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T. Herrling, M. Seifert, K. Jung*

Texture and functional analysis of formulations with Electron Spin Resonance (ESR)

formation of lipid peroxyl radicals which can reduce the PCA signal.

Electron Spin ResonanceElectron Spin Resonance (8) is a method designed for the investigation of free radicals and its properties. Electron spin resonance (ESR) is a widely used basic and materials science investigation method. It gathers information about paramagnetic center, e.g. in molecular or materials sys-tems. There are a number of free radicals called spin labels which can attach them-selves to particular sites in polymer sys-tems and produce spectra which provide information on changes in the chemical and physical characteristics in the neigh-borhood of the site. Cosmetic formulations are designated by their micro texture. Micro texture is the collective term for a material‘s crystallographic parameters and other aspects of microstructure: such as morphology, including size and shape distributions, chemical composition and crystal orientation and relationships[1] . At first, the spin label PCA is brought in the region of interest. Secondly, the sample is stressed by UVAB radiation. The generated lipid radicals reduce the PCA-signal. We register the response of the sample to the reduced PCA signal as a function of irra-diation time for a given irradiance.

n Results

The following results can be registered:

1. Micro texture a) Structure of formulation character-

ized by the LWF (Lipid/Water factor = k-factor)

Together with a UVAB irradiation we

the esr is an analytical method used for material science. together with the spin probe (tempo, pca, ...) and various irradiation times, a time dependent signal is registered which gives information about

structure and functionality of micro emulsion. the creation of a lwF (lipid-water-Factor) is the basis for these investigations. the time dependent behaviour of the curve (k-factor) gives information about the stability and functionality of a formulation. the lwF is typical for every formulation. the lwF=0 is typical for a pure water formulation. the stability of a cream is marked by a high lwF. the influence of antioxidants on the formulation lwF is marked by decreasing lwF. Furthermore, the effect of filter sub-stances leads to a decrease of the lwF, so that the customer gets informa-tion about a better release and distribution of formulation.

n■ Introduction

In the Cosmetics and Personal Care in-dustries, it is essential to understand the relationship between particle properties and functionality to provide the optimum balance in terms of formulation, con-sumer acceptance and application per-formance. The physical properties of the dispersed particles, such as the average particle size, the size distribution, the zeta potential or charge on the particles and their functionality help to characterize a chemical/cosmetic formulation (1–7) .Cosmetic formulations consist mainly of a mixture of water and lipids (poly unsat-urated fatty acids). Both parts generate free radicals under UV radiation. In water hydroxyli(OH)-radicals are generated by radiolysis. The OH-radicals have a short life time of 0,3 ns which is correlated with a half-life way of 1,8nm. To scav-

enge these radicals, a molar concentra-tion of spin trap would be necessary. On the other side all other radicals like lipid alkyl radicals (L., half-life of 10 ns, half-life way of 60 nm), Lipid alkoxyl radicals LO.(1µs, 6 µm), Lipid peroxyl radicals LOO.(1-10s) would be scavenged with an obvious smaller trap concentration. The application of spin probes as radical scavenger needs radicals with a longer life time like Lipid peroxyl radicals. The life time of hydroxyl radicals is too short to scavenge nitroxides like PCA. This is very impressively shown in Fig. 1 by the lipid-water-factor (LWF). For pure water the LWF = 0, whereas for pure lipids the LWF → 1. OH radicals from the radiolysis of water cannot be seen by the reduction of PCA and result in a LWF → 0. The LWF gives also information about the rheology of such a lipid/water formulation like it is shown in Fig. 2. It is decisively seen by the

abstract

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corresponds to a half-life of 1,8nm. It is very hard for a OH.-radical to scavenge a PCA-molecule considering a concen-tration of 0,01 mmol. This is because no signal can be detected in the water part (Fig. 1). The ratio between lipophilic and hydrophilic parts determines the k-factor.Radicals were detected in the lipid phase of a formulation like a cream. In pure water, no radicals can be detected. In cellulose gel the signal comes from the glucose-molecule. No OH. are scavenged because of the short life time. In comparison with known samples, results can be analysed.

b) Different lipid content

Different lipid contents of a formulation can be distinctly seen by this new LWF, as shown in Fig. 2. The LWF changes from LWF = 0,015 for a milk up to LWF=0,063 for a cream. The LWF for a lipid rich cream is very much greater than for milk or lotion. 2. Stabilitya) Rancidity

Rancidity denotes the oxidation of lipids, the generation of lipid-peroxyl radicals (LOO.). If creams are oxidised, no more oxidation is possible. That’s why the LWF is very small. The process is finished and free radicals cannot be registered. For a normal cream, the full radical signal is measured (Fig. 3).

