168 JOURNAL OF COSMETIC SCIENCE As discussed before (15 ), these systems produce high viscosities at very low shear rates or shear stresses ( 100 Pas), and this prevents any creaming or sedimentation of the emulsion. Another problem with many emulsion systems that are weakly flocculated or "struc- tured" to reduce creaming or sedimentation is "syneresis." The "gel network" produced in the system may contract with time (as a result of gravity forces) and some supernatant liquid may be "squeezed out," leaving a clear liquid layer at the top or bottom of the container. To prevent syneresis, one has to optimize the bulk modulus (which is related to the elastic or storage modulus). The latter can be measured using dynamic or oscil- latory techniques that have been described previously (15). CONTROL OF PHASE INVERSION Phase inversion can be of two types: (i) Catastrophic inversion caused by increasing the disperse phase volume fraction above a critical value (mostly above the maximum packing fraction) and (ii) transitional phase inversion produced by changing the condi- tions, e.g., an increase in temperature with emulsions stabilized by ethoxylated surfac- tants. Catastrophic phase inversion can be eliminated by reducing the phase volume of the disperse phase in the emulsion. This is usually not a problem since most emulsion systems are prepared at disperse volume fractions well below the maximum packing fraction. Transitional phase inversion has to be prevented by the proper choice of emulsifier. This problem has been discussed in detail by Shinoda and Saito (16). With 0/W emulsions based on ethoxylated surfactants, phase inversion may take place at a critical temperature (referred to as the phase inversion temperature, PIT). With increas- ing temperature, the polyethylene oxide (PEO) chain becomes dehydrated (as a result of the breakdown of the hydrogen bond between EO and H2O). This results in reduction of the aqueous solubility of the surfactant, and at the PIT the surfactant becomes more oil-soluble and hence suitable for formation of a W/O emulsion. The above-mentioned problem of phase inversion is eliminated when using polymeric surfactants such as hydrophobically modified inulin (HMI). This polymeric surfactant is not oil-soluble at any temperature, and hence by increasing the temperature there is no chance of inversion to a W/O emulsion. As long as no coalescence or Ostwald ripening occurs (as discussed above), the O/W emulsion remains stable up to high temperatures without any phase inversion occurring. CONCLUSIONS This overview shows the main advantages of polymeric surfactants in the stabilization of emulsions for personal care applications. For O/W emulsions, a hydrophobically modi- fied inulin (HMI) graft copolymer is shown to be very effective for stabilization of the emulsions both in water and in high electrolyte concentrations at high temperatures. This is attributed to the multipoint anchor of the HMI at the O/W interface. The loops of polyfructose remain hydrated both in water and in high electrolyte solutions up to high temperatures. For W/O emulsions, an A-B-A block copolymer was the most suitable. A is poly(ethylene oxide) (PEO) (the anchor chain in water droplets), and B is polyhydroxystaric acid (PHS) (the stabilizing chains that are strongly solvated by hy-

EMULSION STABILIZATION 169 drocarbon molecules). Any creaming or sedimentation of the emulsions can be prevented by using "thickeners" (such as hydroxyethyl cellulose or xanthan gum) that produce high viscosity ( 1000 Pas) at low shear stresses or shear rates. The HMI polymeric surfactant also prevents phase inversion, since the molecule is insoluble in oil at all temperatures. ACKNOWLEDGMENT The author is grateful to Orafti Biobased Products for sponsoring the research on the use of INUTEC® SPl for the stabilization of emulsions. For more information on the technical applications of INUTEC® SP 1 in cosmetic formulations the reader can contact Mr Karl Booten of Orafti: e-mail address karl.booten@orafti.corn. REFERENCES (1) Th. F. Tadros and B. Vincent, in Encyclopedia of Emulsion Technology, P. Becher, Ed. (Marcel Dekker, New York, 1983). (2) I. Piirma, Polymeric Surfactants, Surfactant Science Series, No. 42 (Marcel Dekker, New York, 1992). (3) C. V. Stevens, A. Meriggi, M. Peristerpoulou, P. P. Christov, K. Booten, B. Levecke, A. Vandamme, N. Pittevils, and Th.F. Tadros, Biornacrornolecules, 2, 1256 (2001). (4) E. L. Hirst, D. I. McGilvary, and E.G. Percival, J. Chern. Soc., 1297 (1950). (5) M. Suzuki, in Science and Technology of Fructans, M. Suzuki and N. J. Chatterton, Eds. (CRC Press, Boca Raton, Fl., 1993) p. 21. (6) Th.F. Tadros, C. Dederen, and M. C. Taelman, Cosrnet. Toiletr., 112, 75 (1997). (7) Th.F. Tadros, in Polymer Colloids, R. Buscall, T. Comer, and Stageman Applied Sciences, Eds. (London, Elsevier, 1985), p. 105. (8) D. H. Napper, Polymeric Stabilization of Dispersions (Academic Press, London, 1983). (9) P. J. Flory, Principles of Polymer Chemistry (Cornell University Press, New York, 1953). (10) Th.F. Tadros, A. Vandamme, K. Booten, B. Levecke, and C. V. Stevens, Colloids Surf, 250, 133 (2004). (11) J. Esquena, F. J. Dominguez, C. Solans, B. Levecke, K. Booten, and Th.F. Tadros, Langmuir, 19, 10469 (2003). (12) P. Walstra, in Encyclopedia �f Emulsion Technology, P. Becher, Ed. (Marcel Dekker, New York, 1985), Vol. 4, p. 1. (13) Th.F. Tadros, in Ernulsions�A Fundamental and Practical Approach, J. Sjoblom, Ed. (NATO ASI Series, Vol. 363, 1992), p. 173. (14) I. M. Krieger and M. Dougherty, Trans. Soc. Rheol., 3, 137 (1959) I. M. Krieger, Adv. Colloid Interface Sci., 3, 111 (1972). (15) Th.F. Tadros, Adv. Colloid Interface Sci., 108-109, 227 (2004). (16) K. Shinoda and H.J. Saito, Colloid Interface Sci., 30, 258 (1969).