Multiple emulsions in cosmetics

Multiple emulsions are complex systems of emulsions of emulsions [38, 39]. Two main types can be distinguished (i) Water-in-Oil-in-Water (W/O/W) multiple emulsions whereby the dispersed oil droplets contain emulsified water droplets, (ii) Oil-in-Water-in-Oil (O/W/0) multiple emulsions whereby the dispersed water droplets contain emulsified oil droplets. The most commonly used multiple emulsions are the W/O/W which may be considered as Water/Water emulsions, whereby the internal 

For applications in personal care and cosmetics, a wider range of surfactants can be used provided these molecules satisfy some essential criteria such as lack of skin irritation, lack of toxicity on application and safety to the environment (biodegradability of the molecule is essential in this case). 

Florence and Whitehill [38] distinguished between three types of multiple emulsions (W/O/W) that were prepared using isopropyl myristate as the oil phase, 5 % Span 80 to prepare the primary W/0 emulsion and various surfactants to prepare the secondary emulsion (a) Brij 30 (polyoxyethylene 4 Lauryl ether) 2%. (b) Triton X-165 (polyoxyethylene 16.5 nonyl phenyl ether (2%). (c) 3 1 Span 80 Tween 80 mixtures. A schematic picture of the three structures is shown in Fig. 1.34. The most common structure is that represented by (b) whereby the large size multiple emulsion droplets (10-100 pm) contain water droplets 1 pm. A schematic representation of some breakdown pathways that may occur in W/O/W multiple emulsions is shown in Fig. 1.35. 

One of the main instabilities of multiple emulsions is the osmotic flow of water from the internal to the external phase or vice versa. This leads to shrinkage or swelling of the internal water droplets respectively. This process assumes the oil layer to act as a semipermeable membrane (permeable to water but not to solute). The volume flow of water,, may be equated with the change of droplet volume with time dv/dt. 

Lp is the hydrodynamic coefficient of the oil membrane , A is the cross-sectional area, R is the gas constant and T is the absolute temperature, g is the osmotic coefficient of electrolyte solution with concentration c. 

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Taelman, M. C. Loll, P. Multiple Emulsions in Cosmetics in Proc. Cosmet. Confl, Everberg Belgium, 1994, pp. 213-233. 

Garti, N. and Bisperink, C. (1998) Curr. Opin. Colloid Interface Sci., 3, 657 —667. Taelman, M.C. and Loll, P. (1994) Multiple emulsions in cosmetics, in Proceedings of Cosmetics Conference, Everberg, Belgium, pp. 213-233. Isenberg, C. (1978) The Science of Soap Films and Soap Bubbles, Tieto, Clevedon. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

No systematic studies of the use of silicone surfactants as emulsifiers have yet been published. Silicone polyoxyalkylene copolymers with relatively high molecular weight and a high proportion of silicone are effective water-in-silicone oil emulsifiers and a recent study of these copolymers suggests that they stabilize emulsions by a solid-particle mechanism [68]. This type of silicone surfactant has been used to prepare transparent water-in-oil emulsions (often with an active ingredient in the internal phase) for use as deodorants or antiperspirants as well as cosmetics and other personal care products. Their use as drug delivery vehicles has also been claimed. These copolymers can also be used to prepare multiple emulsions not requiring a two-pot process. 

Polyoxyethylene alkyl ethers are nonionic surfactants widely used in topical pharmaceutical formulations and cosmetics, primarily as emulsifying agents for water-in-oil and oil-in-water emulsions and the stabilization of microemulsions and multiple emulsions. 

Several industrial systems involve emulsions, of which the following are worthy of mention. Food emulsions include mayonnaise, salad creams, deserts, and beverages, while personal care and cosmetics emulsions include hand creams, lotions, hair sprays, and sunscreens. Agrochemical emulsions include self-emulsifiable oils that produce emulsions on dilution with water, emulsion concentrates with water as the continuous phase, and crop oil sprays. Pharmaceutical emulsions include anaesthetics (O/W emulsions), hpid emulsions, and double and multiple emulsions, while paints may involve emulsions of alkyd resins and latex. Some dry-cleaning formulations may contain water droplets emulsified in the dry cleaning oil that is necessary to remove soils and clays, while bitumen emulsions are prepared stable in their containers but coalesce to form a uniform fihn of bitumen when apphed with road chippings. In the oil industry, many crude oils (e.g.. North sea oil) contain water droplets that must be removed by coalescence followed by separation. In oil slick dispersion, the oil spilled from tankers must be emulsified and then separated, while the emulsification of waste oils is an important process for pollution control. 

