration used with conventional surfactants (such as alcohol ethoxylates). Emulsions were prepared at 0.5, 1.0, and 2 mol dm- 3 NaCl, as well as in the presence of 0.5, 1.0, 1.5, and 2 mol dm- 3 MgSO 4 . All emulsions containing NaCl did not show any coalescence up to 50°C for almost one year of storage. With MgSO 4 , the emulsions were also stable up to 1. 0 mol dm - 3 . The above-mentioned stability in high electrolyte concentrations is not observed with polymeric surfactants containing poly(ethylene oxide) (PEO) as the stabilizing chain. The difference between the inulin- and PEG-containing chains can be understood if one considers the repulsive energy obtained using these polymeric surfactants. As discussed previously, the mixing free energy, G mix' for two droplets stabilized by A chains with thickness o depends on the value of (1/2 - x) (see equation 2). As discussed previously, when (1/2 - x) is positive, i.e., X 1/2, G mix is positive and the net interaction is repulsive. If X 1/2, G mix is negative, and this leads to incipient flocculation that is normally accompanied by coalescence of the emulsion. The Flory-Huggins interaction parameter Xis related to the solvency of the medium for the chains. In water, both inulin and PEO are strongly hydrated by the water molecules, and hence X 1/2 under these conditions. On increasing the temperature, H-bonds between the chains and water molecules will be broken and the X parameter will increase. However, with both inulin and PEO, this will happen at much higher tem- peratures than those experienced on storage (usually the X parameter is less than 1/2 below 80°C). With inulin, it does not show any dehydration up to 100°C. On addition of electrolytes, dehydration of the chains may take place (salting-out effect), and at a given electrolyte concentration (and type) and temperature, X will change value from 1/2 to 1/2 and G mix will change sign from positive to negative. It seems that the inulin-stabilizing chain can retain its hydration to much higher temperatures and electrolyte concentrations when compared to PEO chains, and this is probably the reason for the higher stability obtained when using the hydrophobically modified inulin as an emulsion stabilizer. To confirm the above-mentioned effects, we have carried out cloud point measurements for PEO with 4000 molecular weight in the presence of various concentrations of NaCl and MgSO4 . Some results were also obtained for a PEO with a molecular weight of 20000 in the presence of NaCl. For comparison, results were also obtained for inulin (INUTEC® N25) in the presence of NaCl and MgSO 4 .