4.2.2 Classification and selection of skin cleanse...
1.Classification of skin cleansers
Particles of dust，metal or non-metal oxides deposited on the skin can be removed by water.However，oil，pus，or dirt in a film form are tightly bound to the skin by electrostatic or even chemical interactions，and often require the use of cleansers to remove them.Through wetting，permeating，emulsifying and dispersing，the cleansers make the dirt escape from the skin and enter the water.After fully emulsifying and solubilizing，the dirt is dispersed in the water，and then it can be removed by repeated rinsing with clean water.The ideal skin cleansers should be harmless to the skin and can keep the surface of the skin clean and moist.According to its chemical properties，skin cleansers are often divided into soap cleanser and synthetic cleanser.
（1）Soap cleanser：It consists of fat，grease and salt，which exerts the cleaning effect by forming soap salts to emulsify the dirt on the skin surface.Because the soap salt component is alkaline（pH 9.0-10.0），it has strong detergency and the sebum film of the skin can be easily removed，which may cause skin irritation by increasing the skin pH and reducing skin tolerance.Modified soaps with added moisturizing ingredients and handmade soaps containing glycerin are gentle in nature and have low skin irritation.
（2）Synthetic cleanser：It is a non-saponified artificial surfactant with some added ingredients including moisturizers，adhesives，and preservatives.According to the chemical characteristics of surfactants，they can be divided into anionic surfactant，cationic surfactant，zwitterionic surfactant，nonionic surfactant，silicone surfactant，etc.，among which the anionic surfactant has the strongest cleaning ability and is also more irritating to the skin.Synthetic cleanser can clean the skin by emulsification and wrapping effect of the surfactants.Moisturizers and emollients added in the formula can form a thin moisturizing film on the surface of the skin to reduce skin sensitivity and alleviate the skin barrier damage caused by surfactants.Transmission electron microscopy investigation shows that the skin lipid and protein structures are superbly protected after the use of synthetic cleansers.Compared with soap cleansers，synthetic cleansers are more gentle in nature and significantly less irritating to the skin.In order to further improve the moderate extractability and low temperature stability of the cleansers，water-soluble polymer components are often added to the cleansers，including acrylate-（methyl）alkyl acrylate copolymer，hydroxyethyl（propyl）cellulose and others.These water-soluble polymer components can be used alone or in combination of two or more.
2.Selection of skin cleansers(https://www.daowen.com)
The selection of cleansers and the frequency of use depend on the skin type，different part of skin and the seasons to use.For example，for oily skin，the amount of cleansers and the frequency of cleaning can be adjusted according to the amount of sebum to make the skin neither greasy nor dry.Oily skin is recommended to choose mild nature oil-control cleansers to avoid excessive cleaning.Excessive cleaning may damage the sebum film and increase the rate of transdermal water loss，which will then stimulate the skin sebaceous glands to secrete more sebum by negative feedback mechanism and cause“oil outside while dry inside”phenomenon.However，those with dry skin should avoid the use of soap-based cleansers.The further increase in transdermal water loss and in skin pH caused by alkalinity of the soap salt will lead to greater damage to barrier function of skin.Skin cleaning for those with sensitive skin only requires bathing with clean water or using very mild cleansers.And using tap water，river water，lake water and other soft water are relatively not irritating to the
skin，so become a better choice for them.
Face should be washed once every morning and evening.The water temperature should be adjusted with seasons'change.
Bathing frequency varies with the intensity of physical activity，seasons，environment，sweating and one's personal habits.Take a bath every 2 to 3 days under normal circumstances and the frequency can be increased to once a day in hot summer or for those who love sports.The water temperature is based on the skin temperature，which can be lower than the body temperature in summer and slightly higher than the body temperature in winter，generally 35-38℃.Excessive washing will reduce the sebum film content，which means less protection and moisturization of the skin，and the skin aging will be promoted.