# Does the value of Surface tension (the value of the constant) change with a change in surface area?

Surface Tension or ϒ (as in gamma) is a constant value for a particular fluid in fixed conditions. When we increase the surface area of the interface, more molecules pop up at the surface and contribute to a greater tension force. So I understand the ϒ remains same because it is energy required to increase a unit surface area. But now consider a solution where there's a fluid say soap in water (below CMC or saturation). When you increase the interface area, there is more scope for soap molecules to appear, which would reduce the surface tension and vice versa. So is it true that ϒ value is constant only for pure liquids and not solutions (even of constant concentrations) because the interface solute concentration is variable?

If true, why do we do problems based on surface tension in soap bubbles taking ϒvalue never bothering about the surface area?
If false, why is there a change in ϒvalue for the surfactant in lung with change in surface area (during inspiration and expiration)?

• $\gamma$ is energy per unit area to create said unit area of interface. – Jon Custer May 26 '15 at 21:47