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.

158 JOURNAL OF COSMETIC SCIENCE Figure 4. Conformation of hydrophobically modified inulin (HMI) at the O/W interface. The alkyl chains are soluble in the oil, and the polyfructose loops extend into the aqueous phase. mation of HMI at the O/W interface is given in Figure 4, whereas that of PHS-PEO at the W/0 interface is given in Figure 5. It is clear from the above description of polymer configurations that for full character- ization of the process of adsorption, it is necessary to know the following parameters, namely, the amount of polymer adsorbed per unit area of the surface, r (mole m- 2 or mg m- 2 ), the fraction of segments in close contact with the surface, p, and the distribution of polymer segments, p(z), from the surface towards the bulk solution. It is essential to know how far the segments extend into the solution, i.e., the thickness of the adsorbed layer. It is important to know how these parameters change with polymer overage (concentration), the structure of the polymer, and its molecular weight. It is also essen- Figure 5. Conformation of PHS-PEO-PHS block copolymer at the W/O interface. The PEO chains are soluble in the water droplets, and the PHS chains extend into the oil phase.