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 .

164 JOURNAL OF COSMETIC SCIENCE PEG 4000 - Cloud point (°C) PEG 4000 - Cloud Point (°C) 120 120 100 &::::=:: : : -+-0 M Nl!CI e --tr-2M NaCl 80 -- -3M NaCl 60 r II --M-4M NaCl -i1E-4.5M NaCl 40 ---Q-5M NaCl 20 " " " -+-SM NaCl 100 80 .s 60 40 (..) 20 1-+-oM Mgso4 I --e-o.SM MgS04 ----6-1M MgS04 -i!E-2M MgS04 0 0 0 0 2 4 6 % PEG 4000 % PEG4000 A B Fi g ure 10. Cloud points of PEO 4000 at various NaCl (A) and MgS04 (B) concentrations as a function of PEO concentrations. Cloud points of 100°C mean that no cloud point could be noticed up to this temperature. Figures lOA and lOB show the results for the cloud points of PEO 4000 at various concentrations of NaCl and MgSO4 , whereas Figure 11 shows the results for PEO 20000. The results for inulin (INUTEC® N25) are shown in Figure 12. It can be seen from the results of Figure 10 that for PEO 4000 in water, the cloud point is around 100°C and it shows very little dependence on concentration (in the range 1-5%). On addition of NaCl, there is a systematic reduction in the cloud point with an increase in NaCl concentration. Also, the cloud point shows a reduction with an increase in PEO concentration from 1 % to 5 % . The results for PEO 20000 showed lower cloud point values when compared with the values obtained with PEO 4000. However, the results for inulin (Figure 12) showed no cloud point up to 4 mol dm- 3 NaCl, indicating an absence of dehydration by this electrolyte. With MgSIO4 , a cloud point could only be measured at 1.5 and 2 mol dm- 3 MgSO4, reaching values below room temperature. Generally speaking, the cloud point is related to the X parameter. When Xis 1/2, the chains should remain solvated by the molecules of the medium and the solution is clear when X 1/2, dehydration of the chains will occur and chain-chain interaction takes 120 100 ,.._ 80 ·- 60 40 .! 20 0 0 PEG 20000 · Cloud points ( ° C) a & 2 liil A )( % PEG 20000 ◊ El A )( 0 4 --..-oMNaCI -G--1M NaCl -lr---2M NaCl --M-4MNaCl ----8-6MNaCl 6 Figure 11. Cloud points of PEO 20000 at various NaCl concentrations as a function of PEO concentration.