Role of soft and hard aggregates in the thermodynamics of lipid dispersions

We study the thermodynamics of a two-dimensional polydisperse ideal gas mod el of different species of aggregates. We show that if these aggregates are distinguished not only-by their sizes but also by their ability to-display shape fluctuations, the system presents dominance of one or other species, depending on the temperature region. This result, which emerges solely fro m the statistics of the model in total absence of interaggregate interactio ns, describes well the observed temperature dependence of light scattering in dispersions of dimyristoyl phosphatidylglycerol, a negatively charged li pid. [S1063-651X(99)13512-8].