138 JOURNAL OF COSMETIC SCIENCE ADVANCES IN EMULSION SCIENCE FOR PERSONAL CARE APPLICATIONS Tharwat Tadros, Ph.D. 89 Nash Grove Lane, Wokingham,, Berkshire RG404HE, U.K. SUMMARY This overview starts with an introduction summarizing the stabilization mechanisms for macroemulsions (those that are currently used in most personal care applications). These systems are thermodynamically unstable since the energy required for their formation exceeds the much smaller entropy of dispersion of the droplets in the medium. To stabilize the emulsions against flocculafion and coalescence, one needs to create an energy barrier between the droplets to prevent their close approach (where the van der Waals attraction is very strong). This can be achieved by electrostatic stabilization, e.g. by the use of ionic surfactants, where the double layers prevent the close approach of the droplets. This stabilization mechanism is not ideal since the double layer repulsion is significantly reduced in the presence of electrolytes. An alternative mechanism is that of steric stabilization, whereby nonionic surfactants or polymeric surfactants may be used. These molecules consist orb chains that are strongly adsorbed (or soluble in oil) and A chains which are soluble in the medium and strongly solvated by the its molecules. Examples are given of the types of A-'B, A-B-A block copolymers that can be used for stabilization of oil-in-water (O/W) or water-in-oil (W/O) emulsions. The stabilization mechanism by these polymeric surfactants is described in terms of the unfavourable mixing of the A chains, when these are in good solvent conditions, and the loss in configurational entropy on overlap of the chains. The second part of the overview deals with the subject of microemulsions, which are thermodynamically stable systems that can be transparent or translucent (with drop sizes in the range 5- 50 nm). These systems are produced spontaneously by mixing oil, water, surfactant and cosurfactant. The driving force for their for their spontaneous formation is the ultralow interfacial tension, in which the surface energy in expanding the interface is overcompensated by the negative entropy of dispersion of the droplets. The reason in using two surfactant molecules for reduction of interfacial tension can

2000 ANNUAL SCIENTIFIC MEETING 139 be accounted for in terms of their co-adsorption at the interface. The methods that can be applied for formulation ofmicroemulsions are briefly described. Finally, the use of polymeric surfactants for formulating microemulsions is described. This has the advantage of avoiding any skin irritation, since these polymeric surfactants cause no disruption of the stratum corneum of the skin. The third part of this overview deals with the topic of nanoemulsions (covering the size range 50 -200 rim) which may also appear transparent or translucent. However, these systems are not thermodynamically stable, but due to their small size they have a long term kinetic stability. This is due to the larger ratio between adsorbed layer thickness and droplet radius, when compared with the values for macroemulsions. Two main procedures could be applied for preparation ofnanoemulsions, namely high pressure homogenization and application of the phase inversion temperature (PIT) principle. The droplets produced using the first technique show a narrower distribution when compared with those obtained using the PIT method. However, nanoemulsions may show growth of droplet size with time as a result of Ostwald ripening (that results from the difference in solubility between the small and large droplets). Two methods may be applied to reduce Ostwald ripening, namely incorporation of a second oil phase with very low solubility and/or incorporation of a polymeric surfactant that is strongly adsorbed at the O/W interface and has low solubility in the continuous phase. The fourth part of the overview deals with liposomes and vesicles which are produced by dispersion of lipids followed by sonication. The principle of the critical packing parameter is applied to predict the formation of vesicles. The latter are useful delivery systems for personal care applications, since nonpolar actives may be solubilized in the hydrocarbon core of the bilayer, whereas polar and Water soluble actives can be intercalated in the aqueous film between the bilayers. Liposomes and vesicles cause no skin irritation since they have similr structure to the lJpid I•ilayer of the stratum corneum. The last section of the overview deal with multiple emulsions which are very useful for formulation of personal care products with several active ingredients. The use of polymeric surfactants for preparation of stable multiple emulsions is described and the criteria of stability is summarized. The multiple emulsion can also be stabilized using thickeners to prevent creaming or sedimentation.