Density jumps across phase transitions in soft-matter systems

We investigate the magnitude of density jumps across phase transitions in soft-matter systems composed of macromolecular particles, like star polymers or colloidal suspensions. The standard route to predict phase transformations is to start from an effective interaction potential between these macroparticles and map the phase diagram onto that of the corresponding effective one-component system. Using density-functional perturbation theory, we demonstrate that this procedure leads to wrong density jumps if the number of microscopic degrees of freedom is coupled to the number of macroparticles. In particular, the microscopic degrees of freedom can drive a macroparticle phase transition to be isochoric, i.e., to occur without any jump in the macroparticle density.