Usually the film is substantially complete and can accept no more solute molecules when the bulk concentration attains some small value, and hence the surface concentration undergoes little change when the bulk concentration is increased further over a wide range. From Eq. (14) we can therefore expect that over this range a plot of the surface tension of the solution against the logarithm of the bulk concentration should be linear. It will deviate from linearity both at low and at high concentrations, in both cases yielding a slope of smaller absolute magnitude.