On the gas-liquid phase separation and the self-assembly of charged soft dumbbells

We investigate the phase behaviour of charged soft dumbbells, particles composed of two soft but oppositely charged sites, as a function of the separation between the sites. Through successive umbrella sampling Monte Carlo simulations, we evaluate the equilibrium particle density of states. For elongated dumbbells, we recover the expected critical behaviour. However, reducing the elongation we are unable to locate the gas-liquid critical point in the accessible region of temperature and density. Correspondingly, the self-assembly of the particles in ring structures becomes dominant with respect to the formation of chain-like structures. Our results enrich the debate on the competition between self-assembly and phase separation and contribute to the long-standing dilemma about the putative gas-liquid criticality in dipolar fluids.