b) Antioxidants

Antioxidants prevent the generation of lipid peroxyl radicals Antioxidants reduce the possibility of radical generation (generation of lipid peroxyl radicals – LOO.). Antioxidants scavenge the free radicals (lipid peroxyl or hydroxyl radicals) so that they can-not scavenge PCA. We measure a signal curve of a normal cream compared to the rancid sample (Fig. 4). 3. SunscreenThe description of sunscreens by free rad-icals is shown in Fig. 5. If the base is irradi-ated lipid, radicals are generated and the PCA-signal is reduced by this amount. By the application of UV-filter the irradiance is minimized and so the LWF is decreased. The base is the origin of generated free

measure the time dependent reduction of the reporter molecule as result of radical increase. According to the sur-roundings and micro texture we find different radical movement behaviour which is typical for this formulation. The aim of the investigation is the mag-netic moment of test paramagnetic substance PCA which is introduced with stirring homogeneously in the cosmetic formulation. With the photon energy of UVB and UVA (6 mW UVB, 22 mW UVA), ROS (LOO., OH.) are generated. The generated ROS react with nitroxide PCA to the corresponding hydroxyl-amine which is ESR-silent. The distri-bution of lipids (lipid –peroxyl-radicals LOO.) and the nitroxide PCA describe the structure of the cosmetic formu-lation. The time dependent reduction

Fig. 1 Radical generation in more lipophilic and more hydrophilic formulations marked by the LWF (Lipid/Water Factor = rate constant »k«)

CerapartsANTIAGE

of the nitroxide PCA as a function of ir-radiation time t is a measure of micro structure and rheology of cosmetics. This value documents the lipid content (whole cosmetic formulation), the PCA-content and the interacting of influencing forces. So the rate constant k is higher more rigid formulations like creams.The structure of formulations can be dis-tinguished for their generation and life time of free radicals. This information is included in the rate constant »k« of the formulation. For that reason the sample is UV irradiated to generate free radicals. Lipids in a water/lipid emulsion like in cream are transformed to lipid-peroxyl-radicals (LOO.) with a life time of 1-10 s. For the hydrophilic part of the sample OH.-radicals were generated by radiolysis of water with a life time of 0,3 ns which

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lipid radicals. As a function of filter quality and quantity the number of generated free radicals is decreased from k = 0,0406 to k= 0,0119 corresponding to an SPF = 20. The addition of antioxidants will decrease the k-value to k = 0,0091. Also the effect of antioxidants can be clearly seen by reducing the k-value from k = 0,0119 to k = 0,0091.

n■Discussion and conclusion

The ESR is a spectroscopic method, which detects free radicals. Using PCA as a spin marker combined with time dependent UV irradiation we can get information about the micro texture, the stability (rancidity), the effect of antioxidants and UV filters. phases adopted by Especially the poly-

morphism of lipid-water systems describes types of liquid-crystalline lipids in water and factors which control phase stabil-ity. Information about the micro texture, for instance, can shed light on the release of substances. The character of the for-mulation from cream to lotions can be distinguished up to differences between the LWF (k-factors). Every cosmetic for-mulation can be measured corresponding to its properties with regard to the activi-ties which are done (aging, temperature, moisturizing, radiation, etc). ESR cannot give information about the reaction part-ner but on the reaction itself. UV irradiance generates radicals (ROS), the properties of which give information about the texture (free electrons). The rate constant (LWF) is different for the formulations. So the rate constant (LWF) is higher for rigid formula-tions. In the case of aging, the formulation is oxidised and no free radicals are gener-ated during UV. Taking the example of a rancid cream, this becomes obvious. Only a minimum of free radicals is generated. To prevent this reaction, antioxidants were used which react with the ROS and prevent the reduction of PCA. UV-filter reduces the irradiance and the number of photons which generate free radicals. Antioxidants have the same effect and reduce the num-ber of free radicals. The application of this technique enables new characterisation and insight in the functionality of the for-mulations and gives new input, such as an optimum of release and distribution of in-gredients for the texture of formulations.

Fig. 4 Reduction of the possibility of radical generation by the addition of 1% of green tea.

Fig. 3 The k-factor (LWF) for a normal and a rancid cream.

Fig. 2 Radical generation (decrease of PCA) in different lipid formulation

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Literature

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* Author´s address:GEMATRIA Test Lab GmbH,

Pestalozzistraße 5-813187 Berlin

Email@gematria-test-lab.com

n

Fig. 5 Radical generation for parts of a sunscreen base, base + UV-filter, additionally AOX

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