Fig 6.6 shows the effect of adding different materials on the surface tension of water . On adding hydrophilic solutes that are present more in the bulk than the interface, the surface tension increases. While lyophobic solutes, that dislike the bulk aqueous phase, decrease the surface tension somewhat, huge reduction of surface tension takes place through surfactant addition.Cosurfactant addition further reduces the surface tension, while it is seen that alcohol, if added along with surfactant and cosurfactant can lead to ultra low surface tension.
At high concentration of solute(surfactants), interfacial tension does not changemuch due to formation of micelles. Hence may not be a good representation and needs to be replaced with activity. Micelles form only after a particular concentration of solute (i.e surfactants) called critical micelles (delete) concentration (CMC). CMC is achieved because there is an energy penalty in micelles formation. Before reaching the CMC, thesurface tension changes strongly with the concentration of the surfactant. After reaching the CMC, the surface tension change becomes very small. Thus from a graph of concentration of surfactant versus the change in surface tension, the CMC for the system can be evaluated.

[2] For , micelles form as a result of saturation and the activity of monomer surfactant remains constant.