where the derivative on the right is known as the surface entropy. One can evaluate the surface entropy by investigating the manner in which the surface tension varies with temperature.
Reports of surface tension are often given for the situation in which the second phase is air or the vapor of the material. For example, at 20°C, the reported surface tension of benzene against air is 28.85 dyne/cm, whereas the reported surface tension of benzene against its vapor is reported to be 28.89 dyne/cm. In Table 1.1 are data on the surface tension of water against air.
Surface tensions of liquids against liquids are called interfacial tensions. Such interfacial tensions can be very different from the surface tensions of liquids against air. For example, the interfacial tension of water against benzene is 35 dyne/cm at 20°C, whereas the surface tension of water against air is 73.05 dyne/cm, and that of benzene against air is 28.85 dyne/cm.