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

156 JOURNAL OF COSMETIC SCIENCE for the surface activity of the PEO-PPO-PEO triblock at the O/W interface is probably "rejection" anchoring. The PPO chain is not soluble in water or most oils. Several other di- and tri-block copolymers have been synthesized: diblocks of polystyrene block-polyvinyl alcohol triblocks of poly(methylmethacrylate)-block polyethylene ox- ide-poly(methyl methacrylate) diblocks of polystyrene-polyethylene oxide and triblocks of polyethylene oxide-polystyrene-polyethylene oxide. An alternative (and perhaps more efficient) polymeric surfactant is the amphipathic graft copolymer consisting of a polymeric backbone B (polystyrene or polymethyl methacry- late) and several A chains ("teeth") such as polyethylene oxide. The graft copolymer is referred to as a "comb" stabilizer, and the polymer forms a "brush" at the O/W interface. The copolymer is usually prepared by grafting a macromonomer such as methoxy polyethylene oxide methacrylate with polymethyl methacrylate. Typical commercially available graft copolymers are Atlox 4913 and Hypermer CG-6 supplied by ICI. The "grafting into" technique has also been used to synthesize polystyrene-polyethylene oxide graft copolymers. These molecules are not commercially available, and they are not approved for use in personal care and cosmetic preparations. Recently a novel graft copolymer based on a naturally occurring polysaccharide, inulin (polyfructose), has been synthesized (3). Inulin is a polydisperse polysaccharide consist- ing mainly, if not exclusively, of �(2- 1) fructosyl fructose units (Fm) with normally, but not necessarily, one glucopyranose unit at the reducing end (GF 0) (4,5). To produce the amphipathic graft copolymer, the chains were modified by introduction of alkyl groups (C4 - C18) on the polyfructose backbone. The structure of the molecule (hydro- ponically modified inulin, HMI) is illustrated in Figure 2. In this structure, the alkyl groups represent the B chains (that are randomly distributed on the sugar backbone on primary hydroxyl functions as well as on the secondary ones), which become strongly adsorbed on an oil droplet. The sugar chain forms the stabilizing chain, as this is highly water-soluble. These graft copolymers are surface-active, and they lower the surface tension of water and the interfacial tension at the oil/water interface. t;t �N r 2 Off n � CH� OH (GFn) Figure 2. Structure of hydrophobically midfield inulin (HMI).