Multiple emulsions1 are complex systems in which the drops of the dispersed phase themselves contain even smaller dispersed droplets that normally consist of a liquid that is miscible, and in most cases identical, with the continuous phase. They are, therefore, emulsions of emulsions. In cosmetics, these systems can prevent degradation of an active ingredient and release it at a controlled rate. This article reviews the different techniques for preparing multiple emulsions. These techniques are more complicated than for simple oil-in-water (o/w) or water-in-oil (w/o) emulsions, but may be worth the extra effort for formulators wishing to protect and deliver sensitive actives.
In multiple emulsions, the internal and external phases are alike and an intermediate phase separates the two like phases. The intermediate phase is immiscible with the two like phases. For example, in water-in-oil-in-water (w/o/w) multiple emulsions, a w/o emulsion is dispersed in a water-continuous phase. An emulsifier is present to stabilize the emulsions and various ionic and nonionic surfactants are available for this purpose. Lipophilic (oil-soluble, low HLB) surfactants are used to stabilize w/o emulsions, whereas hydrophilic (water-soluble, high HLB) surfactants are used to stabilize o/w systems.
Potential applications for multiple emulsions are well-documented and many of these applications have been patented.2-6 The important applications are in cosmetics (see Cosmetics Using W/O/W Multiple Emulsions), pharmaceuticals and foods. For example, in cosmetics they have a fine texture and a smooth touch upon application, and they are aimed for slow and sustained release of active matter from an internal reservoir into the continuous phase (mostly water). They can serve as an internal reservoir to entrap matter from the outer diluted continuous phase into the inner confined space. They can also improve dissolutions or solubilization of insoluble materials. Due to these properties, multiple emulsions find applications related to protecting sensitive and active molecules such as vitamins C and E from the external phase—a process called antioxidation. Patents from the past 15 years indicate that work is now being carried out on the stability and preparations aspects of multiple emulsions, so one can expect many more applications to emerge in the near future.
Multiple emulsions generally consist of three components: the oil phase, the aqueous phase and an emulsifier. Each of these components and the method in which each component is prepared and combined contributes to the type and stability of emulsions. Hameyer et al.7 described multiple emulsions consisting of two different interfaces that require two sets of different types of emulsifiers. In o/w/o multiple emulsions the set of emulsifiers for the internal interface must be hydrophilic, while the set of emulsifiers for the external interface must be hydrophobic. For w/o/w multiple emulsions, the opposite is true; the inner emulsifiers are hydrophobic while the outer ones are hydrophilic. In many cases, a blend of two or more emulsifiers in each set is recommended for better stabilization results. Based on these observations, Hameyer et al. concluded that multiple emulsions require at least two emulsifiers with very different HLB values, as shown in Table 1.
In early reports,8 the preparation of multiple emulsions used only one set of emulsifiers and an inversion process. Such preparations were done in one step, but the stability was often questionable. It was difficult to control the distribution of the emulsifiers within the two interfaces. Fast migration of the emulsifiers between the phases destabilized the emulsions.
In most recent emulsion formulations, the emulsions are prepared in two steps. In the first step, a high-shear homogenization is applied to the water that is added to the solution of oil and hydrophobic emulsifiers to obtain a stable w/o emulsion. In the second step, the w/o emulsion is gently added with stirring (not homogenization) to the solution of water and hydrophilic emulsifiers.
The droplet size distribution of a typical classical multiple emulsion ranges from 10 μm to 50 μm. An emulsifying device such as a homogenizer or a stirring device such as a votator may also be used. Vigorous agitation at a rate of approximately 3000 to 8000 rpm is effective. The types of agitators used for this process include: those with agitating blades arranged near the bottom, suction type agitators with agitating blades extending throughout the chamber interior, agitators of mere suction mixing type, homomixers, homogenizers and votators.
It is recommended to use an agitator in which the impact applied to the upper w/o emulsion layer is weak. The authors note that when homogenizers are used to mix the upper layer, the emulsions break down; however, slow hand stirring with a glass stirrer often yields stable w/o/w multiple emulsions. To facilitate the subsequent emulsification, the w/o emulsion is preferably heated to 50–80°C. The aqueous phase is mixed with the above-mentioned w/o emulsion and the mixture is agitated with the agitator at 250 rpm for 5 min and then treated with a homogenizer to obtain the intended w/o/w emulsion having a very fine texture.
Some more sophisticated preparation processes have been reported in the literature. Vigie9 described a lamellar phase dispersion process derived from the process to obtain liposome-like vesicles with nonionic emulsifiers. This process can be used only when the constituents form a lamellar phase by mixing with water in definite proportions. This procedure is advantageous because it requires only a simple emulsification step. The mesophase formed by an ideal ratio of lipophilic emulsifiers in water is thermodynamically stable and can be obtained rapidly and easily. The method’s main limitation is the fact that most emulsifiers do not form lamellar phases. When the lamellar mesophase exists, the HLB of the mixture of emulsifiers is often too high, which is disadvantageous for the stability of a multiple emulsion because higher HLB emulsifiers do not show any affinity for the oil phase, in the authors’ experience.
