154 JOURNAL OF COSMETIC SCIENCE required to expand the interface �A y (where �A is the increase in surface area when the bulk oil is subdivided into small droplets and y is the interfacial tension), which is positive, is much higher than the entropy of dispersion T �S (where T is the absolute temperature and �S is the increase in entropy due to the formation of a large number of droplets). From the second law of thermodynamics, the free energy of formation of the emulsion is given (1): (1) With macroemulsions �A y � - T �S, �G is positive, emulsification is non- spontaneous (energy is required to form the emulsion), and the system is thermody- namically unstable. This leads to a number of breakdown processes, as is schematically illustrated in Figure 1. Creaming and sedimentation may occur as a result of gravity when the density of the droplets is different from that of the medium and the droplet size is large (whereby the Brownian motion is not sufficient to overcome gravity). Flocculation may occur as a result of insufficient repulsive energy between the droplets. Ostawald ripening is due to the difference in solubility between the small and large droplets. Coalescence is the result of thinning and disruption of the liquid film between the droplets. Phase inversion may occur under conditions whereby the surfactant be- comes soluble in the oil phase, and hence a W/O emulsion is produced. In order to overcome the above-mentioned breakdown processes, a number of stabili- zation mechanisms are necessary such that the emulsion remains stable over a long 0 Oo o o 0 ° 0 oo o O Oo 0 0 0 0 o Q Ooo 0 •·· · •• • •• • • • • •• .. •.· . •• • ••• •• Figure 1. Schematic representation of the breakdown processes of emulsions.

EMULSION STABILIZATION 155 period (usually 2-3 years at various temperatures). This paper, discusses the various stabilization mechanisms that are required for prevention of strong flocculation, coales- cence, and Ostwald ripening. This is best achieved using polymeric surfactants, which is the main objective of the present paper. A summary will also be given for the methods that can be applied to prevent creaming or sedimentation and phase inversion of the emulsion. STRUCTURE OF POLYMERIC SURFACTANTS AND THEIR CONFORMATION AT INTERFACES The simplest type of a polymeric surfactant is a homopolymer, which is formed from the same repeating units: poly(ethylene oxide) (PEO) and poly(vinylpyrrolidone) (PVP). Homopolymers have little surface activity at the oil/water (0/W) interface. In general, homopolymers are not the most suitable emulsifiers. A small variant is to use polymers that contain specific groups that have high affinity to the surface, e.g., partially hydrolyzed poly(vinyl acetate) (PV Ac), technically referred to as poly(vinyl alcohol) (PVA). Commercially available PVA molecules contain 4-12% acetate groups. The acetate groups give the molecule its amphipathic character on a hydrophobic surface (such as oil droplets), the polymer adsorbs with preferential attach- ment of the acetate groups on the surface, leaving the more hydrophilic vinyl alcohol segments dangling in the aqueous medium. Partially hydrolyzed PVA molecules exhibit surface activity at the 0/W interface. Polymeric surfactants of the block (A-B or A-B-A) or graft (BA 0 ) type are essential materials for the preparation of many emulsion systems, particularly in personal care products. A block copolymer is a linear arrangement of blocks of varying composition (2): Diblock - poly A - block poly B ~~A~~~~~ ~----~B-- Triblock - poly A - block poly B - poly A --A---~~ ~~~~B-~~- ~~~~~A~~ A graft copolymer is a non-linear array of one B block on which several A polymers are grafted: ------ B ----- \ \ \ \ \ AA A A A Most block and graft copolymers have low critical micelle concentrations (cmc), and in many cases it is not easy to measure the cmc for these block and graft copolymers. Several examples of block and graft copolymers may be cited: triblock polymeric surfactants "Pluronics" (BASF) or "Synperonic PE" (ICI) and two poly-A blocks of PEO and one block poly-B of polypropylene oxide (PPO). Several chain lengths of PEO and PPO are available. Tri blocks of PPO-PEO-PEO (inverse "Pluronics") are also available. Polymeric triblock surfactants can be applied as emulsifiers and dispersants. The hydrophobic PPO chain resides at the hydrophobic surface, leaving the two PEO chains dangling in aqueous solution (providing steric stabilization). The above-mentioned triblocks are not the most efficient emulsifiers, and the PPO chain is not sufficiently hydrophobic to provide a strong "anchor" to an oil droplet. The reason