Foaming agents are surfactants that act on a liquid surface to help it form and hold the foam or improve the stability of the foam. Their function in personal care products is to mostly improve the user experience (like facilitating rich foaming in toothpaste, soap, and shampoo to enhance cleaning effectiveness and smell).
Foaming agents can be found in a multitude of products, such as:
Toothpaste: Foaming substances like Sodium Lauryl Sulfate (SLS) are usually used in toothpaste to help form dense foam and provide cleaning power.
Soap: Foaming ingredients are used in soap to lower surface tension and thus, generate more foam and clean better.
Shampoo: Foaming agents such as Sodium Lauryl Ether Sulfate (SLES) are added to shampoos to create rich and durable foam to cleanse hair and scalp.
Foaming agents can be used in personal care products not only for the functionality and user-experience, but also for marketing purposes. But to be clear, foaming agents will help the cleaning function but do not actually act in cleaning. Rather, they infer the performance of the product by increasing the user experience.
Basic Principles of Foam Formation
Foaming agents work basically by lowering the surface tension of liquids and deflating air bubbles into a stable foam structure. Here is a detailed explanation:
Surface Tension Reduction
Foaming solutions tend to be made up of surfactants, which can drastically decrease the viscosity of the fluids. This force of attraction between liquid molecules is called surface tension, and this makes the liquid surface shrink down to as small a surface area as possible. When the surfactant molecules stick to the liquid surface, they dampen the favourable interactions of the liquid molecules and thus the surface tension. The way this works allows gas to get into the liquid and bubble.
Stabilization of Air Bubbles
Surfactants both diminish surface tension and also stabilize air bubbles by laying a thin film on the surface of the bubbles. This film helps prevent bubbles from collapsing (forming a solid) keeping foam steady. Furthermore, surfactants minimize the movement and escape of the liquid film and improve the foam stability.
Factors Influencing Foam Stability and Texture
Depending on surfactant type and concentration, temperature, pH and liquid hardness, foam stability and feel are affected. For instance, the chemistry of the surfactant can vary depending on the temperature, and thus how active it is. What’s more, various surfactants (anionic, cationic, nonionic) make foaming easier. All in all, synthetic surfactants make a better foam than natural surfactants.
What Makes Soap Foam?
Some reasons soap foam comes from these factors are soap chemical composition, hardness, and pH of water, and differences between natural and synthetic soaps.
The Chemistry of Soap and Its Foaming Properties
Soap molecules are amphiphilic, that is, they both have hydrophilic (water-attracting) and hydrophobic (oil-attracting) ends. The hydrophilic end grabs water molecules, and the hydrophobic end grabs oils and fats. When soap is mixed in water, they form micelles, microclusters of molecules whose hydrophobic ends gather heaped and whose hydrophilic ends project outward. This architecture lowers surface tension, making it easy to make foam. What’s more, the saturated fatty acids in soap, such as lauric acid and myristic acid help to make the soap quickly foam and have better cleaning abilities.
Role of Water Hardness and pH in Soap Foam
The calcium and magnesium ions in water define water hardness largely. When soaps are used in hard water, calcium and magnesium ions combine with alkaline components to create insoluble precipitates that cause the soap not to foam. Soft water (low-hardness water), on the other hand, has fewer calcium and magnesium ions, and so soap will be able to make much foam easily. And also, the soap pH is also a factor in how well it foams. The soap is alkaline, and the higher the pH, the better it will cling to make stable foam.
