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, π or π s, and is equal to γ 0- γ where γ 0 is the surface tension of the clean surface and γ that of the covered surface.
Sources:
Green Book, 2nd ed., p. 63 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1972, 31, 49. 'Extended chain crystals of linear high polymers' on page 59 (https://doi.org/10.1351/pac197231010049)
PAC, 1983, 55, 1251. 'Interphases in systems of conducting phases (Provisional)' on page 1264 (https://doi.org/10.1351/pac198355081251)