When oil floats on water, the surface tension of water

When oil floats on water, it forms a thin layer or film that significantly reduces the surface tension of the water. Surface tension arises from the imbalance of molecular forces at the liquid-air interface [t1]. Water has a high surface tension (approximately 72 mN/m), while most oils have much lower surface tension values [t1][t7]. When oil spreads over the water surface, the oil molecules interact with the water molecules at the interface, balancing some of the inward attractive forces that previously caused high tension [t1][t4]. This interaction creates an interfacial layer that effectively decreases the overall surface tension of the system [t1]. This principle is historically noted for its ability to 'smooth' waves, as the reduced surface tension prevents the wind from easily catching and building up the water surface [t3].

Sources

1. [1] https://www.sciencedirect.com/science/article/abs/pii/S1359029412000374