In this blog post, we’ll examine the difference between regular soap and exfoliating soap, and explore how soap and surfactants remove dirt and impurities from the skin.
How does soap remove dirt?
Soap is what we use to wash our hands when we come home after being out. In everyday life, when our hands get dirty, we often wash them with soap. However, you’ve probably come across something called “scrub soap” at some point. Scrub soap is commonly used in bathhouses when exfoliating the skin. So, what exactly is the difference between regular soap and scrub soap?
Whether it’s regular soap or exfoliating soap, the basic purpose is to help wash away contaminants, sebum, and dead skin cells from the skin’s surface with water. The substances that play a key role in this cleansing action are called surfactants. A single surfactant molecule has both a hydrophilic part—which is attracted to water—and a hydrophobic part—which is attracted to oil. People often describe relationships that don’t mix well as being “like oil and water.” This is because water is polar, whereas oil is primarily a nonpolar substance, so they do not mix well. Similarly, the hydrophilic portion of a surfactant molecule interacts well with water, while the hydrophobic portion interacts with substances that are insoluble in water, such as oil or sebum. Thanks to this structure, surfactants help oil-based impurities on the skin’s surface adhere to them so they can be washed away with water.
What is skin grime made of?
Similarly, the outermost layer of our skin is the stratum corneum, which protects the skin. The stratum corneum consists of dead skin cells (keratinocytes) and lipids—such as ceramides, cholesterol, and fatty acids—that fill the spaces between them, and it is responsible for the skin’s barrier function. Therefore, it is difficult to categorize the skin itself as simply hydrophilic or lipophilic. What we commonly refer to as “dirt” does not simply mean dead skin cells shed from the skin. Dirt can be broadly defined as the mixture of dead skin cells, sebum, sweat, and various external contaminants—such as dust—that remain on the skin’s surface. Surfactants disperse or emulsify these contaminants and substances—such as sebum—that do not dissolve easily in water, allowing them to be removed along with the water.
Looking at the cleansing mechanism of soap, surfactant molecules first approach the sebum and contaminants on the skin’s surface. At this point, the hydrophobic portions interact with sebum or oily contaminants, while the hydrophilic portions interact with water. Combined with the flow of water and the physical force of rubbing with hands or a washcloth, this causes the contaminants adhering to the skin’s surface to be removed. When the concentration of surfactants is high enough, the surfactant molecules can aggregate to form structures called micelles. In micelles, the hydrophobic portions face inward while the hydrophilic portions face the water, which helps disperse oily substances—that are not easily soluble in water—into the water. Therefore, when you wash with soap and rinse thoroughly with water, sebum and contaminants on the skin’s surface are removed along with the surfactants. However, since micelles are extremely small structures, they are not visible to the naked eye; if any visible substances are floating in the wash water, they are likely various contaminants or other particles dispersed in the water rather than the micelles themselves.
What is the difference between regular soap and exfoliating soap?
As shown here, the basic cleansing principles of regular soap and exfoliating soap are not significantly different. However, products marketed as exfoliating soaps sometimes contain plant-based ingredients—such as green tea, kelp, and mung beans—or ingredients that aid in exfoliation, in addition to standard cleansing agents. Since the types and concentrations of these additional ingredients vary by product, it cannot be assumed that all exfoliating soaps work in the same way. In particular, it is difficult to conclusively state that ingredients such as green tea, kelp, or mung beans widen the skin’s pores to allow soap particles to penetrate deeper into the skin. If an exfoliating soap demonstrates cleansing power or exfoliating effects, it is more appropriate to view this as the result of a combination of factors: the surfactants and exfoliating agents contained in the product, as well as the physical friction applied while scrubbing away dirt in the bath. While the process of physically removing dead skin cells can help exfoliate the skin, it is important to exercise caution, as rubbing too vigorously or exfoliating repeatedly can damage the skin barrier and lead to dryness or irritation.
Furthermore, not all soaps have the same effect on the skin. Traditional soaps are generally alkaline, which can affect the skin’s pH and barrier function, and some cleansers may strip the skin of too much oil and moisture, leading to dryness or irritation. In contrast, synthetic cleansers known as “syndets” allow for the adjustment of pH and cleansing agents to better suit the skin, which can help reduce skin irritation in some cases. Therefore, a product cannot be considered good simply because it is a “soap that removes dirt well”; it is also important to choose a cleansing strength that matches your skin condition.
Scrub soaps, known for their specialized dirt-removing properties, share the basic cleansing principles of regular soap but often contain additional ingredients—depending on the product—to enhance exfoliation or cleansing power. When scrubbing away dirt in the bathhouse, the effect isn’t solely due to the soap’s cleansing action; it’s also influenced by the process of the skin becoming wet and the stratum corneum absorbing moisture, as well as the physical action of rubbing with a scrub towel. Therefore, if you use a scrub bar at the public bathhouse, you should ensure that the product is gentle on your skin and avoid scrubbing too vigorously. While it’s a good idea to check that you’ve brought a scrub bar with you, it’s most important not to strip away the protective barrier your skin needs.
