(Continued from Part I)
Cosmetics including deodorants, shampoo, skin care and perfumes are an integral part of daily life.54, 60 Today’s consumers perceive skin, hair and sun products a from a value-added perspective and are willing to pay for additional benefits.61-63 However, the cosmetic formulator cannot just make a standard formula and build on it. Appearance and feel must be considered, and formulations should provide long-term benefits to the consumer. Considering these requirements, microemulsions emerge as a tool for multiple benefits. They improve appearance, elegance and stability,64 are easy to apply, and useful to control the release of actives.65
Most importantly, due to the presence of polar, non-polar and interfacial domains, they enable the encapsulation of ingredients with varying solubility; for example, in cases where hydrophobic cosmetic actives might be drawn into the oil phase, making them difficult to dissolve into aqueous phases. Microemulsions therefore enhance the penetration of actives into the skin and pores.66 In fact, there has been a significant interest in the role of microemulsions as novel drug delivery systems, and even at selected lengths, along the intestinal track.67, 68
The following sections review more recent applications of microemulsions. Additionally, the delivery segment goes into greater depth and provides suggestions for optimization.
The outermost stratum corneum is mainly concerned with barrier function. It is important to keep this layer flexible, which is achieved by controlling water content. Water content is intimately linked with the lamellae in the intracellular spaces of the stratum corneum. The drastic reduction of water levels leads to a dilation of the peripheral blood capillaries, leaving the skin dry, itchy and red. In addition, wrinkling and damage can compromise this layer and cause moisture loss.
Normally, moisturizing agents and active ingredients address these issues. However, moisturizing or nourishing solutions typically must be applied separately from other functions, e.g., cleansing, since it is challenging for products to provide all benefits at once. To overcome this limitation, microemulsions can be employed.69, 70
Microemulsions are more desirable than suspensions, dispersions or macroemulsions for several reasons. Emulsions are thick and tend to leave an oily patch on skin, which upon contact with external dirt, forms a greasy state and heavy, tacky feel. Microemulsion globules, due to their smaller size, penetrate the skin more deeply, effectively moisturizing while the aqueous phase washes away the dirt. Besides their delivery capabilities, microemulsions form a protective layer on skin that lasts for hours, they improve the flow of product across the skin, and they replenish natural skin oils. Lastly, they have a high level of clarity, which is esthetically pleasing for consumers.
Microemulsions also can improve the dispersions of some materials. For example, Parmer et al. produced stable neem oil-based microemulsions71 using alkyl phenolethyxolate as the nonionic emulsifier was nonionic. This invention was mainly appreciated for its alcohol-free formula, which is suitable for skin care and cure products.
Fine Dispersion Sprays
Microemulsions also can play a major role in sprays. Benner et al. proved it by making stable o/w, microemulsion-based sprays with inorganic pigments as emulsifiers.72 This is because microemulsions hold actives in a more finely dispersed form than simple emulsions so.
Slavetcheff et al. devised a hydroalcoholic microemulsion comprised of water, C4 alkanol, vitamin-based oils and terpenes73 to help impart water-insoluble vitamins and essential oils to the body. This form dries more quickly than solution-based formulas and gives a cooling sensation.
Baur et al. invented ester-based o/w micoremulsions with polyglycol ester as the emulsifier and another lipophilic internal phase. In this way, vitamins A, E, K, D and their derivatives could be dissolved in the lipophilic component and imparted to the skin74 to treat wrinkles, burns, etc.
Many others, such as Pereira et al., Wieshe, Sato, Yoshiki, etc., devised microemulsion-based cosmetic formulas that performed much better than corresponding emulsions or solutions.75-77 Doescher et al. prepared o/w microemulsions for cosmetic and dermatological light screening, which also turned out to be good bases for sprayable sunscreens or insect repellent.78 Ma et al. made stable, clear, antiperspirant microemulsions,79 and the Gillete Company made clear, silicon-based emulsions with water.80
Microemulsions can enhance the pentration of actives into the skin, thus their potential for drug delivery has gained interest.
How About in Hair?
Hair care products also have been reinvented using microemulsions. Compared with shampoos or oil-based products, microemulsions impart better effects in terms of combability, elasticity, styling, detangling and esthetics. Schroder et al., for example, produced highly viscous, gel-like microemulsions designed to keep the formula from dripping, allowing it time to act on the hair.81
Bergmann et al. synthesized a microemulsion based hair treatment that significantly improved the delivery of conditioning agents into hair,82 in turn improving wet and dry combing. It consisted of water-insoluble, amino-containing compounds containing silicone. This treatment is stable and is currently used in shampoos, conditioners, fixtures, lotions, hair sprays and more.
Since shampoos and styling agents normally make the hair and scalp dry, oil must be reapplied. However, microemulsions enable both oil nourishment as well as the removal of unwanted elements in one benefit system. Dow Corning Corp., for example, devised amino functional, silicone microemulsion-based clear shampoos;83 and Dalry et al. made transparent microemulsions for use in hair conditioners.84
Procter & Gamble also recently developed stable, microemulsion-based hair styling compositions that provide good style retention, restyling benefits and improved hair esthetics.85 These formulations were based on non-silicone hair styling polymer and a liquid hydrophilic non-polymeric cationic compound having at least one quaternary ammonium molecule. Likewise, numerous U.S. patents for hair bleaches and colorants incorporate microemulsions.86-88
Soothing Shaved Skin
Another area where microemulsions are effective is shaving lotions and foams, and after-shaves. The high amount of alcohol in these formulas typically is a source of irritation but microemulsion based formulas provide good pre-shaving lubrication through the oil phase. They also help to keep skin supple and wet after the shave.
