Surfactants have both hydrophilic and lipophilic groups in their molecules, a property also known as amphiphilic.

Due to this amphiphilic property, they can form micelles in water at appropriate concentrations – the hydrophilic head is attracted outward by water, and the lipophilic tail is repelled by water and thus faces inward.

These micelles have an important role in skin care products.

During cleansing, oil stains are pulled by the lipophilic groups to the inside of the micelle, and the whole micelle is carried away by water. In an oily environment, they can form an inverse micelle, where the head is inside and the tail is outside.

The strength of surfactant and the size of critical micelle concentration are closely related to its hydrophilicity, and the hydrophilicity is determined by the interaction and joint determination of hydrophilic and lipophilic groups, for this reason, Griffin proposed to express the hydrophilicity of surfactants by the hydrophilic-lipophilic balance (HLB), which is a measure of the degree of balance between hydrophilic and hydrophobic groups in terms of size and strength.