All of the above processes are influenced by the nature of the two emulsifiers used to prepare the multiple emulsion. Most reports on multiple emulsions are based on conventional nonionic surfactants, but unfortunately most of these surfactant systems produce multiple emulsions with Hmited shelf-Uves, particularly if the system is subjected to large temperature variations. During the past few years, multiple emulsions have been formulated using polymeric surfactants for both the primary and multiple emulsion preparation. These polymeric surfactants proved to be superior over conventional nonionic surfactants in maintaining the physical stability of the multiple emulsion, such that today they may be applied successfully to the formulation of agrochemical multiple emulsions. The results obtained using these polymeric surfactants offer several potential applications in formulations. The key in the latter cases is to use polymeric surfactants that are approved by the FDA for pharmacy and food, by the CTA for cosmetics, and by the EPA for agrochemicals. 

Eccleston, G. M. 1990. Multiple phase oil-in-water emulsions. /. Soc. Cosmet. Chem. 41 1-22. 

Multiple emulsions may be interesting ways for releasing bioactive compounds in a controlled rate, useful in cosmetic, pharmacy, agricultural, and industrial chemicals nevertheless, their commercial applications have been limited due to their thermodynamic instability and unexpected fast release of encapsulated bioactive molecules (Yoshida et al., 1999 Beer et al., 2013). 

Multiple emulsions usually refer to series of complex two-phase systems that result from dispersing an emul sion into its dispersed phase. Such systems are often referred to as water-in-oil-in-water (W/OAV) or oil-in-water-in-oil (O/W/0) emulsions, depending on the type of internal, intermediate, and continuous phase. Multiple emulsions were early recognized as promising systems for many industrial applications, such as in the process of immobilization of proteins in the inner aqu eous phase (37) and as liquid membrane systems in extraction processes (38). W/O/W emulsions have been discussed in a number of technical applications, e.g., as prolonged drug-delivery systems (39-44), in the context of controlled-release formulations (45), and in pharmaceutical, cosmetic, and food (46) applications. 

While a great deal of information has been pubhshed over the years on the theoretical and practical aspects of emulsion formation and stabilization, until recently little has been said about more complex systems generally referred to as multiple emulsions. Multiple emulsions, as the name implies, are composed of droplets of one liquid dispersed in larger droplets of a second liquid, which is then dispersed in a final continuous phase. Typically, the internal droplet phase will be miscible with or identical to the final continuous phase. Such systems may be w/o/w emulsions as indicated in Figure 11.13, where the internal and external phases are aqueous or o/w/o, which have the reverse composition. Although known for almost a century, such systems have only recently become of practical interest for possible use in cosmetics,... 

As with simple emulsions, multiple emulsions will also be sensitive to breaking due to physical abuse. Shear and shock sensitivity must always be considered. In some applications, however, such sensitivity may be advantageous. In a cosmetic skin cream, for example, the shear imposed on the system... 

Choice of surfactant(s) for the preparation of multiple emulsions can, in principle, be made from any of the four classes of surfactants discussed in Chapter 3, although nonionics tend to be materials of choice because they are more easily tailored to meet the needs of the system. The choice will be determined by the characteristics of the final emulsion type desired, such as the natures of the various phases, additives, and solubilities. In many applications (e.g., foods, drugs, cosmetics), the choice may be further influenced by such questions as toxicity, interaction with other addenda, and biological degradation. In a given system, several different surfactants may perform adequately in terms of stability, but produce different types of multiple emulsions (A, B, or C in Figure 11.15), so that the choice will depend on application as well as function. 

As discussed before (2), multiple emulsions (w/o/w or o/w/o) are ideal systems for application in personal care products for the following reasons. First, one can dissolve ingredients in three different compartments. For example with w/o/w multiple emulsions, one can incorporate two different water-soluble additives (proteins, enzymes, and vitamins) and an insoluble ingredient (e.g., perfume). Second, they can be applied for sustained release by control of the breakdown process that occurs on application. Third, they allow one to produce the same cream consistency required for many cosmetics, e.g., by incorporation of a thickener or gelling agent in the outer continuous phase. 

The last part of the chapter dealt with the preparation of stable water-in-oil-in-water (w/o/w) multiple emulsions that are suitable for application in cosmetics. The main criterion for producing stable w/o/w systems is to use two polymeric surfactants one with a low HLB number for preparation of the primary w/o emulsion and one with a high HLB number for preparation of the final w/o/w multiple emulsion. The primary emulsifier should produce a viscoelastic film fhat prevents leakage from the internal water droplets to the outside continuous phase. It will also ensure high stability (minimum coalescence) of the internal water droplets. The secondary emulsifier should also provide an effective barrier to prevent flocculation and coalescence of the multiple emulsion droplets on storage. It is also essential to balance the osmotic pressure of the internal aqueous droplets and the outside continuous medium. 

Yazan Y, Aralp U, SeUler M, Grossiord JL. PVP in multiple emulsions. Cosmet Toilet 1995 110 53-57. 

The consistency of the multiple emulsion, which is very important for cosmetic applications, can be evaluated using rheological methods, in the same manner as used for emulsions (see above). These methods can be applied for the primary as well as the final multiple emulsion, which may also contain a gel phase. 

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