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 correlat
ions - 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.
