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| Newsletter | Cosmetic emulsions: stabilizing mechanisms with the help of modern analytical and physico-chemical measurements | Achim Ansmann |
Abstract
Principally, cosmetic emulsions follow the classical rules of stability which are driven by Stoke's law and the theories of attractive and repulsive forces. Modern analytical and physico-chemical tools allow new insights into the characterization and optimization of emulsions. By adopting interfacial tension measurements the most beneficial emulsifier mixtures and manufacturing process may be found. Process developments of high pressure emulsification, cold manufacture or phase inversion technology enlarge the portfolio of manufacturing stable emulsions. In addition, new emulsifier-free, polymer stabilized systems and Pickering emulsions for pigmented emulsions are discussed.
Introduction
Several theories of attractive and repulsive forces give insight into stability during formulation development. The sedimentation stability of cosmetic emulsions follows Stokes' law which has been set up in 1851. It explains the influence of viscosity n of the continuous phase, of droplet size rp and of difference in densities between continuous and dispersed phase on the velocity omega of sedimentation of the dispersed droplets. Basically, high viscosity, small droplets and little density difference increase stability.
As the droplet size is very important and often the only variable in formulation development, it is necessary to measure it and understand its variation by changes in the manufacturing process. During the last years modern analytical and physico-chemical methods have been developed and allow a perfect investigation of cosmetic emulsions. Methods of particle size evaluations, interfacial tension and rheological measurements as well as calorimetric methods are extremely valuable in understanding the mechanisms in droplet and lamellar gel formation and need to be used in development and production processes of emulsions.
Droplet Size of Cosmetic Emulsions
Several possibilities are available to measure the droplet size in cosmetic emulsions. Microscopic inspection especially polarization microscopy enables the observation of lamellar gels and their stability in cosmetic emulsions. Because of anisotropy (1) these lamellar gels exhibit Maltese Cross like structures close to the individual droplets (Figure 10). In scientific investigations, laser diffraction measurements of o/w systems are well established and give information on droplet size and distribution of droplet sizes. These data may explain phenomena like Oswald ripening and predict emulsion stability. Figure 1 shows an ideal monomodal peak of very fine droplets, i.e. about 100 nm (2).
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The new electron holotomographic method exposes the specimen to accelerated electrons. Fresnel diffraction, i.e. interference between non-affected and diffracted components of the beam, produces an in-line hologram like impression. Thus, the technique of holotomography can create three-dimensional reconstructions from two-dimensional projection images. As an example, this method offers three-dimensional views into gel networks which have entrapped ingredients, e.g. emollients (Figure 2). Because of its straight forward development, the gels can be used as building blocks for light and elegant formulations (3).
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The final sensorial performance of emulsions mainly follows its gel rheology (thixotropy) and the composition of emollients according to spreading cascade (Figure 3). This principle (4, 5) helps to achieve elegant and aesthetic smoothness in cosmetic emulsions by balancing the gap between highly spreading emollients (green) and slowly spreading ones (red) and introducing medium rich emollients (blue).
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View on Interfacial Tension
The creation of fine droplets is mainly driven by reduced interfacial tension between continuous and dispersed phase and mechanical energy input. The interfacial tension is strongly related to the emulsifier system. When comparing w/o and o/w emulsifiers, the magnitude of reduction of interfacial tension of o/w emulsifiers is higher and usually correlates inversely to the temperature (Figure 4). In o/w systems the introduction of mechanical energy at higher temperature is favorable.
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W/O emulsifiers exhibit an opposite correlation behavior of interfacial tension to temperature (Figure 5). Basically, no inverse relation as with o/w emulsifiers could be recognized. Thus, for a most efficient use of homogenizers the medium range of temperatures (50-60° C) is optimal in w/o emulsions (6,7). It is the region of lowest interfacial tension.
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In so-called phase inverted emulsions the interfacial tension is extremely low at the inversion temperature (Figure 6) and is related to the existence of micro emulsion phase at the temperature of inversion from o/w to w/o system (Figure 7). This outstanding behavior (8) results in very small droplets with only a little input of mechanical energy.
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View on Calorimetric Behavior of Emulsions
Lamellar and liquid crystals as building principle of emulsions have been established by numerous scientists (9-11). Kinetic calorimetric investigations not only give information about the stability of liquid crystals, but also show the interaction with other ingredients. Investigations of lamellar gels based on Alkylglucoside emulsifiers and Cetearyl Alcohol (Figure 8) indicated that the stability of lamellar gels increased with the polarity of the emollient phase (5).
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The differential scanning calorimetry (DSC) diagram exhibits no change of the lamellar gel (Figure 9) before and after three months of storage in the cream with Medium Chain Triglycerides (MCT). This was confirmed by microphotographic inspection (Figure 10).
