Why are the values of the surface tension of most organic liquids similar?

The values of the surface tension of most organic liquids are of the same order of magnitude. The explanation is that surface tension is governed by London dispersion forces, which are independent of the permanent dipole moment of the molecules. The surface tension of organic liquids (with the exception of polymers and polymer solutions) depends on the ionization potential and the diameter of the molecule only. These parameters vary slightly for organic liquids.