Example

Consider liquid, its vapors and the solute as a two-phase two-component system. Let the properties of the vapor be represented by []' and the properties of the liquid phase  by []".

(1) represents solvent and

(2) represents solute.

If the solute is non-volatile (surfactants are usually non-volatile), then . Further, since concentration of solvent in vapor phase is very small compared to that in the bulk liquid phase, we have .

From the above discussion we have

If we consider the bulk solubility of the surfactant to be small then

This is a limiting case implying that all the surfactant has gone to the surface.