Aggregate formation in fluids with bounded repulsive core and competing interactions

Soft matter systems often exhibit an intriguing morphology, related to the formation of intermediate range order structures. A rationale proposed for this behavior is the presence of competing short-range attraction and long-range repulsion. On the basis that effective interactions among the centers of mass of soft macromolecules often result in a finite short-range repulsion, we here consider a model fluid in which competing interactions supplement a repulsive core that, at difference from the models investigated up to now, has a bounded nature. We study the structural and phase behavior of this model through a theoretical approach and by computer simulation. We find that the structure factor exhibits a low-k peak, which is particularly relevant at low densities, where our system is formed by polydisperse clusters. A distinctive feature characterizing our model fluid is the presence of a peak situated between the low-k peak and the first diffraction peak. We show that this second pre-peak is associated with the enhancement of the relative population of the second coordination shell with respect to the first one, generated at intermediate densities by the long-range repulsion. As it concerns the phase behavior, the system investigated undergoes, for weak long-range repulsion, a liquid-gas phase transition. Upon increasing the strength of the long-range repulsion, such transition becomes less evident and eventually disappears. At high densities, the system undergoes freezing into a clustered solid, with multiply occupied crystal sites.