We have shown that the thermodynamics of a 2D multicomponent ideal gas model describes well the thermal behavior of light-scattering (LS) data in lipid dispersions of dimyristoyl phosphatidylglycerol (DMPG) [C. Goldman, K. A. Riske, and M. T. Lamy-Freund, Phys. Rev. E 60, 7349 (1999)]. The components (species) of the gas are distinguished by size and by their ability to display shape fluctuations. Here, the model is extended to include the presence of thermalized impurities bound to the fluctuating species. We show that with this modification the model accounts also for additional LS data from DMPG under variation of ionic strength.