Surfactant is a very difficult concept to define for human skin, eyes, hair, especially for the skin and eyes, so far there is no unified standard. The irritation or sensitization of surfactants to mucous membrane is mainly caused by three factors:


(1) Dissolubility refers to the dissolubility of surfactants to the moisturizing components (such as NMF), intercellular lipids and free amino acids and fats in the cuticle. The excessive dissolution of these ingredients will damage the skin oil and the surface layer, reduce the skin's water holding capacity, and cause cell scurf to fall off, resulting in skin tightness, tingling or dryness. What's more, surfactants can not only peel off cells, but also dissolve cells. For example, SDS is a very effective dissolving agent of biofilm.


(2) Permeability refers to the ability of surfactants to penetrate through the skin, which is considered to be one of the causes of various skin inflammation. The infiltration of surfactants changes the original structure of the skin and the compatibility between adjacent molecules, resulting in contact dermatitis, dermatitis, skin irritation and even allergic reactions, resulting in erythema and edema on the skin. The stimulative effect of surfactants on skin and mucous membrane was the most cation, followed by anion, and the least nonionic and zwitterionic.


(3) Reactivity refers to the adsorption of proteins by surfactants, which leads to protein denaturation and changes of skin pH conditions. The results show that the reactivity of PEG is lower than that of LAS.






Two point one


Methods of evaluating moderation


There are many methods to evaluate the gentleness of surfactants, but there is no unified standard. At present, there are two kinds of general methods to evaluate the mildness: in vivo test and in vitro test. For the sake of safety and meeting the needs of animal protection movement, the in vitro test method is strongly advocated at present, but most of the legislation still takes the results of in vivo test as the test standard.






2.1.1


in vivo






(1) Draize rabbit skin test: Draize rabbit skin test is mainly conducted on rabbit skin, sometimes on the skin of rats or Dutch pigs. After coating the skin with various concentrations of samples, observe the comprehensive score of erythema and edema (see Table 4). Draize rabbit skin test can be used as an independent test or as a preparatory test for human skin test. Some results of rabbit skin test are listed in Table 5. In vivo experiments are mainly carried out on human skin, rabbit skin and rabbit eye mucous membrane. Driaize rabbit skin test and Draize rabbit eye test are two commonly used methods. Sometimes we also use the Duhring chamber test or cupshaping test, that is to say, patch test on the flexion side of the front wrist of the human body, and observe the phenomenon of erythema and edema caused by the intermittent test of surfactants on the human body. There is also a method of hand immersion, that is, hand immersion in a certain concentration of surfactant solution to simulate scrubbing action or dishwashing action, after a certain period of time to test the skin surface sebum exfoliation rate or protein solubility before and after immersion.






(2) In the Draize rabbit eye test, 0.1ml of the test solution was applied to the cornea of the white rabbit, and the stimulation results were observed at a certain interval of time and a comprehensive evaluation was made (see Table 5). It can be seen from table 6 that the tolerance of eye mucosa to surfactants is the lowest with cations (0.5% - 1.0%), followed by anions, and the highest with non-ionic surfactants.