Phase diagram of heteronuclear Janus dumbbells

Using aggregation-volume-bias Monte Carlo simulations along with successive umbrella sampling and histogram re-weighting, we study the phase diagram of a system of dumbbells formed by two touching spheres having variable sizes, as well as different interaction properties. The first sphere (h) interacts with all other spheres belonging to different dumbbells with a hard-sphere potential. The second sphere (s) interacts via a square-well interaction with other s spheres belonging to different dumbbells and with a hard-sphere potential with all remaining h spheres. We focus on the region where the s sphere is larger than the h sphere, as measured by a parameter 1 ≤ α ≤ 2 controlling the relative size of the two spheres. As α → 2 a simple fluid of square-well spheres is recovered, whereas α → 1 corresponds to the Janus dumbbell limit, where the h and s spheres have equal sizes. Many phase diagrams falling into three classes are observed, depending on the value of α. The 1.8 ≤ α ≤ 2 is dominated by a gas-liquid phase separation very similar to that of a pure square-well fluid with varied critical temperature and density. When 1.3 ≤ α ≤ 1.8 we find a progressive destabilization of the gas-liquid phase diagram by the onset of self-assembled structures, that eventually lead to a metastability of the gas-liquid transition below α = 1.2.