Principle of Surface Tension Reduction by Surfactants

In order to reduce the surface tension of water, for example, the surfactant molecules have hydrophilic and lipophilic groups. Due to the repulsion between the lipophilic groups and the water molecules, in order to find the lowest energy form of existence, the wholesale surfactant molecules are first arranged on the surface of an aqueous solution with the hydrophilic groups facing down (contact with water molecules) and the lipophilic groups facing up (contact with air molecules). Additional surfactant molecules move into the solution only when there is no room at the liquid surface to accept them. The hydrophilic radical is attracted downwards by water molecules, but this attraction is clearly weaker than the attraction between water molecules because the polarity of the hydrophilic radical is weaker than that of water molecules; the lipophilic radical is attracted upwards by air molecules, and this attraction is stronger than the attraction between air molecules and water molecules because the relatively larger volume allows the lipophilic radical to come into contact with more air molecules, and the weaker polarity also allows the lipophilic radical to better interact with the air molecules. Lipophilic radical and air molecules are fused and attracted to each other.

Overall, the downward gravitational force is weakened and the upward gravitational force is increased, and the force imbalance of surfactant agent molecules is weaker than that of water molecules, so that the surface of an aqueous solution with an arrangement of surfactant molecules is clearly weaker than the surface of water molecules alone, where the force imbalance is weaker and, macroscopically, the surface tension is reduced.

The surface of an aqueous surfactantsolution consists of water molecules and surfactant molecules. The attraction between surfactant produced by the surfactant manufacturers and water molecules is weaker than the attraction between water molecules and, as the surfactant molecules are larger than the water molecules, they are closer together. With the distance decreasing, the attraction between them also decreases.Therefore the attraction between surfactant molecules is also weaker than that between water molecules.

In general, in the surface of an aqueous solution with surfactant molecules, the attraction between the molecules is reduced and the taut state of the surface is reduced, which enables the surfactant to reduce the surface tension.