The change of interfacial tension caused by addition of a given species to a base solution. When an area of liquid covered with a spread substance is separated from a clean area of surface by a mechanical barrier, the force acting on unit length of the barrier is called the surface pressure, $\pi$ or ${\pi }_{\text{s}}$, and is equal to ${\gamma }^0-\gamma$ where ${\gamma }^0$ is the surface tension of the clean surface and $\gamma$ that of the covered surface.