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By comparison to results with mineral oil it could be concluded that with increased polarity of emollients the stability of liquid crystals improves. This knowledge affects the stability of lamellar creams and allows to influence parameters like delivery properties for active ingredients.
It needs to be mentioned that lamellar gels are sensitive to shear which could be introduced by stirring or homogenization at low temperature during production. This sensitivity often correlates with chain length distribution and metastability of lamellar gels with regard to crystal formation at low temperature. How complex the phase diagram and its understanding could be can be seen in (Figure 11). It exhibits isotropic phases (low and high viscous) at higher GMS concentration and elevated temperature (> 70O C ), lamellar phases (double layer like "neat" or vesicular ones) between 70 - 50 ° C. At lower temperatures the meta-stable phase exists either as lamellar gel or crystals dispersed in water (12).
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View on Pickering Emulsions
The principle of Pickering emulsions (Figure 12) can be used in stabilizing emulsions which contain solid particles. Sun care formulations often contain micronised pigments of Titanium Dioxide or Zinc Oxide as sun protecting ingredients. These particles contribute to stability or instability.
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In Pickering emulsions which are stabilized by solid particles only the contact angle of the solid particle with the interface decides the type of emulsion (o/w or w/o) and the stabilization energy (Figure 13). Other factors like particle size of the solid particle, size of droplets and solid particle concentration need to be investigated for optimal stability. Coating of pigments has an important impact on contact angle (Figure 14) and the stability potential of a Pickering emulsion (13).
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Summary
In discussing the possibilities and results of modern analytical and physico-chemical measurements, new insights for characterization and optimization of cosmetic emulsions and their structure have been found. For example, interfacial tension measurements and correlation with temperature help select the most beneficial emulsifier mixtures and manufacturing process. In addition, it could be shown that new emulsifier-free, polymer stabilized emulsions are valuable and sensorially beneficial. The stabilization mechanism of Pickering emulsions, i.e. pigmented emulsions, is of importance in Sun Care emulsions, but needs very careful formulation development.
This article was presented at the CHI Conference, Gostiny Dvor, Moscow in September 2003 and published in SÖFW 5 Vol. 130 (2004) 2-7 (Title: A Modern View on Stabilisation Mechanisms of Cosmetic Emulsions)
Acknowledgement
Part of the presentation at the CHI Conference in Moscow was provided by the courtesy of Prof. R. Daniels, Braunschweig. Other investigations have been contributed by Marc Beuché, Cognis France, Rolf Kawa and Björn Klotz, Cognis Germany.
Literature
1. M. Weuthen, Fat Sci. Technol. 97, 209-211 (1995)
2. M. Beuche, pers. communication
3. Cosmedia SP, Cognis GmbH & Co. KG
4. U. Zeidler, Über das Spreiten von Lipiden auf der Haut, Fette,Seifen, Anstrichm., 87, 403-408 (1985)
5. A. Ansmann, R. Kawa, E. Prat, A. Wadle; Modern Cosmetic Emulsion - Technology and Assessment, SÖFW 120, 158-161 (1994)
6. R. Kawa, pers. communication
7. Th. Förster, B. Guckenbiehl, A. Ansmann, H. Hensen; Neuartige Körperpflegemittel auf Basis von Mikroemulsionen mit Alkylpolyglycosiden, SÖFW 122, 746-753, Vol. 11 (1996)
8. T. Foerster et al., J. Dispersion Sci. Techn. 13, 183-193 (1992)
9. S. Friberg, K. Larsson, "Liquid Crystals" Vol. 2, 173, Acad. Press (1976)
10. H. Junginger et al., J. Soc. Cosmet. Chem. 35, 45-57 (1984)
11. M. Nielsen et B. Drews, SÖFW 127, 8-12 (2001)
12. G. Schuster, Parf. Kosmetik 58, 353-363 (1977)
13. R. Daniels, pers. communication
Author
Dr. Achim Ansmann obtained his doctorate in Organic Chemistry. In 1975 he assumed a position in Basic Research at Henkel KGaA and was responsible for product development Skin Care at Henkel-Schwarzkopf and later for the development of new raw materials in Skin Care. In 1999 he assumed responsibility at Cognis Deutschland GmbH & Co. KG as head of Technical Service and Application for Skin Care and has been Director of Personal Care Technology North Europe since 2003. He has been a long-time member of the German Society of Cosmetic Chemists (DGK), Experts Group Skin Care and organizes symposia and advanced trainings for the DGK.
Author's Address:
Dr Achim Ansmann
Cognis Deutschland GmbH & Co. KG
40551 Duesseldorf
Germany
Email: Achim.Ansmann@cognis.com
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