Molecular association of heteronuclear vibrating square-well dumbbells in liquid-vapor phase equilibrium

Encapsulation of spherical nanoparticles by colloidal dimers

We study by Monte Carlo simulation the coating process of colloidal dimers onto spherical nanoparticles. To this end we investigate a simplified mixture of hard spheres (the guest particles) and hard dimers formed by two tangent spheres of different sizes (the encapsulating agents) in an implicit-solvent representation; in our scheme, the range of effective interactions between the smaller particle in a dimer and a guest sphere depends on their relative size. By tuning the size and concentration of guests, under overall dilute conditions a rich phase behavior emerges: for small sizes and/or low concentrations, the preferred arrangement is compact aggregates (capsules) of variable sizes, where one or few guest particles are coated with dimers; for larger sizes and moderate guest concentrations, other scenarios are realized, including equilibrium separation between a guest-rich and a guest-poor phase. Our results serve as a framework for a more systematic investigation of self-assembled structures of functionalized dimers capable of encapsulating target particles, like for instance bioactive substances in a colloidal dispersion.

Cluster formation and phase separation in heteronuclear Janus dumbbells

We have recently investigated the phase behavior of model colloidal dumbbells constituted by two identical tangent hard spheres, with the first being surrounded by an attractive square-well interaction (Janus dumbbells, Munaó et al 2014 Soft Matter 10 5269). Here we extend our previous analysis by introducing in the model the size asymmetry of the hard-core diameters and study the enriched phase scenario thereby obtained. By employing standard Monte Carlo simulations we show that in such 'heteronuclear Janus dumbbells' a larger hard-sphere site promotes the formation of clusters, whereas in the opposite condition a gas-liquid phase separation takes place, with a narrow interval of intermediate asymmetries wherein the two phase behaviors may compete. In addition, some peculiar geometrical arrangements, such as lamellæ, are observed only around the perfectly symmetric case. A qualitative agreement is found with recent experimental results, where it is shown that the roughness of molecular surfaces in heterogeneous dimers leads to the formation of colloidal micelles.

Phase separation and self-assembly of colloidal dimers with tunable attractive strength: from symmetrical square-wells to Janus dumbbells

We numerically investigate colloidal dimers with asymmetric interaction strengths to study how the interplay between molecular geometry, excluded volume effects and attractive forces determines the overall phase behavior of such systems. Specifically, our model is constituted by two rigidly-connected tangent hard spheres interacting with other particles in the first instance via identical square-well attractions. Then, one of the square-well interactions is progressively weakened, until only the corresponding bare hard-core repulsion survives, giving rise to a "Janus dumbbell" model. We investigate structure, thermodynamics and phase behavior of the model by means of successive umbrella sampling and Monte Carlo simulations. In most of the cases, the system behaves as a standard simple fluid, characterized by a gas-liquid phase separation, for sufficiently low temperatures. In these conditions we observe a remarkable linear scaling of the critical temperature as a function of the interaction strength. But, as the interaction potential approaches the Janus dumbbell limit, we observe the spontaneous formation of self-assembled lamellar structures, preempting the gas-liquid phase separation. Comparison with previous studies allows us to pinpoint the role of the interaction range in controlling the onset of ordered structures and the competition between the formation of these structures and gas-liquid condensation.