Water molecules strongly attract each other through hydrogen bonding.  At the surface, the unbalanced forces results in a strong compressive force down into the bulk which simulates a tight, cohesive “skin” at the interface.  When surfactant molecules migrate to that interface, they disrupt this structure and lower the surface tension in several ways.

First, they replace the high-energy water surface with a lower-energy surfactant-covered interface. Second, the intermolecular attractive forces between the hydrophobic moieties of the surfactant are weaker than the cohesive forces between water molecules, reducing the overall interfacial tension. Third, as surfactants accumulate at the surface they generate a surface pressure, analogous to vapor pressure, that helps expand and spread the interface.

Together, these effects explain why even small amounts of surfactant can dramatically change how liquids wet, spread, and interact with surfaces.