Effects of Aqueous Surfactant Solutions

Surfactants act at the boundary between two substances (interfaces) and change the interfacial properties. For example, when surfactants are present at the interface between water and oil, they enable these two substances—normally immiscible—to mix. In a similar way, surfactants make water more compatible with water-repellent surfaces (Figure 4).
When water is dropped onto the surface of a mosquito, it is repelled and does not wet the surface. This occurs because surface tension—the cohesive force among water molecules—strongly resists spreading. The addition of surfactants reduces surface tension, allowing water to spread and wet the surface of a mosquito’s body.

Kao discovered that by utilizing these properties of surfactants, it becomes possible to wet the highly hydrophobic surfaces of mosquito bodies and wings. Furthermore, comparative studies of various aqueous surfactant solutions revealed that surfactants with greater surface tension–reducing capacity can wet mosquito surfaces more effectively (Figure 5).