JOURNAL OF COSMETIC SCIENCE 106 CONCLUSION AND PERSPECTIVES The focus of this article is to summarize the state-of-the-art understanding of surfac- tants’ penetration into the skin, which have been studied by many researchers for decades. Nevertheless, an explicit surfactant penetration model still could not be given so far. It is likely that different penetration hypotheses play a role simultaneously, and a combi- nation of all the mechanisms enables surfactant penetration into the skin. With respect to mild cleansing, the addition of nonionic/amphoteric surfactants, hydrophilic polymers, or humectants such as glycerol can minimize the skin penetration of anionic surfactants, reducing the occurrence of skin irritation. On the other hand, a complete prevention of surfactant penetration into the skin is diffi cult and challenging, implying that surfactants can be used as penetration enhancers for transepidermal-active delivery. ACKNOWLEDGMENTS The author is grateful for the opportunities provided by the College of Pharmacy, Shenyang Pharmaceutical University, and the ViaX Online Education. REFERENCES (1 ) K. Holmberg, B. Jönsson, B. Kronberg, and B. Lindaman, “Introduction of surfactants,” in Surfactants and Polymers in Aqueous Solution, 2nd Ed., John Wiley & Sons Ltd., (2002), pp. 1–23. (2) A. Mehling, M. Kleber, and H. Hensen, Comparative studies on the ocular and dermal irritation poten- tial of surfactants. Food Chem. Toxicol., 45, 747–758 (2007). (3) Y. Nakama, “Sufactants,” in Cosmetic Science and Technology, 1st Ed., Elsevier Ltd., (2017), pp. 231–224. (4 ) A. Seweryn, Interactions between surfactants and the skin – theory and practice, Adv. Colloid Interf. Sci., 256, 242–255 (2018). ( 5) L. Zhang, X. Zhang, P. Zhang, Z. Zhang, S. Liu, and B. Han, Effi cient emulsifying properties of glyc- erol-based surfactant, Colloids Surf. A, 553, 225–229 (2018). (6 ) P. López-Mahía, S. Muniategui, D. Prada-Rodríguez, and M. C. Prieto-Blanco, “Surfactants and deter- gents,” in Encyclopedia of Analytical Science, 2nd Ed. Elsevier Ltd., (2005), pp. 554–561. (7) D. Bajpai, A. Mishra, J. Clark, T. Farmer, Synthesis, chemistry, physicochemical properties and indus- trial applications of amino acid surfactants: a review, Compt. Rendus Chem., 21, 112–130 (2018). (8) Y . Yu, J. Zhao, and A. E. Bayly, Development of surfactants and builders in detergent formulations. Chin. J. Chem. Eng., 16(4), 517–527 (2008). (9) J . Steber, “The ecotoxicity of cleaning product ingredients.” in Handbook for Cleaning/Decontamination of Surfaces., 1st Ed., Elsevier B.V, (2007), Vol. 2, pp. 721–746. (10) D . Zhao and Y. Wan, The synthesis of mesoporous molecular sieves. Stud. Surf. Sci. Catal., 168, 241– 300 (2007). (11) M . Teresa Garcia, E. Campos, A. Marsal, and I. Ribosa, Fate and effects of amphoteric surfactants in the aquatic environment, Environ. Int., 34, 1001–1005 (2008). Table VI Skin Aqueous Pore Radius and Normalized Pore Number Density Resulted by Various Solutions Solution Average pore radius, rpore (A) Normalized pore number density, (ε/τ)normal SDS 33 ± 5 7 ± 1 SDS with 10% glycerol 20 ± 5 3 ± 1 PBS control 20 ± 3 1 10% glycerol 11 ± 4 0.5 ± 0.1 See references 68., 69., 70., 71., 72., 73.

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