Grossiord et al.10 described an emulsified microemulsion process. The idea is to use a surfactant to disperse an oil phase within water; this forms the first liquid phase, the so-called L2 phase in the terminology of ternary phase diagrams. This L2 phase is further emulsified with the external continuous water phase to form the multiple emulsion. The problem with this process is the lack of evidence that microstructures are in fact formed and that it does, indeed, lead to multiple emulsions. Moreover, there is insufficient evidence that the internal phase, an L2 phase of submicrometer droplets, remains after the second emulsification process.
It seems that in some cases, the process is a well-characterized two-step emulsification that leads to relatively large multiple-emulsion droplets. This process should be more carefully evaluated. Proving that the internal phase actually survives as submicrometer droplets might bring a breakthrough to this field because the sizes of the external droplets could be reduced to values below 1 μm, allowing the formation of indictable multiple emulsions.
Higashi et al.11 described a new method of producing w/o/w multiple emulsions by a membrane emulsification technique. This method permits the formation of monodispersed liquid microdroplets containing aqueous microdroplets to form a w/o/w system. In this method, the aqueous internal phase is mixed with an oil phase containing a lipophilic emulsifier. The mixture is sonicated to form a w/o emulsion and the upper chamber of a special apparatus is filled with an emulsion.
The external aqueous phase containing the hydrophilic emulsifier is continuously injected into the lower chamber to create a continuous flow. Nitrogen gas fed into the upper chamber initiates permeation of the w/o droplets through the controlled-pore glass membrane into the emulsifying chamber, forming a w/o/w multiple emulsion. The emulsion is progressively removed from the apparatus.
Factors affecting Preparation of Multiple Emulsions
Herbert’s12 work attracted a great deal of attention to multiple emulsion systems, but it soon became obvious that if multiple emulsions were to succeed commercially, the materials and methods of preparing them would have to be studied systematically to determine the type of emulsifying agents to be used, the effect of phase volumes on the formation and stability of multiple emulsions, and the effect of processing procedure on formation and stability of multiple emulsions.
Effect of emulsifying agents: Matsumoto et al.13 concluded that the yield of a w/o/w emulsion is influenced by the concentration of nonionic emulsifying agents used in both steps of the emulsification. Increasing the concentration of the hydrophilic emulsifier used in the outer phase decreased the yield of the multiple emulsions.
Effect of phase volume: Fukuda14 obtained increasingly better yields as the dispersed w/o phase volume increased to a level of about 50% (see Figure 1). Accordingly, in preparing the w/o/w emulsions that consist of a w/o emulsion as a dispersed phase and an aqueous solution of a water-soluble emulsifier as a dispersion medium, the volume percentage of the w/o emulsion may reach up to 75% by volume. Thus, the volume percentage of an aqueous solution of the water-soluble emulsifier may be reduced to 25% by volume and the w/o/w emulsion is extremely stable even when such a great amount of w/o emulsion is dispersed in the aqueous solution of the water-soluble emulsifier.
Effect of shear: Fukuda14 demonstrated that the optimal period of time required for agitation in preparing w/o/w emulsions by adding a w/o emulsion to an aqueous solution of the water-soluble emulsifier varies depending on the emulsification equipment used. The current authors note that it should not exceed 1 hr because both the efficiency of agitation and the average diameter of particles in the dispersed phase decline over time. Therefore, it is preferable to predetermine the agitation time for the emulsification equipment used so that the preparation rate will be as high as possible. In one example from Fukuda, 10 min of agitation by a homomixer allows the preparation rate of the multiple emulsions to be kept at high level almost constantly, while agitating for 15 min remarkably reduces the rate (Figure 2). In short, it is better to avoid long periods of agitation.
W/O/W Multiple Emulsions for Cosmetics
Emulsions, particularly w/o and o/w emulsions, have been widely utilized as a fundamental form for various chemical products. Cosmetics such as creams and lotions mostly use w/o or o/w emulsions as their fundamental form. In cosmetics, w/o/w multiple emulsion systems are used for sustained release of fragrance, prolonged skin moisturization, protection of sensitive biologicals and to protect incompatible materials from interacting with each other.
