First, we need to understand surface tension. The molecules on the surface of the liquid are pulled by the internal liquid molecules. This pulling force makes the surface of the liquid tend to shrink, like a tight elastic film. This is surface tension. For example, water droplets appear nearly spherical on lotus leaves because the surface tension of water makes the water droplets minimize their surface area.
The molecular structure of surfactants is special. It has a hydrophilic head and a hydrophobic tail. When Dish Soap is added to water containing oil, the hydrophobic tail of the surfactant molecule will be inserted into the oil because the oil is hydrophobic. The hydrophilic head faces the water phase. In this way, the surfactant molecules surround the oil particles.
With continuous stirring or wiping, more surfactant molecules gather around the oil, so that the oil particles are divided into smaller oil droplets. The surface of these small oil droplets is covered by surfactant molecules. Because the surfactant reduces the surface tension between the liquid and the oil, the oil droplets can be well dispersed in the water to form an emulsion.
This emulsion can be washed away by water, thereby achieving the purpose of cleaning. For example, when washing greasy dishes, the surfactant in Dish Soap will emulsify the oil on the dishes, so that the oil will no longer adhere to the surface of the dishes and will be washed away with the water. At the same time, the surfactant can also prevent the oil from settling on the surface of the dishes again, so that the dishes remain clean after washing.