Edwards et al., for example, prepared a polyorganosiloxane based microemulsion foam having a particle size of about 0.14 microns.89 This formulation improved the lubrication, richness and stability of the foam during shaving. It also gave good skin feel, anti-pore-clogging benefits and foam rinseability post-shaving.
Many others, e.g., Feng, Diec and Hill, developed miscellaneous microemulsion matrices for specific purposes.90-92 In fact, in Hill’s method, a clear, stable microemulsion was formed by combining water, a volatile cyclic methyl siloxane and a silicone polyether surfactant.
Besides the described areas, microemulsions are finding application in protein, peptide and drug delivery. As noted, microemulsions form a stable, feasible and pain-free method of delivery, as opposed to injections. Jones and coworkers assessed a radio-labelled anti-TNF receptor Fc-fusion protein applied to shaved skin via a w/o microemulsion, where it immediately was observed in the dermal layer. Thereafter, it shifted sidelong to distal areas of the skin and muscle.93 Many additional studies point to similarly interesting applications,94 the latest of which describes an all-inclusive microemulsion that can deliver a range of small peptides to large proteins (100-150,000 Da).
W/O drug delivery systems (DDSs) protect water-soluble moieties from the body’s metabolism.95 In microemulsion-based DDSs, these entities can be delivered into the target organ without being affected by metabolism or physical barriers.96, 97 Furthermore, they do not require high temperature processes, and hence are good for encapsulating heat-sensitive drugs. Such microemulsions normally are comprised of fatty acid esters as the oil phase and isopropanol as the co-surfactant.
The capability of microemulsion DDSs is exemplified in the delivery of vasopressin in rats.95 Using a microemulsion delivery base, researchers found the concentration of vasopressin delivered tripled that delivered by an aqueous base. In spite of such a prospect, however, DDSs have not gained industry response. The reasons may be a lack of knowledge of the intricate drug structures in solution, and the partition coefficient of the drug molecules between the oil and aqueous phase. The digestibility of the remnant oil is another question. Further research is being conducted to sort through these issues.98
Revealing Protein Processes
As per the work of Adachi et al.,99 dioctyl sodium sulfosuccinatea based w/o microemulsions are good media for the elucidation of proteins. The authors describe this approach as a phase transfer method, in which the impact of surfactant molecules on the adsorption of protein is studied. This adsorption is then related with time-dependent solubilization processes.
Hilhorst et al. inspected various aspects of enzyme and protein extraction using reverse micelles.100 Different theoretical models have been suggested, and conformational changes of proteins in both normal and reverse microemulsions of different charge types also have been reported.101-105
Another recent development is the pseudo phase model of microemulsion. This is used to quantitatively explain the diverse kinetic behavior of various amines. Gracia Rio et al. studied the shifting kinetics of the nitroso- group from (N-methyl-N-nitroso-p-toluene sulfonamide) to several amines, using varieties of water (and) dioctyl sodium sulfosuccinate (and) isooctane microemulsion basea.106
Researchers also used the pseudo phase model to explain the odd kinetics of solvolysis of many compounds. This is because the kinetics are better explained using steady state theory than normal second order kinetics.
Microemulsions in gel also format can serve as a reaction medium. Reports suggest the use of lipase-containing microemulsion based gels to synthesize a variety of different esters on a lab scale under mild conditions.107-109 Both regio- and stereo-selective results have successfully been achieved.
It also has been found that microemulsions are the best media for biomimetic systems, assemblies and good models for highly conductive thin films and photosynthesis.110, 111 Nanoparticles, for example, often are manufactured by reverse microemulsion-based synthesis as it produces a much narrower particle size distribution.112, 113 Most bismuth oxyhalides are produced via microemulsion based routes, too. These compounds play a vital role in components of color filters, catalysis, electronics, etc.
Interestingly, the use of multiple microemulsions increases synthesis yield. In this, each reactant is dissolved in a separate vessel containing the same set of microemulsions. The nanoparticles are formed by intermicellar exchange of the reactants present in the separate reverse micelles. W/O microemulsions have a clear-cut advantage over other methods for preparing nanoparticle catalysts on supports, such as electron beam lithography, colloidal lithography and spin coating, as they can be formed at room temperature and pressure conditions.114
As Analytical Reagents
Finally, microemulsions have the potential to revolutionize the analytical aspects of chemistry in general and they are finding their way onto the shelves of chemical labs. The use of microemulsions as well as semi-microemulsions for the determination of metals has been reported.115-118 Furthermore, global studies have shown that microemulsion-based formulations greatly enhance spectro-analytical processes. They can be used as spectral-shifting agents, are good dissolving media and they amplify intensity.119
Microemulsion application even can extend to areas of laser excited photoionization spectroscopy.120 The latest advent in this field is microemulsion electro kinetic chromatography (MEEKC), which has been used to analyze the hydrophobicity of solutes.121, 122
Microemulsions present a means to convert the challenges faced by the industry into step-change opportunities. As a relatively recent discovery, microemulsions have grown and found importance in a variety of chemical and industrial processes. They enable the mixing of different phases and ingredients into single personal care formulas, they effectively deliver actives in a cost-efficient manner, and they can control the relative dispersion of formulas, e.g., in spray applications.
The field of microemulsions is far from being fully utilized, hence many researchers are continually intrigued by and challenged to exploit their immense potential in multiple areas. Microemulsions show exceptionally low interfacial tension, thermodynamic stability and giant interfacial area.123, 124 Thus, they provide the market with an opportunity for growth and many unique preparations.
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