Taelman and Loll noted that widely used w/o emulsion-type cosmetic creams have very smooth appearance, high cleansing effect and excellent emollient effect, but they a give sticky or oily feel because the external oil phase is exposed to skin on application. The emulsion stability is reduced if water content exceeds 50% by volume in a w/o emulsion.15
O/W emulsion-type cosmetic creams provide excellent extensibility and a well-refreshed feeling when then are applied on skin15 but they are less effective at cleansing and emolliency than w/o emulsion-type creams. Because emulsion-type lotions are generally made as a form of o/w emulsions, they exhibit the disadvantages described above as well as instability often caused by the problem of creaming. W/O/W multiple emulsions may eliminate the disadvantages of w/o and o/w emulsions while simultaneously providing a very fine texture and a smooth touch upon application to the skin. This explains their use in the production of cosmetics.15
In a more general discussion of w/o/w multiple emulsions, Simon observed that the amount of water or aqueous solution to be added may be properly determined depending on the type of products to be manufactured but in general is in the range of 30–75% by volume of the w/o emulsions.16
It is preferable to make the water phase volume greater than 30% (vol/vol) in order to fully utilize the advantages of the w/o/w emulsions; if the water phase volume is less than 30% (vol/vol), the properties of the resulting w/o/w emulsions become similar to those of the o/w emulsions. On the other hand, if the water phase volume in w/o emulsions exceeds 75% (vol/vol), stability of the w/o emulsion itself decreases and it may reduce the preparation rate of the multiple particles.
In order to fully utilize the advantages of the w/o/w emulsions, the maximum amount of the oil component should be 52.5% by volume in the w/o/w. On the other hand, the minimum amount is not specifically determined but is preferably 7.5% by volume for manufacturing products such as cosmetics.16
Florence and Whitehill noted that the higher the proportion of multiphase-emulsified particles in the w/o/w emulsions, the higher the viscosity of the multiple emulsions.17 They reasoned that the true volume proportion of the dispersed phase increases as the proportion of multiphase-emulsified particles increases, causing a proportional change in viscosity. The viscosity of the continuous phase and dispersed phase volume can be easily controlled, enabling the production of emulsion-type lotions with a wide range of viscosities.
O/W/O Multiple Emulsions for Cosmetics
An o/w emulsified composition further emulsified and dispersed in an oil phase is called an o/w/o multiple emulsion and has become important in various cosmetic applications. A stable o/w/o emulsion is used in the cosmetics and dermatological fields, especially for the controlled release of active principles for the purpose in particular of cleansing, treating, protecting or moisturizing the skin, mucous membranes and keratinous fibers and more particularly for the purpose of treating dry skin.18
The most frequently encountered mechanism for destabilization of multiple emulsions is the migration of oil from internal droplets to the oily external phase through the intermediate aqueous layer, either by simple diffusion of oil through the aqueous membrane or by rupture of the aqueous film. This allows the internal oil droplets to coalesce with other internal oil droplets that are now in the external continuous oil phase.
Various approaches have consequently been envisaged to improve the stability of multiple emulsions. One approach suggested by Sekine et al. consists of introducing one or more gelling polymers into the oily internal or external phase, the role of which is to limit, on a long-term basis, the movement of oil from the internal phase to the external phase.18 However, polymers capable of gelling oils are not very common and do not have good cosmetic properties, according to the authors. They also intensify the feeling of greasiness and are sticky during and after application to the skin. The multiple emulsions obtained exhibit the drawback of being sticky and of taking a long time to penetrate into the skin. The need, therefore, remains for stable o/w/o emulsions that are pleasant to use on the skin and that do not have the disadvantages of those of the prior art.
One advantage of o/w/o emulsions is that their oily continuous phase makes it possible to form a lipid film at the skin surface, which prevents transepidermal water loss and protects the skin from external attack. The greasy effect of such emulsions can be avoided by incorporating novel light oils that are nongreasy to the touch, such as low viscosity silicone oils or certain esters of fatty acids and of fatty alcohols with a short chain. With such simple adjustments it is possible to exploit the advantages available from multiple emulsions of the o/w/o type.
Patent literature4 lists the following preparation procedures for several w/o/w multiple emulsions used in cosmetic products.
Cleansing cream: The cleansing cream shown in Formula 1 was prepared by heating components A to 50°C. Component B was added to the mixture and homogenized. Separately, C was heated to 50°C and added to AB. D was added to ABC at a rate of 20 mL/min under agitation at 6000 rpm. In the course of this step, the o/w emulsion was inverted to obtain a w/o emulsion. 100 parts by weight of the w/o emulsion was dispersed in 20 parts by weight of a 10% aqueous solution of polyoxyethylene sorbitan monolaurate. The dispersion was emulsified again with a homogenizer at 80°C to obtain a cleansing cream comprising the w/o/w-type multiple emulsion. The obtained cleansing cream reportedly was nongreasy and realized a fresh feeling upon use. The cream also was reported to have excellent spreadability and cleansing effects and a stable emulsion state.
