[kelvinLTR]

In a sum, we are given the variation of surface tension with concentration of a surfactant and asked to calculate the surface excess concentration and surface pressure. The lecturer gave us the guide line as

1. Calculate natural logarithms of the concentration and plot that against the surface tension (To get the Gibbs Adsorption Isotherm(GAI))

2. Get the magnitude of the graph( which will be a constant) and substitute into the GAI and get the surface excess concentration (The temperature was given)

3. Substitute Surface excess concentration to 2D equation of state  {ΠA=nRT} and find Π(Surface pressure)

The problem I have is : The answer we are getting for Π is a constant for any concentration. But since Π=γ⁰-γ and γ changes with concentration, shouldn't Π change as well?

[γ is the surface tension]

I tried google thinking I do not understand when I can apply GAI and 2D equation of state but couldn't find a good enough resource.

Any help is appreciated.