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).

EMULSION STABILIZATION 157 They will also adsorb on the oil droplets with the alkyl groups strongly attached (multipoint anchoring), leaving the polyfructose chains dangling in solution and prob- ably forming large loops. As we will see later, these graft copolymers can produce highly stable emulsions, in particular at high electrolyte concentrations. An example of com- mercially available HMI is INUTEC® SPl (produced by Orafti, Belgium). Another A-B-A block copolymer for stabilization of W/O emulsions has been developed by Uniqema (ICI), namely Arlacel P135 (6). The A chains are poly(l2-hydroxy stearic acid) (PHS), whereas the B chain is poly(ethylene oxide (PEO), i.e., PHS-PEO-PHS. A schematic representation of the structure of the polymer is given in Figure 3. The polymer has a weight average molecular weight, M w , of 6809 and an average molecular weight, M 0 , of 3499 (polydispersity of 1.94). It has an HLB number of 5-6, which makes it suitable for W/0 emulsions. The PHS chains (the stabilizing chains) are highly soluble in most hydrocarbon solvents and strongly solvated by their molecules. These chains extend in the oil, giving a layer thickness of the order of 10 nm. This makes the molecule ideal for steric stabilization (see below). The PEO chain (the "anchor chain") is highly soluble in water and in moderate electrolyte solutions, and this gives very strong adsorption at the W/0 interface. Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to knowing how these molecules act as stabilizers. Most basic ideas on adsorption and conformation of polymers have been developed for the solid/liquid interface (7). The same concepts may be applied to the liquid/liquid interface, with some modification whereby some parts of the molecule may reside within the oil phase, rather than simply staying at the interface. Such modification does not alter the basic concepts, particularly when one deals with stabilization by these molecules. The process of polymer adsorption involves a number of interactions that must be separately considered. Three main interactions must be taken into account, namely, the interaction of the solvent molecules with the oil in the case of O/W emulsions, which need to be displaced for the polymer segments to adsorb, the interaction between the chains and the solvent, and the interaction between the polymer and the surface. Apart from knowing these interactions, one of the most fundamental considerations is the conformation of the polymer molecule at the interface. These molecules adopt various conformations, depending on their structure. A schematic representation of the confor- / + (PEO chain) Figure 3. Schematic representation of the structure of PHS-PEO-PHS block copolymer.