60 PRODUCT PROTECTION O/W EMULSION A
4.0 3.0 2.0 1.0 0.0
O/W EMULSION B 3.0 1.4 0.6 Vehicle
0.5% dermofeel Toco 70 non GMO + 0.2% dermofeel AP MB
0.5% dermofeel AP MB
4.0 3.0 2.0 1.0 0.0
3.1 2.5 1.0
Vehicle
0.5% dermofeel Toco 70 non GMO + 0.2% dermofeel AP MB
0.5% dermofeel AP MB
Figure 3: Oxidation stability values of raw material DF AP MB alone and in combination with DF Toco 70 non GMO in two model O/W Emulsions with DF GSC SG (O/W Emulsion A) or distearyldimonium chloride (O/W Emulsion B) as emulsifiers. DF AP as a single substance in O/W Emulsion B provides reliable product protection. In this case it is even slightly more effective than in combination with DF Toco 70 non GMO. If DF AP MB is used in O/W Emulsion A, there is a significant deterioration in oxidation stability. Here, ascorbyl palmitate shows a prooxidative effect and therefore cannot be used as a single substance as a product-protecting antioxidant. The addition of DF Toco 70 non GMO significantly stabilizes the oxidation stability of O/W Emulsion A and detrimental effects caused by single DF AP are fully compensated without any further loss in product protection efficacy
in the composition of their interphase (referred to as ‘O/W Emulsion A’ and ‘O/W Emulsion B’), we have shown that isomers of tocopherols (a/β/γ/δ-tocopherols differ slightly in their molecular configuration) behave differently in the presence of positively or negatively- charged interphases. The interphase of O/W Emulsion A is
mainly composed of Dermofeel® (hereafter known as DF) GSC SG (INCI: Glyceryl Stearate Citrate), a negatively-charged emulsifier from the group of natural glyceryl esters.15
The
interphase of O/W Emulsion B consists mainly of distearyldimonium chloride, a cationic emulsifier from the group of quaternary ammonium salts. With the help of our newly established
oxidation measurement method, it could be shown that DF Toco 70 non GMO (INCI: Tocopherol; Helianthus Annuus (Sunflower) Seed Oil) with its relatively high content of β/γ/δ- tocopherols provides reliable product protection in all emulsifier systems investigated.16 In particular, in combination with the
negatively-charged interphase of O/W Emulsion A, the oxidative stability value is three times higher than the vehicle formulation without antioxidant and two times higher than the sample with DF TocoBalance (INCI: Tocopherol; Helianthus Annuus (Sunflower) Seed Oil) (Figure 2; results for O/W Emulsion A).17 DF TocoBalance is derived from
rapeseed and provides a balanced blend of product-protecting β/γ/δ-tocopherols and a-tocopherol isomers known for their antioxidant skin protection. Interestingly, the antioxidant efficacy of the
tocopherols used is reversed when combined with a positively-charged interphase (Figure 2, results for O/W Emulsion B). As before, both tocopherol products were able to efficiently stabilize the systems. However, in this case, the tocopherol with the higher a-tocopherol content (DF TocoBalance) was the antioxidant of choice, with an oxidative stability value
PERSONAL CARE May 2023
approximately five times higher than that of the corresponding vehicle formulation (Figure 2, results for O/W Emulsion B). A comparison of the two vehicle
formulations, to which no antioxidant was added, reveals that the system with the negatively-charged emulsifier (O/W Emulsion A) exhibits 1.4 times higher oxidation stability than the system with the positively-charged emulsifier (Figure 2). Thus, our results show that the charge
of an emulsifier has a decisive influence on the oxidation stability of a cosmetic emulsion and that a cationic emulsifier, in contrast to an anionic emulsifier, adds additional oxidative stress to the formulation. A possible explanation for our observation comes from Kancheva and Kasaikina, who found that a cationic emulsifier promotes the decomposition of hydroperoxides and thus increases the level of oxidative stress.18 Our observation that the tocopherol
product with a higher a-tocopherol content (DF TocoBalance) performed particularly well in a system with a cationic interphase is supported by the literature. Yoshida et al demonstrated that the a-tocopherol consumption induced by CuCl2
was faster in
micelles stabilized with an anionic emulsifier than in micelles stabilized with a cationic emulsifier. Moreover, the antioxidant or prooxidant
properties of a-tocopherol in O/W Emulsions are influenced by the level of oxidative stress. At high levels, such as observed in our O/W Emulsion B (Figure 2, vehicle), a-tocopherol tends to act as an antioxidant, whereas at low oxidative stress in the O/W Emulsion A it acts more as a prooxidant.18
This makes the a-tocopherol-
rich DF TocoBalance, among others, a perfect antioxidant for cationic systems.18,19,20
What factors promote the efficacy of the co-antioxidant ascorbyl palmitate in cosmetic formulations? In order to exploit the full potential of tocopherols and especially a-tocopherols in
a system with a low level of oxidative stress, it is useful to add certain co-antioxidants. A prominent example is ascorbic acid and its derivatives.21 As effective as they are for the skin and
against oxidative deterioration, the reactivity of the raw materials is by no means trivial and therefore poses a challenge to the formulator. This is confirmed by our results for DF AP MB (INCI: Ascorbyl Palmitate).22 In contrast to a system with a negatively-
charged interphase (Figure 3, O/W Emulsion A), ascorbyl palmitate in a system with a positively-charged interphase (Figure 3, O/W Emulsion B) shows reliable product protection properties. This combination is especially attractive because it unites skin protection and product protection properties. However, if the formulator plans
to use ascorbyl palmitate in a system with a negatively-charged interphase, the combination with tocopherols is recommended (Figure 3, O/W Emulsion A). The results obtained so far demonstrate
that in certain systems, both tocopherols and ascorbyl palmitate alone have a positive influence on oxidative product protection, and that their effects can be combined to obtain a maximum beneficial impact or to completely compensate for the negative effect of one component (Figure 3). However, for a true synergistic effect, the antioxidant must interact positively with another ingredient in the system. The amount of O2
consumed is directly
related to the progression of oxidation. It is influenced by the overall system and the ingredients or reactive components it contains. Tocopherols, especially atocopherols, cause this steady consumption of O2
to accelerate
over time. In the antioxidant reaction with fatty acids, tocopherols themselves have the potential to react as radicals, actively promoting oxidation. Figure 4 shows that the average weekly
oxygen levels in the presence of tocopherols decreased steadily over time. This observation indicates a continuous increase in prooxidative
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Relative oxidation stability
Relative oxidation stability
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