In this blog post, we’ll examine how the surfactants in detergents remove grease stains—which are difficult to remove with water alone—focusing on two key cleaning mechanisms: rolling-up and liquid crystal formation.
Why Can’t Grease Stains Be Removed With Water Alone?
When something gets on our clothes, we usually rinse it off quickly with water, and if it doesn’t come clean with water alone, we use soap or detergent to wash it. In everyday life, stains on clothes can be divided into those that are water-soluble and those that are not; soap or synthetic detergents are used to remove water-insoluble stains. Most people are probably aware that the surfactants in soap or synthetic detergents are what remove these stains. However, the specific process by which surfactants remove stains is not well understood.
How do surfactants remove stains?
There are several mechanisms by which surfactants remove grease stains that are difficult to remove with water alone. Among these, the two primary mechanisms in which surfactants play an active role are “rolling-up” and “lyotropic liquid crystal formation.” Understanding these two principles makes it easier to grasp how detergents remove grease.
What is interfacial tension?
To understand rolling-up, you must first understand the concept of interfacial tension. Interfacial tension is a physical property related to the tendency of two different substances to minimize the area of their common interface. Surface tension can be viewed as a form of interfacial tension that occurs at the interface between a liquid and a gas. Therefore, we can understand the basic principles of interfacial tension through examples of surface tension.
Why does surface tension occur?
Between the molecules that make up a liquid, attractive forces and repulsive forces—which arise when the distance between molecules becomes too close—act simultaneously. Molecules inside a liquid experience attractive and repulsive forces in all directions in a balanced manner, so they do not feel any particular force. In contrast, molecules at the liquid’s surface experience relatively stronger attractive forces acting toward the interior of the liquid. As a result, surface molecules experience a net force directed inward, and consequently, surface energy proportional to the surface area exists at the surface. Liquids exhibit a tendency to minimize this surface energy, which is referred to as surface tension. Interfacial tension is a concept that extends this principle beyond just liquid surfaces to various interfaces, such as liquid-liquid, solid-liquid, and solid-gas.
How does rolling-up occur?
Rolling-up is a stain-removal mechanism that plays a crucial role in the laundry process. This process refers to the entire sequence, starting from the initial state where stains are firmly attached to the fibers and ending with the final state where the stains are completely separated from the fiber surface. Energy is required for this transformation to occur, and the amount of energy needed is closely related to the difference between the fiber-stain interfacial tension in the initial state and the detergent solution-stain interfacial tension and detergent solution-fiber interfacial tension in the final state. In other words, if the energy difference resulting from the change in interfacial tension is sufficient, rolling-up proceeds, allowing the stain to detach from the fiber.
What role do surfactants play?
In this process, surfactants serve to lower the interfacial tension between the detergent solution and the dirt, as well as between the detergent solution and the fiber. Surfactant molecules possess both a hydrophilic group, which binds well with water, and a lipophilic group, which binds well with oil. During the washing process, the hydrophilic groups orient toward the water-containing detergent solution, while the lipophilic groups orient toward the dirt or the fiber. When surfactants position themselves at the interface between two substances in this way, the interfacial energy decreases and the interfacial tension is reduced. As a result, the bonding force between the dirt and the fabric weakens, allowing the dirt to detach easily from the fabric surface.
How does liquid crystal formation help remove dirt?
Another mechanism in which surfactants play an active role is liquid crystal formation.
When the surfactant concentration is sufficiently high and the dirt contains many components that interact well with the surfactant, the surfactant molecules adsorb densely onto the dirt’s surface. As a large amount of surfactant accumulates on the surface, some molecules penetrate into the dirt, resulting in a higher surfactant concentration inside the dirt. Consequently, osmosis causes the detergent solution and water to penetrate into the stain as well, gradually softening it. The state in which the stain has been softened by the penetration of surfactants and the detergent solution is called the liquid crystal state; in this state, the stain breaks apart into smaller pieces or deforms easily, allowing it to be readily removed by the mechanical forces applied during the washing process.
Why is this effect more pronounced in drum washers? A certain amount of time is required for the liquid crystal state to form. In the past, it was often argued that the cleaning effect resulting from liquid crystal formation was limited in impeller-type washers, which had relatively short wash cycles. However, today, with the diversification of wash times, detergent concentrations, water temperatures, and washing methods, the liquid crystal effect can manifest sufficiently depending on the conditions. In particular, when high-concentration detergent is applied directly to heavily soiled clothing or when using a drum washing machine with a relatively long wash cycle, the likelihood of surfactants penetrating deep into the soiled areas increases, which can be even more effective in removing stains.
