Surfactants help the herbicide spray stick onto the weeds instead of falling onto the soil immediately.
Surfactants which are also called surface-acting agents are used in herbicide formulations. The surfactants' role is to reduce the surface tension of water.
As herbicide sprays are mainly made of water, surfactants that are added help to target the weed instead of falling instantly onto the soil. Polyethoxylated tallow amine (POEA) is the surfactant added to the Roundup weed-killer.
This substance can also boost the cancer-causing properties of glyphosate, an active ingredient of Roundup because of which Roundup is considered to be more carcinogenic than Glyphosate alone.