Studying the impact of emulsifiers on SC lipids by confocal Raman microspectroscopy

Emulsifiers are widely used in face washes, shower gels, body lotions and many more cosmetic and pharmaceutic products. Some of them are suspected to show irritating effects and to harm the skin barrier function. It is debated that this might be due to either extraction of lipids from the stratum corneum and/or by disturbance of their highly ordered structure. PEG-ethers are a group of emulsifiers which are widely used in pharmaceutics and cosmetics. We thus chose to investigate this group of emulsifiers regarding their impact on SC lipids. We hypothesized that a skin irritating effect may be related to the HLB-value of the emulsifiers. We thus selected emulsifiers with different chain length of the acyl group and different numbers of PEG groups. The emulsifiers were dissolved or dispersed in water and applied to the skin ex vivo in Franz diffusion cells. Subsequently, the stratum corneum was isolated and measured by confocal Raman microspectroscopy (CRM). This method was selected as it enables simultaneous measurement of SC lipids content and organization. We found that the amount of lipids extracted from the SC correlates with the PEG-chain length of the emulsifier. A maximum lipid extracting capacity was found for PEG-40-cetyl ether. Emulsifiers with shorter as well as longer PEG-chains extracted less lipids. Further, we were able to show that lipids extraction from SC was paralleled by a disruption of the lipid conformation. CRM was found to be a powerful tool in this context as it provided answers to both research questions at the same time while being a lot faster and convenient compared to conventional analytical methods.

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