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High Skin Cleansing and Skin-mild Properties Both Achieved by Use of Surfactants

Interface Science

The basic performance requirements for skin cleansing are "high cleansing power of sebum stains" and "mildness to skin". Surfactants, the main components of detergents, are easily compatible with oils, thus they adsorb to sebum stains and change them to a state where they can be easily removed. However, since adsorption to skin also occurs, a surfactant with high cleansing performance may reduce moisture and cause irritation. On the other hand, poor cleansing performance requires rubbing with hands or a towel to remove sebum stains, which also results in irritation to the skin.
Until now, it has been considered difficult to achieve both "high cleansing power" and "mildness to skin" in the same product. However, Kao has discovered that the surfactant "alkyl ether carboxylate (EC)" can achieve both at a high level. EC demonstrates a special intermolecular interaction with liquid fatty acid, the main component of sebum stains, and converts that into a surfactant, allowing the stain to be removed by only a flow of water. This provides a high level of cleansing power that allows removal of sebum stains without rubbing the skin with hands or a towel (Figure). In addition, since EC has a strong interaction with sebum, it tends to adsorb more to sebum stains than to skin. Due to this property, it has a milder effect on skin than other surfactants. EC is applied as the main base of the facial cleanser "Biore" and the body cleanser "Biore u", and supports the Kao skin care philosophy of "cleaning without causing strain to skin".

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Figure. Cleaning experiment with aqueous surfactant solution for sebum stains. A certain amount of model skin oil stain colored with carbon black was applied to the upper arm and the cleansing effect was simply observed by slowly pouring an aqueous surfactant solution from above.
(a) Pre-experimental state. (b) After EC aqueous solution activity. (c) After conventional surfactant aqueous solution activity.
EC allows dirt to be removed just by flushing, while dirt is not removed with conventional surfactants unless the skin is rubbed.

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