335 Application of Reverse Micelles in Cosmetic Formulations REVERSE MICELLE When surfactant concentration in an oil phase or nonpolar solvent reaches its CMC, reverse micelles are formed. The surfactant head groups gather to form an aqueous core while their hydrophobic tails point toward the bulk solvent phase. The structure of a spherical reverse micelle is shown in Figure 2. Size of reverse micelles are estimated through measuring the water content in the system (22). Karl-Fischer titration is usually conducted for measuring the water content of reverse micelle systems. Generally, the size of reverse micelles is from 1 nm to 10 nm (23). Cylindrical reverse micelles may also form depending on the composition of micellar systems and surrounding conditions. The water-to-surfactant ratio and the surfactant concentration in the bulk phase are important factors that determine the shape and size of reverse micelles formed (19). Regardless of shape, reverse micelles are very useful for the solubilization of polar compounds in nonpolar solvents. Reverse micelle systems usually contain a low amount of water. The water molecules are encapsulated in reverse micelles and dispersed in a nonpolar solvent (24). The polar compounds are encapsulated in the water cores of reverse micelles (17, 25). The suspension and dispersion of polar soluble ingredients in oil phases is made possible by utilizing reverse micelles formation (15). Reverse micelle systems are often used in the extraction of biological compounds, enhanced oil recovery, and the formation of nanoparticles. Reverse micelles are also used in the investigation of biomaterial due to their resistance to water properties (26). Reverse micelles can be produced at room temperature, thus making it more advantageous compared to other nanoparticle systems (27). Extraction using reverse micelles is usually done in two stages: the forward extraction and the backward extraction (28). Forward extraction involves selective dissolution of bioactive compounds into the reverse micelles, and backward extraction involves the release of encapsulated compounds into the fresh aqueous phase (22). Reverse micelle extraction is particularly effective at extracting charged compounds. The possibility of biodiesel production using diesel-based, reverse micelle was also reported (Nguyen et al., 2010). Reverse micelle systems have potential in formulating innovative cosmetic products. The following sections will focus on discussing the applications of reverse micelles in relation to the cosmetic field. Figure 2. Structure of a reverse micelle (43).
336 JOURNAL OF COSMETIC SCIENCE REVERSE MICELLE IN COSMETICS Reverse micelles can be applied in cosmetic formulations to encapsulate hydrophilic active ingredients. Encapsulation in reverse micelles can protect the active ingredients and preserve their activities. The solubilization and encapsulation of proteins and dyes in reverse micelles show potential improvements in cosmetic formulations (23, 29). Furthermore, the application of reverse micelles in cosmetic skin care products shows the enhanced penetration of active ingredients into the SC and through the epidermal barrier (17). In addition, reverse micelle extraction has been used to extract various active ingredients, such as plant extracts, for use in cosmetic formulations. DIRECT APPLICATIONS IN COSMETICS The direct inclusion of reverse micelles in cosmetic formulations is possible yet they are limited in their function. This means that reverse micelle systems may not be suitable for all skin care formulations. Substantial dirt such as sebum and cosmetic residues can be removed from the skin’s surface by using cosmetic products formulated with oil-based emulsion or microemulsion (9). Oil-based liquid cleansers have the advantage of retaining the oiliness on skin’s surface after cleansing. This is not achievable using typical water-based cleansers as they contain only a small amount of oil. This can be a reason for introducing reverse micelles into the cleansing formulations since the reverse micelles tend to form in nonpolar solvents such as oil (15). The reverse micelles in the formulations can improve the cleansing property through encapsulation. Examples of these cosmetic products are cleaning oils, gels, and lotions. Another cosmetic formulation that can benefit from using reverse micelles is lipstick. Lipsticks in general are made up of oil-based formulations which have a lower moisture content. To increase the moisturizing function of the lipsticks, reverse micelles are employed to encapsulate water-soluble collagen in the oil phase of the lipstick formulations (30). Reverse micelles allow hydrophilic active ingredients to be included in cosmetic formulations, especially the oil-based ones. The components used must be mild to the skin and nontoxic. Examples of such reverse micelle systems are AOT in orange essential oil (31), AOT in Isopropyl myristate (IPM), and AOT in Methyl laurate (ML) (32). These reverse micelle systems utilize nontoxic solvents and do not include co-surfactants. IL-AOT–based reverse micelle in IPM or ML shows potential in cosmetic applications (33). The advantage of IL is that it can be tailored to have certain properties, it has less toxicity, and it is environmentally friendly. The mixture of lecithin, sophorolipids, and rhamnolipids in IPM or ML is able to form reverse micelles at an appropriate mixture ratio (34). This reverse micelle system shows good tolerance toward changes in temperature and electrolyte concentration, thus is useful for designing more robust formulations. Inclusion of biosurfactants also helps improve the biocompatibility, sustainability, and biodegradability of the formulation. All of the aforementioned reverse micelle systems can be used to encapsulate hydrophilic active ingredients and are suitable to be used in cosmetic formulations. The encapsulation of various active compounds and drugs in reverse micelles has been reported. A food-grade lecithin reverse micelle system was used to encapsulate gallic acid, p-hydroxybenzoicacid, protocatechuic acid, and tyrosol (34). It was found that the antioxidant activities of the encapsulated active compounds are preserved. AOT/ Phosphatidylcholine (PC) and AOT/ Monoolein (MO) reverse micelles in soybean oil can solubilize ascorbic acid,
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