Soap. Computer artwork showing how the molecular structure of soap works to separate water (blue) and oil (centre, yellow). Soap is a surfactant - a compound that lowers the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. The surfactant molecules have oil-soluble tails that project into the oil, while the water-soluble heads (round) remain in contact with the water, thus isolating the oil within the water.