EMULSION STABILIZATION 165 INUTEC®N 25 - Cloud points (°C) INUTEC®N 25 - Cloud points (°C) 120 120 100 • • • -+-OM NaCl --1MNaCI 80 --6-2M NaCl .t) �3M NaCl . s ----4M NaCl ] 40 --t--5M NaCl $ $ $ --e-6M NaCl (.J 20 � 100 a 6 a (.J -+-OM MgS04 � 80 .t) ---&-1 M Mg$04 .s 60 • � a �1.5MMgS04 40 ----2M MgS04 (.J 20 0 0 2 4 6 0 0 2 4 6 % /NUTEC®N 25 % /NUTEC®N 25 A B Figure 12. Cloud points of INUTEC® N25 at various NaCl (A) and MgSO,1 (B) concentrations as a function of INUTEC® N25 concentrations. place resulting in cloudiness. The cloud point depends both on polymer concentration as well as the molecular weight with increase in molecular weight and concentration, the cloud point generally decreases, and this can be illustrated from the results shown in Figures 10 and 11. With PEO at 2 mol dm - 3 NaCl, the cloud point is about 60°C, and if the polymer concentration will reach, for example, 20%, the cloud point could be lower than 50°C. Thus, for emulsion stabilizers based on PEO, stability cannot be maintained at 2 mol dm- 3 NaCl. With MgSO4 , the situation is even worse, as shown in Figure 10: at 5 % PEO 4000, the cloud point is lower than RT at 1 mol dm - 3 , and hence instability will be more serious with this electrolyte. However, for inulin the cloud point can be maintained at about 100°C up to 4 mol dm - 3 NaCl, and hence one would expect stable emulsions at temperatures exceeding 50°C up to this electrolyte concen- tration. With MgSO4 stability can be maintained at high temperatures up to 1 mol dm- 3 . Thus, these cloud-point measurements give conclusive evidence of the unique behavior of polymeric surfactants based on inulin. The polyfructose chain remains hy- drated up to high temperatures and in the presence of high electrolyte concentrations. This makes this polymeric surfactant a very useful candidate for the stabilization of emulsions in high electrolyte concentrations when compared with emulsions prepared using polymeric surfactants based on PEO. Similar results were also obtained when using cyclomethicone as the oil: 50/50 0/W emulsions prepared using 2 (w/v)% INUTEC® surfactant showed stability at RT and 50°C for more than one year. Emulsions prepared in the presence of 1 mol dm- 3 NaCl and MgSO4 were also stable up to 50°C for more than eight months. From the above discussion, it is clear that using HMI as an emulsion stabilizer will eliminate any strong flocculation or coalescence of the emulsion both in water and in high electrolyte concentrations. This can be attributed to a number of effects: (i) the multipoint attachment of the polymer by the several alkyl chains that are grafted on the backbone, ensuring strong adsorption ("anchoring") at the O/W interface (ii) strong hydration of the polyfructose "loops" (in between the alkyl chains) that dangle in solution, ensuring a X 0.5 both in water and high electrolyte concentrations (iii) high-volume fraction (concentration) of the loops at the interface, recent results using polystyrene latex dispersions (11) showing an adsorbed-layer thickness in the region of

166 JOURNAL OF COSMETIC SCIENCE 4 nm (and, therefore, the concentration of the polyfructose in this layer is high and this increases the free energy ox mixing according to equation 2) and (iv) enhanced steric stabilization as suggested by Napper (8) for polymers with multi-attachment points. The hydrophobically modified inulin surfactant can also stabilize the emulsions against Ostwald ripening. The latter arises from the difference in solubility between the small and large droplets. During storage, oil molecules will diffuse from the smaller droplets (which have higher solubility due to their higher curvature) to the larger droplets (with lower solubility due to the smaller curvature). This process can be prevented by using polymers that adsorb very strongly at the O/W interface. As discussed by Walstra (12), this strong adsorption results in an increase in dilatational elasticity, thus reducing the process of diffusion from the small to the large droplets. W/O EMULSIONS STABILIZED WITH PHS-PEO-PHS BLOCK POLYMER W/O emulsions (with the oil being Isopar M) can be prepared using PHS-PEO-PHS block copolymer at a very high water volume fraction (0.7). These emulsions have a narrow droplet size distribution, with an average radius, R, of 183 nm. They also remain fluid up to high-volume fraction (0.6), as is illustrated in Figure 13, which shows the viscosity-volume fraction curves for the emulsion (13). The effective volume fraction, 'Peff, was calculated from the relative viscosity, 'llr, using the Douherty-Krieger equation (14): 200 150 100 � 50 0-1------....-------....---------------------t 0.4 0.5 0.6 0.7 0.8 0.9 volume fraction Figure 13. Viscosity-volume fraction curves for W/O emulsions stabilized with PHS-PEO-PHS block copolymer. o, experimental points ■, calculated using equation 4.