Why can soap and washing powder wash clothes?

Cleaning is the process of removing stains from solid surfaces. The washing effect is mainly based on the comprehensive effect of surfactants to reduce the interfacial surface tension. After the dirt is soaked in detergent (surfactant) solution for a certain period of time, the surfactant obviously reduces the surface tension of water, so the oil stain is easily wetted. When the surfactant is wetted by water, it permeates the dirt surface, so that the dirt and the components in the detergent solution are dissolved. After friction, washing, stirring and other mechanical actions, dirt will be emulsified, dispersed and dissolved in the washing liquid, and part of it will float to the surface with the generated foam, and then it will be washed repeatedly with clean water to achieve the purpose of decontamination.

Not only soap, but also the decontamination of any detergent is due to the following functions:

(1) Wetting can make textiles or other solid substances wet by water. For this reason, any detergent must be a surfactant, which can reduce the surface tension of water and make water molecules easily penetrate into fabric tissues.

(2) After being surrounded by soap or detergent molecules, emulsified dirt is easily dispersed into liquid to form emulsion. So detergent is also needed to reduce the surface tension of water.

(3) Dispersion or peptization This requires detergent molecules to be dispersants, which turn oil stains and dirt into dispersed colloidal solutions, mainly relying on the adsorption attraction of the electric double layer formed by detergent and water.

(4) Protective effect protects colloidal particles from aggregation.

(5) The ability to carry dirt to prevent it from redepositing on the fabric.

(6) Foaming power is a common phenomenon in washing process, but it is not the main property of surfactant in detergent. Foaming is determined by the directional adsorption of detergent molecules on the interface, which is closely related to the decrease of surface tension and indirectly increases the dirt carrying capacity of detergent.