Nonaqueous Emulsions: History and Current Specialized Applications

Until the mid-1970s, practical applications for nonaqueous emulsions in personal care were almost nonexistent due to formulation difficulties and inelegant properties. However, beginning in the early 1980s, specialized nonaqueous emulsions have attracted technical interest as potential vehicles and delivery systems for personal care and cosmetic products. The reason for this transition has been the development and availability of a broad range of silicone-based emulsifiers, silicone polymers and other polymeric emulsifiers. These materials have enabled the use of nonaqueous emulsions as aesthetic vehicles and delivery systems for a range of distinctive cosmetic and personal care products. This article will briefly review their history and evolution into current-day specialized applications.

Early Emulsions

In a 1963 article¹ from the Journal of Pharmaceutical Science, the authors reported that only stearate ester surfactants induced emulsification, either in glycerin-in-oil systems or when added to olive oil. According to the article abstract, representatives of seven classes of nonionic surfactants and their combinations were tested, by hand methods of trituration, for their ability to induce the emulsification of glycerin and olive oil. No relation was apparent between hydrophilic-lipophilic balance (HLB) values and emulsifying capacity, method of mixing or emulsion type; however, the chemical nature of the surfactant appeared to affect the method of mixing and emulsion type.

In another article,² appearing in a 1968 Journal of the Society of Cosmetic Chemists, a departure from traditional methods and concepts of emulsion technology is described, which was said to result in the development of a family of nonaqueous emulsions possessing interesting and unique properties. In this work, glycerin, propylene glycol and polyethylene glycol 400 were used as the polar phase, and olive oil as the nonpolar phase. Representative anionic, cationic and nonionic surfactants were employed, and the authors showed that conventional theories and methods were not readily translatable to the nonaqueous systems in many instances. “For example, emulsion type and method of preparation seem to be more closely related to the chemical nature of the surfactant than to other types of classification such as the HLB system,” states the abstract.

The same article reported that the chemical nature of surfactant can be correlated with the emulsion type and/ or method employed in forming the emulsion. In addition, only derivatives of stearic acid effectively produced glycerin-in-oil emulsions; all emulsions prepared with sodium stearate as the emulsifier were the polyol-in-oil type. Further, it was noted that only preparations containing sorbitan monostearate, alone or in combination with polysorbate 60, formed stable or semi-stable emulsions. From this author’s limited lab studies, conducted back in 1968, sodium stearate at ~2.00% dissolved in glycerin was found effective in preparing stable glycerin-in-mineral oil emulsions, some of which exhibited chromatic properties at room temperature.

Nonaqueous Emulsion Fundamentals

Selecting the right solvents for nonaqueous emulsions is important, thus a theoretical basis for selecting and predicting their respective miscibility, as well as surfactant behavior, is required.^3–6 It has been proposed^{7, 8} that a liquid capable of replacing water in an emulsion should have an appreciable polarity to make it immiscible with oils and a good solvent for the solvophilic portions of the surfactant molecules.

Hydrogen bonding in the polar liquid is expected to play a role in solvating both ionic and nonionic surfactants, and in the formation of a hydrogen-bonded network in the liquid itself. In each nonaqueous surfactant system, the concept of hydrophilicity should be replaced by solvophilicity, thus defining a new scale that incorporates interactions specific to that solvent. Numerous other studies have been reported in the pharmaceutical literature, some of which are listed in the references.^9–11

Silicone Emulsifiers

The introduction of silicone emulsifers during the early 1980s has gained increasing importance over time. Their use has made it feasible to prepare nonaqueous glycol-in-silicone emulsions with good aesthetics and stability properties. This group of organomodified silicones represents a very modern class of emulsifiers that are polymeric or oligomeric surfactants, possessing more than one hydrophilic and hydrophobic functional group that is able to attach to the interface of emulsions with several groups. These multifunctional polymeric emulsifiers can be used at relatively low concentrations and reportedly are efficient in generating highly stable emulsions.¹²

As explained in the literature,¹² the distinctive functionality of silicone emulsifiers is based on their ability to function differently from organic emulsifiers. To perform properly, a silicone emulsifier must satisfy three requirements: It must migrate to the interface between the two phases, stay at that interface and stabilize the repulsion forces of the two phases. While typical organic emulsifiers are amphiphilic molecules, the action of the silicone emulsifier is the result of functional groups alongside the silicone backbone, which results in lower interfacial tension and a more robust, flexible film at the interface. This characteristic is exemplified by dimethicone copolyol emulsifiers such as cyclomethicone (and) PEG/PPG-18/ dimethicone, and also by lauryl PEG/ PPG-18/18 methicone.