Massage cream: Components A in Formula 2 were heated to 50°C. B was added to A and then C was added to AB while agitating with a homomixer at 6000 rpm. D was heated to 50°C and added successively to ABC to obtain a w/o emulsion. 100 parts by weight of the w/o emulsion was dispersed in 20 parts by weight of a 10% aqueous solution of polyoxyethylene sorbitan monooleate. The dispersion was emulsified again with a homogenizer at 80°C to obtain a massage cream comprising the w/o/w multiple emulsion.
Again, patent literature19 provides examples of preparation procedures for several o/w/o multiple emulsions used in cosmetic products.
Nourishing cream for dry skin: An o/w primary emulsion in Formula 3 was prepared by adding A to B while agitating with a homogenizer at 6000 rpm. 100 parts by weight of the obtained emulsion was dispersed in 20 parts by weight of C, the oily external phase. The dispersion was emulsified again with homogenizer to obtain a nourishing cream comprising the o/w/o multiple emulsion.
Moisturizing cream: An o/w primary emulsion in Formula 4 was prepared by adding A to B while agitating with a homogenizer at 6000 rpm. 100 parts by weight of the obtained emulsion was dispersed in 18 parts by weight of C, the oily external phase. The dispersion was emulsified again with homogenizer to obtain a moisturizing cream comprising the o/w/o multiple emulsion.
Multiple emulsions have been known for more than three decades and were extensively studied over the past 20 years. The internal phase is an excellent reservoir for active matter that needs protection and can be released at a controlled rate but the size of the droplets and the thermodynamic instability were significant drawbacks of this technology.8
In cosmetics, multiple emulsion systems are used for sustained release of fragrance, for prolonged skin moisturization, for protection of sensitive biologicals, and to protect incompatible materials from interacting with each other. What is required is a more thorough study of emulsifying agents to give more stable particles in various sizes.
1. C Fox, An introduction to multiple emulsions, Cosmet Toil 101(11) 101–112 (1986)
2. WO/1993/000007, Stable double emulsions containing finely divided particles, FL Thill (Jan 7, 1993)
3. US Patent 5,322,704, Method for preparing a multiple emulsion, AG Gaonkar, assigned to Kraft General Foods Inc. (Jun 21, 1994)
4. US Patent 4,985,173, Process for producing a w/o/w type multiple emulsions for medicines, cosmetics, etc, Y Takahashi, T Yoshida and T Takashi, assigned to Meiji Milk Products Co., Ltd. (Jan 15, 1991)
5. US Patent 5,424,181, Process for preparing photographic emulsions having a low fog level, assigned to Eastman Kodak Co. (Jun 13, 1995)
6. US Patent 6,022,547, Rinse-off water-in-oil-in-water compositions, CA Herb et al, assigned to Helene Curtis Inc. (Feb 8, 2000)
7. P Hameyer and R Klaus, Emulsifiers for multiple emulsions, Cosmet Toil 111(7) 39–48 (1996)
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10. Multiple Emulsions: Structure, Properties and Applications, J Grossiord et al, eds, Paris: Editions de Sante 57–80 (1998)
11. S Higashi et al, Arterial-injection chemotherapy for hepatocellular carcinoma using monodispersed poppy-seed oil microdroplets containing fine aqueous vesicles of epirubicin, Cancer 75 1245–1254 (1995)
12. US Patent 5,478,561, Process for the preparation of stable complex multiple emulsions of water-oil-water systems, L Ferrero, assigned to Lancaster Group AG (Dec 26, 1995)
13. S Matsumoto, Y Kida and D Yonezawa, An attempt to prepare w/o/w type emulsions, J Colloid Interface Sci 57 353–361 (1976)
14. US Patent 4,254,105, Multiple emulsions having a form of water/oil/water phase and process for preparation thereof, and multiple emulsion type cosmetics, H Fukuda, assigned to Lion Dentifrice Co., Ltd. (Mar 3, 1981)
15. M Taelman and P Loll, Multiple emulsions in cosmetics, Cosmetics Conference Proceedings, Barcelona 213 (1994)
16. US Patent 6,358,500, Stable w/o/w emulsion and its use as a cosmetic composition, P Simon, assigned to L’Oréal (Mar 19, 2002)
17. AT Florence and D Whitehill, The formation and stability of multiple emulsions, Internat J Pharmaceutics 11 277–308 (1982)
18. US Patent 6,150,425, O/W/O type multiphase emulsion, T Sekine et al, assigned to Shiseido Co., Ltd. (Nov 21, 2000)
19. US Patent 6,346,256, Stable o/w/o emulsions and its use as a cosmetic and/or dermatological composition, P Simon, assigned to L’Oréal (Feb 12, 2002)