  We study model protein solutions and colloidal suspensions in the temperature
range whereupon the nature of the system changes from a homogeneous fluid to a
"cluster fluid". It is commonly assumed - as deduced by the behavior of the
structural correlations - that this microphase separation sets as a continuous
process. We challenge such assumption and demonstrate that the entropy shows a
discontinuous behavior across a well-defined temperature threshold, providing a
neat signature of the modifications occurring in the fluid. All predictions,
obtained in the framework of a refined theory of the liquid state, are
systematically assessed against extensive Monte Carlo simulations. We emphasize
the broad generality of our conclusions, and the importance of accurate
theoretical treatments to characterize the thermodynamic properties of soft
materials.
