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
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J. Cosmet. Sci.) 57, 153-169 (March/April 2006) Principles of emulsion stabilization with special reference to polymeric surfactants THARWAT TADROS, 89 Nash Grove Lane, Workingham) Berkshire RG40 4HE, U.K. Accepted for publication November 17, 2005. Synopsis This overview summarizes the basic principles of emulsion stabilization with particular reference to poly- meric surfactants. The main breakdown processes in emulsions are briefly described. A section is devoted to the structure of polymeric surfactants and their conformation at the interface. Particular attention is given to two polymeric surfactants that are suitable for oil-in-water (O/W) and water-in-oil (W/O) emulsions. For O/W emulsions, a hydrophobically modified inulin (HMI), obtained by grafting several alkyl groups on the backbone of the inulin (polyfructose) chain, is the most suitable. For W/0 emulsions, an A-B-A block copolymer of polydroxystearic acid (PHS), the A chains, and polyethylene oxide (PEO), the B chain, is the most suitable. The conformation of both polymeric surfactants at the 0/W and W/0 interfaces is described. A section is devoted to the interaction between emulsion droplets containing adsorbed polymer surfactant molecules. This interaction is referred to as steric stabilization, and it is a combination of two main effects, namely, unfavorable mixing of the A chains, referred to as the mixing interaction, Gmix, and loss of configurational entropy on significant overlap of the stabilizing chains, referred to as elastic interaction, Gc1. The criteria for effective steric stabilization are summarized. O/W emulsions based on HMI are described, and their stability in water and in aqueous electrolyte solutions is investigated using optical microscopy. Very stable emulsions can be produced both at room temperature and at 50°C. The reason for this high stability is described in terms of the multipoint anchoring of the polymeric surfactant (by several alkyl groups), the strong hydration of the inulin (polyfructose) chains, and the high concentration of inulin in the adsorbed layer. W/O emulsions using PHS-PEO-PHS block copolymer can be prepared at a high volume fraction of water, (p, and these emulsions remain fluid up to high (fl values (0.6). These emulsions also remain stable for several months at room temperature and at 50°C. The last two sections are concerned with the problems of creaming or sedimentation and phase inversion. Creaming or sedimentation can be prevented by the use of "thickeners" in the continuous phase. These molecules produce non-Newtonian systems that will have a very high residual or zero shear viscosity. The latter, which may exceed 1000 Pas, can prevent any creaming or sedimentation. Syneresis of the emulsions can also be prevented by control of the bulk (or elastic) modulus of the system. Phase inversion in 0/W emulsions can also be prevented using HMI, since this polymeric surfactant is not soluble in the oil phase. As long as coalescence and Ostwald ripening are prevented, the emulsions can remain stable for very long times both at room temperature and at 50°C. INTRODUCTION Many personal care and cosmetic products are formulated as oil-in-water (0/W) or water-in-oil (W/0) emulsions. These systems are only kinetically stable since the energy 153
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.

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