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Issue
39 — July 2005 |
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| Issue
39 | |  | | |
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| Category | | Title | | Author |
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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).
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).
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).
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.
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.
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.
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).
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).
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).
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.
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).
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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