The more foam, the stronger the cleaning ability? Don't be fooled by advertisements anymore!

What factors do you usually consider when buying detergent? Many detergent advertisements emphasize the foaming feature when promoting their products to illustrate that the washing effect is very good.

But does the amount of bubbles really have anything to do with how well the clothes are washed? What are the ingredients in the laundry detergents and dishwashing liquids we use for cleaning?

Too long to read

1. In the days when soap was the main washing product, foam was an important measure of washing power;

2. It is not objective to judge whether a detergent is strong or not by the amount of foam;

3. Excessive foam or long-lasting foam is not conducive to washing various utensils;

4. In addition to "foam", we also need to pay attention to inorganic salt additives, organic additives, fluorescent whitening agents and enzyme preparations. Their content is not high, but it should not be underestimated.

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Detergent "upgrade"

As early as thousands of years ago, people knew to use natural products in nature, such as honey locust, tea seed cake, safflower seed, horse millet, etc. to wash clothes. However, the sources of these natural products are limited, so they cannot be produced in large quantities.

Soap appeared at the end of the 17th century. It has the properties of wetting, emulsifying, foaming, solubilizing and suspending in water, and shows remarkable cleaning effect. But soap at that time could not be used in hard water (referring to water containing more soluble calcium and magnesium compounds), because the calcium and magnesium ions in hard water can make soap lose its washing ability, and also form calcium soap and magnesium soap on fabrics, making the fabrics yellow and gray.

After World War II, the petroleum industry provided raw materials and intermediates for detergents, promoting the rapid development of synthetic detergents.

Synthetic detergent is a compound product with washing function made by mixing synthetic surfactants and auxiliary components. It is easy to dissolve, saves time and effort to use, has strong detergency, and requires small dosage, and is gradually replacing soap.

Why can detergent remove stains?

This is related to the molecular structure of the active ingredients (mainly surfactants) contained in the detergent components.

When a liquid comes into contact with other substances, the forces acting on the surface molecules of the liquid are different from those on the internal molecules.

For example, when liquid and air come into contact, the molecules inside the liquid are uniformly attracted by the surrounding molecules; but for the molecules on the surface layer of the liquid, the attraction of the air above the molecules is smaller, so the attraction of the molecules inside the liquid to the molecules on the surface layer is greater than that of the air.

Illustration of the different gravitational forces on molecules inside and on the surface of a liquid. Image from the author

In general, the molecules in the surface layer are attracted by the interior of the liquid phase and tend to migrate into the interior of the liquid phase. Therefore, the surface of the liquid tends to shrink inward and take on a spherical shape (such as dewdrops on grass leaves).

Dew on grass leaves, image from the author

This contraction force acting vertically on the unit length along the liquid surface is called surface tension. The existence of liquid surface tension makes it difficult for the liquid to play the role of wetting, penetration, emulsification, decontamination, etc., and it is impossible to perform effective washing.

For this reason, a substance is often added to the liquid to reduce the surface tension of the liquid. This substance is a surfactant.

When the surfactant is dissolved in water, the hydrophilic part of the molecule tends to remain in the water, while the hydrophobic part tends to escape from the encirclement of water molecules and rise above the water surface.

Each surfactant molecule has this tendency, which will inevitably cause many surfactant molecules to be distributed on the surface, resulting in a significant decrease in the surface tension of the aqueous solution.

There are many types of surfactants, and their applications are extremely wide. They are often used as detergents, foaming agents, emulsifiers, dispersants, solubilizers, etc. They can also be used for sterilization, corrosion inhibition, antiseptic, fabric softening and antistatic.

The more bubbles, the stronger the ability?

Foam is a phenomenon in which air is dispersed in liquid. Through stirring and other effects, air enters the detergent solution, and the active molecules or ions of the detergent can be adsorbed on the interface between the air and the solution in a directional manner. The hydrophilic groups are surrounded by water molecules, forming a thin film on the interface, namely foam.

When the detergent molecules adsorbed on the surface of the bubbles reach a certain concentration, the bubble wall forms a solid film that is not easy to break or merge. Since the relative density of bubbles is less than that of water, the bubbles rise to the liquid surface and adsorb a layer of active molecules on the liquid surface.

Therefore, on the bubble wall that escapes from the liquid surface, the hydrophobic groups of the two layers of active substance molecules are facing the air.

Schematic diagram of foam formation, image from the author

Traditionally, in the era when soap was the main detergent, foam was an important measure of detergency. Foam has a selective adsorption effect on dirt, especially solid dirt particles, thus facilitating washing.

For example, there is a foam decontamination method that uses the principle of solid adsorption on bubbles to effectively wipe away dirt particles on the surface of the car without scratching the paint on the surface of the car. Therefore, when formulating synthetic detergents, it is often considered to adjust the foam size of the formula and add foam enhancers and foam stabilizers.

However, it is not objective to evaluate the effectiveness of a detergent based on its foaming power and foam stabilizing power. Excessive foam or foam that does not disappear for a long time is not conducive to washing various utensils.

Especially in washing machines, it is unwelcome to use detergents with more foam, and foam suppressants need to be added. Therefore, some low-foam detergents are effective in reducing or minimizing environmental pollution.

In addition to surfactants, synthetic detergents also contain some organic and inorganic detergent auxiliary ingredients according to a certain formula combination. Their content is not high, but their role should not be underestimated.

1. Inorganic salt additives can improve the suspension ability of dirt and prevent dirt from being deposited on the fabric.

2. Organic additives can remove metal ions in hard water.

3. Fluorescent brighteners only serve as optical brightening and color correction and cannot replace bleaching.

4. Enzyme preparations can protect color, remove particulate dirt, maintain whiteness and soften fabrics.

The main ingredients of laundry detergent, such as surfactants and enzyme preparations, are marked on the outer packaging. Image from the author

It can be seen that with the emergence of modern detergents, foam is gradually losing its original meaning. However, for some consumers, they still hope that detergents have rich foam, which is mainly a psychological effect, because foam makes people feel the presence of detergent.

This is similar to the misconception that thick detergents are more concentrated and thinner detergents are less concentrated.

But in fact, washing and rinsing are very complicated processes, and the relationship between foam and the detergency of detergent is not a definite relationship.

Author: Lu Wensheng, Institute of Chemistry, Chinese Academy of Sciences

References

[1] Tang Yumin, ed., Synthetic Detergents and Their Applications, May 2006, China Textile Press.

[2] Chen Zongqi, Wang Guangxin, and Xu Guiying (eds.), Colloid and Interface Chemistry, August 2001, Higher Education Press.

[3] GB/T 5327-2008, Terminology of surfactants.

[4] Jiang Long, ed., Introduction to Colloid Chemistry, January 2002, Science Press.

[5] Liu Yun, Detergents: Principles, Raw Materials, Processes and Formulas, September 1998, Chemical Industry Press.

[6] Hua Zhangxi and Xu Qing, eds., Handbook of Detergent Enzyme Application, July 1999, China Light Industry Press.

[7] GB/T 13174-2008 Determination of detergency and washing cycle performance of detergents for clothing.

This article is produced by Science Popularization China, produced by Lu Wensheng (Institute of Chemistry, Chinese Academy of Sciences), and supervised by the Computer Network Information Center of the Chinese Academy of Sciences

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