Why does foaming Hand Soapfoam when pressed?

In life, laundry detergent, shower gel, and shampoo are all items we need for daily cleaning. They come in different types. Some can produce rich foam by rubbing your hands together, while others are non-foaming. People also have different preferences when it comes to choosing between foam-free and foam-free types. For the majority of foam lovers, rich and delicate foam is synonymous with "clean washing". But did you know that there are many scientific secrets hidden behind the little bubbles?

How do bubbles arise?

Foam is by definition a dispersion system formed by insoluble gas dispersed in a liquid or molten solid. Bubbles are ubiquitous in daily life. For example: if you shake a mineral water bottle, fleeting bubbles may appear on the water; if you add some soapy water to the bottle, you will find that compared to the "water bubbles" that burst quickly before, the bubbles filled with soapy water will appear. The bubbles produced in the bottle last longer and are not easy to burst. The same is true for many people's childhood fun of "blowing bubbles".

The reason why these two kinds of bubbles are different in "lifespan" is that they are different manifestations of "drainage effect". We all know that due to gravity, water will flow downwards, so the water in the outer layer of the foam will naturally flow downwards. However, because some substances present in soapy water can resist the drainage effect, the soapy water foam can last for a long time. exist.

The mysterious substance in soapy water that can resist the drainage effect is the famous surfactant, which has a well-known name - industrial MSG. It has a rich variety, with more than 10,000 species. It is found in insecticides, pesticides, laundry detergents, soaps, and shampoos. Therefore, someone once said: "If there were no surfactants, 90% of chemical workers would be unemployed."

In terms of composition, surfactant molecules have a typical two-end structure: one end is a hydrophilic and oil-repellent group, which is easily soluble in water, and is called a hydrophilic group; the other end is lipophilic and hydrophobic. The group is insoluble in water but soluble in oil, and is called a hydrophobic group. This two-end structure gives the surfactant a cleaning function. The hydrophilic groups of surfactants interact more strongly with water, forming a hydrophilic outer layer, while the hydrophobic groups repel water, so the surfactant molecules are oriented in the water.

The beauty of this arrangement is that it weakens the attraction between water molecules and reduces the surface tension of water. To put it simply, this can not only make the clothes wet quickly and fully, but also promote the dispersion and emulsification of stains and fall off the surface of the clothes.

When the surface of the water is filled with surfactants, the surface tension of the water is reduced to a minimum, and other surfactant molecules enter the solution to form micelles. Micelles are countless surfactant molecules arranged in an orderly manner, in a spherical or rod-shaped aggregation, with the hydrophilic groups facing out and the lipophilic groups facing in.

In the process of bubble formation, surfactants act as foaming agents and foam stabilizers. When the foaming agent is added to the aqueous solution, its hydrophilic group and water molecules strongly attract each other and move towards the water layer; while the hydrophobic group is in the air or inside the bubbles and is directionally arranged on the interface layer or surface, thereby reducing the interfacial tension or surface tension. The foaming agent is conducive to the formation of small bubbles with even distribution and moderate pore size, and ensures that the bubbles rise to form a foam layer.