Surfactants function as a crucial component in reducing surface tension in liquids. Their unique structure allows them to change how the liquid’s molecules interact at the surface or interface. Here’s how they accomplish this:

Surfactant molecules are amphiphilic—meaning they have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This dual nature allows them to interact uniquely with the molecules of a liquid (typically water in aqueous solutions).

Without surfactants, a liquid’s surface molecules have a stronger attraction to the molecules in the bulk of the liquid than to the air or gas above, leading to surface tension. When surfactants are added to the liquid, the hydrophilic part of each surfactant molecule is attracted to the water, while the hydrophobic part repels water and is attracted to air. The hydrophobic tails of the surfactant molecules disrupt the uniform attraction among the water molecules at the liquid’s surface, thereby decreasing the surface tension.

As surfactants congregate at the liquid-gas interface, they occupy space and limit the freedom of movement of the liquid’s surface molecules, impeding them from interacting as strongly with each other. This further reduces surface tension.

The alignment of surfactant molecules at the surface also leads to a decrease in the cohesive forces among the water molecules. The less cohesive these molecules are on the surface, the lower the surface tension of the liquid will be.

In conclusion, surfactants decrease surface tension in a liquid by interacting differently with the liquid’s molecules due to their structure and surface activity. They insert themselves between the liquid molecules at the surface, reducing the cohesive forces amongst them, and disrupting the surface tension. This action allows for enhanced spreadability and forms the basis for the creation of bubbles in foams and helps emulsify oils in detergents.