Why does colored soap have white bubbles?
Why does soap make foam?
To understand why soap forms foam, it helps to imagine a glass of water. If you stir with a spoon, air bubbles form in the water, which rise to the surface and burst there. If you now add soap, most of the rising air bubbles remain surrounded by the soapy water, and a layer of soap foam is formed.
So-called surfactants in the soap are responsible for this, without which our hands or dishes would not get clean. Surfactant molecules are created in soap production when oils or fats are mixed with a lye. They consist of both a fat-attracting and a water-attracting part of the molecule: a chain of hydrocarbons binds organic residues such as fat and dust, and a water-soluble end piece enables the dirt to be rinsed off in water.
Without soap, the surface of the water in the glass is stretched like an extended piece of cling film. In soapy water, however, the surfactant molecules push themselves between the water molecules by stretching their fat-attracting chains out of the water, while the water-loving heads remain in the water. This reduces the surface tension. If a soap bubble rises to the edge of the liquid, it does not burst, but remains trapped in the soap solution. Over time, the soapy water sinks to the bottom so that the air can escape and the foam slowly disappears.
Foam whisk: natural soap, detergent and Co.
For a handcrafted natural soap, producers, whether professionals or laypersons, usually use vegetable oils and fats and cook them with caustic soda, a chemical that is contained in pipe cleaners, for example. This process is called soap boiling. In natural soaps that are particularly gentle on the skin, there is an excess of fats that do not react with the caustic soda to form a soap surfactant. How much a natural soap foams depends on which fats are used. Coconut oil and castor oil, for example, produce a particularly large amount of foam.
Liquid soaps and shampoos that do not contain traditional soap surfactants, but rather surfactants made from synthetically produced fatty acids, so-called detergent substances, foam even more. In the list of ingredients you can recognize them by the names "sodium laureth sulfate" (in German: sodium lauryl ether sulfate) or "ammonium lauryl sulfate" (ammonium lauryl sulfate). Nowadays, detergents for textiles contain almost exclusively active detergents, since soap surfactants in hard water form fluffy white calcium and magnesium salts - the "lime soap" - and prevent the formation of foam. They also create a basic environment that can damage the textile fibers. However, detergent formulations often contain a small amount of soap so that it does not foam too much.
Why is lather white?
Whether natural soap, detergent or shower gel - the foam is always white, even if the product itself is colored. Colors come about when the incident light is either completely or partially absorbed (absorbed) or deflected (scattered) by an object. Each color corresponds to a certain wavelength of light. The sun emits light with all wavelengths and therefore appears white. When sunlight hits the many differently sized soap bubbles in a layer of foam, hardly any of it is absorbed. Instead, most of the light is scattered in all possible directions on the curved surfaces of the bubbles. The deflected light rays hit the surrounding bubbles and are further scattered. Almost all wavelengths are thrown back, and our eyes perceive the color of the soap foam as white. Even the pink bath foam gives us mountains of white foam in the tub.
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