EMULSION STABILIZATION 167 (4) where [T)] is the intrinsic viscosity that is equal to 2.5 for hard-spheres and 'P p is the maximum packing fraction that was found to be 0.84 (which is a reasonable value for a polydisperse system). From 'P eff and 'P, the adsorbed layer thickness, 8, was calculated using equation 5: (5) where 8 was found to be -10 nm at 'P = 0.4 and decreased with an increase in 'P as a result of the possible interpenetration and/or compression of the PHS chains. The emulsions remained stable both at room temperature and 50°C for several months, and there was no evidence of flocculation and/or coalescence, as assessed by using rheological measurements for concentrated W/0 emulsions. This stability is expected since the anchor chain (PEO) is soluble in water and insoluble in oil, thus ensuring lack of displacement of the polymer molecules. The PHS chains remained strongly solvated by the oil molecules both at room temperature and at 50°C, and this ensured effective steric stabilization. CONTROL OF CREAMING OR SEDIMENTATION OF EMULSIONS Creaming or sedimentation can be controlled by addition of "thickeners" in the con- tinuous phase. Most of these thickeners are high-molecular-weight polymers such as hydroxyethyl cellulose (or its hydrophobically modified version, referred to as associative thickeners), xanthan gum, alginates, carbomers (e.g., Carbopol), etc. All these materials produce non-Newtonian systems above a critical concentration. This non-Newtonian behavior can be expressed from plots of shear stress, CT (Pa), as a function of shear rate, 'Y (s - \ as is illustrated in Figure 14, referred to as psudopolastic flow. 17/Pa.s cr/Pa y/s-1 y/s-1 Figure 14. Shear stress and viscosity as a function of shear rate for a psudoplastic system.

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-