Some silicone emulsifiers such as cyclopentasiloxane (and) PEG/ PPG-18/18 dimethicone have earned a reputation in personal care by making new product forms possible, particularly clear gels for antiperspirant applications. These materials belong to the dimethicone copolyol family of silicones, which is broadly used in personal care products.¹² Nonaqueous emulsions of silicones are useful delivery systems, particularly when the presence of water initiates a process that changes the nature of the cosmetic composition and adversely affects the stability of active ingredients contained therein, such as ascorbic acid, enzymes, vitamins and antioxidants. Stable emulsions of polyethylene glycol in paraffin oil or in isopropyl isostearate are just two examples of unusual nonaqueous emulsions that can be stabilized with silicone emulsifiers.¹² Silicone oils have been found to be particularly desirable components of these cosmetic compositions because the materials impart a dry, smooth and uniform feel among other benefits, such as increasing apparent luster.¹³

Silicone Emulsifiers in Use

By employing volatile silicones as the continuous phase and selecting the polyol of choice, it is possible to manufacture personal care products that not only protect storage-sensitive actives, but also possess a selective mechanism for the transport of active substances; exert a moisturizing effect via the nature of the polyol phase; and do not leave an unpleasant, greasy film on the skin.¹⁴ Examples of nonaqueous formulations that utilize silicone emulsifers are shown in Formula 1 and Formula 2.

Another example in the literature¹⁵ describes an anhydrous color cosmetic composition comprising at least one resveratrol derivative and particulates, and an anhydrous emulsion skin care composition. Among the silicone surfactant emulsifiers cited in the patent application are PEG/PPG-18/18 dimethicone, bis-cetyl PEG/PPG-14/14 dimethicone, PEG-11 methyl ether dimethicone, PEG-9 dimethicone and lauryl PEG-9 polydimethylsiloxyethyl dimethicone (see Cited Silicone Surfactants for complete list).

Finally, Kiehl’s line-reducing concentrate (see Formula 3) is a good example of a nonaqueous emulsion based on silicone emulsifier, silicone crosspolymer and cylomethicone. Here, lauroyl lysine is employed to improve the cosmetic attributes of this emulsion concentrate.

Conclusion

Nonaqueous systems that were merely a curiosity years ago in chemistry texts have been refined and developed into new, aesthetic and functional commercial products with the advent of silicone emulsifier technology. Emulsion types utilized include nonaqueous oil-in-polyol and polyol-in-oil emulsions, some of which are transparent or translucent in appearance. Commercial products include antiperspirant gels, makeup formulations, skin treatment serums in which ascorbic acid has been stabilized and a range of other topical treatment products. With further refinements and advances in the availability of innovative silicone emulsifiers, silicone polymers and acceptable volatile silicone and nonsilicone additives, the horizon for the development of functional cosmetics and topical delivery systems is bright.

References

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1. JD McMahon, RD Hamill and RV Petersen, Emulsifying effects of several ionic surfactants on a nonaqueous immiscible system, J Pharm Sci 52 1163–1168 (1963)
   
2. RV Peterson and RD Hamill, J Soc Cos Chem 19 627–640 (1968)
   
3. RD Hamill and V Petersen, Effect of surfactant concentration on the interfacial viscosity of a nonaqueous system, J Pharm Sci 55 1274– 1277 (1966a)
   
4. RD Hamill and V Petersen, Effects of ageing and surfactant concentration on the rheology and droplet size distribution of a nonaqueous emulsion, J Pharm ScI 55 1269–1277 (1966b)
   
5. RD Hamill, FA Olson and RV Petersen, Some interfacial properties of a nonaqueous emulsion, J Pharm Sci 54 537–540 (1965)
   
6. KW Reichmann and RV Petersen, Temperature studies with nonaqueous emulsions, J Pharm Sci 62 1850–1856 (1973)
   
7. A Imhof and DJ Pine, Stability of nonaqueous emulsions, J Colloid Interface Sci 192 368–374 (1997)
   
8. A Imhof and DJ Pine, Ordered macroporous materials by emulsion templating, Nature 389 948–951 (1997)
   
9. U.S. Pat 5,110,606, Nonaqueous microemulsions for drug delivery, RP Geyer and V Tuliani, assigned to Athena Neurosciences Inc (May 5, 1992)
   
10. V Jaitley, T Sakthivel, G Magee and AT Florence, Formulation factors in the design of oil in oil emulsions, J Drug Del Sci Tech 14 113–117 (2004)
    
11. U.S. Pat 6,271,295, Emulsions of silicones with nonaqueous hydroxylic solvents, Powell et al, assigned to General Electric Co (Aug 7, 2001)
    
12. Silicone surfactants: Emulsification, in ed RM Hill, Silicone Surfactants (Surfactant Science Series, vol. 86), Marcel Dekker Inc, New York, ch 8 (1999) pp 209–239
    
13. G Dahms and A Zombeck, New formulation possibilities offered by silicone copolyols, Cosm & Toil 110(3) 91–95 (1995)
    
14. J Newton, C Stoller and M Starch, Silicone technology offers novel methods for delivering active ingredients, Dow Corning Corp, available at dowcorning.com/content/publishedlit/27-1153-01.pdf (Accessed Feb 26, 2013)
    
15. U.S. Pat 20090035243, Anhydrous cosmetic compositions containing resveratrol derivatives, A Czarnota et al (Feb 5, 2009)