Inspired by experimental studies of short-ranged attractive patchy particles, we study with computer simulations the phase behavior and the crystalline structures of one-patch colloids with an interaction range equal to 5% of the particle diameter. In particular, we study the effects of the patch surface coverage fraction, defined as the ratio between the attractive and the total surface of a particle. Using free-energy calculations and thermodynamic integration schemes, we evaluate the equilibrium phase diagrams for particles with patch coverage fractions of 30%, 50% and 60%. For a 60% surface coverage fraction, we observe stable lamellar crystals consisting of stacked bilayers that directly coexist with a low density fluid. Inside the coexistence region, we observe the formation of lamellar structures also in direct NVT simulations, indicating that the barrier of formation is low and experimental realization is feasible. For sufficiently strong interactions, these structures spontaneously assemble from the fluid in simulations, suggesting that they might also easily form in experimental systems. In the Janus case, i.e. at 50% surface coverage fraction, no lamellar structures are formed, and the stable crystals are similar to those that have been found previously for a longer interaction range (i.e. 20% of the particle diameter). At 30% coverage fraction, we identify novel ‘open’ crystal structures with large unit cells of up to 14 particles that are stable in the strong interaction limit.

We perform Monte Carlo simulations of a simple one-patch colloidal model to investigate the cluster formation and the phase behavior of the system on changing the width of the patch. We investigate the parameter region where the coverage (defined as the ratio between attractive and total surface) varies from 50% (the Janus case) to zero (hard-sphere). Simulation results indicate that on decreasing the coverage, particles self-assemble into clusters of different shapes, from micelles close to the Janus case, to one and two dimensional aggregates (wires and lamellae) for smaller coverage. Close to the hard-sphere limit, small micelles and dimers dominate the scene. We never find evidence of a gas–liquid (colloidal-rich/colloidal-poor) phase separation: it confirms that self-assembly into clusters which expose to their neighbors mostly repulsive surfaces suppresses phase separation and stabilizes cluster phases.

We perform a systematic application of the hybrid particle-field molecular dynamics technique [Milano, et al., J. Chem. Phys., 2009, 130, 214106] to study interfacial properties and potential of mean force (PMF) for separating nanoparticles (NPs) in a melt. Specifically, we consider Silica NPs bare or grafted with Polystyrene chains, aiming to shed light on the interactions among free and grafted chains affecting the dispersion of NPs in the nanocomposite. The proposed hybrid models show good performances in catching the local structure of the chains, and in particular their density profiles, documenting the existence of the “wet-brush-to-dry-brush” transition. By using these models, the PMF between pairs of ungrafted and grafted NPs in Polystyrene matrix are calculated. Moreover, we estimate the three-particle contribution to the total PMF and its role in regulating the phase separation on the nanometer scale. In particular, the multi-particle contribution to the PMF is able to give an explanation of the complex experimental morphologies observed at low grafting densities. More in general, we propose this approach and the models utilized here for a molecular understanding of specific systems and the impact of the chemical nature of the systems on the composite final properties.

The self-assembly process in a water solution of an amphiphilic polydimethylsiloxane-b-polyethyleneoxide (PDMS–PEO) diblock copolymer was investigated by means of small-angle X-ray scattering (SAXS) experiments in the concentration region below (and near) the critical micellar concentration (c.m.c. = 0.007 g cm⁻³). In the highly diluted region, at the copolymer concentration of c = 0.002 g cm⁻³, the early stage of the self-assembly process was characterized by the formation of small (primary) micellar units (with a radius of R = 2.7 nm) with core–shell morphology, which coexisted with larger supramolecular aggregates of entangled micelles (with an average radius of R = 9.5 nm). The increase in the copolymer concentration (to c = 0.005 and c = 0.01 g cm⁻³) caused increase in the sizes of both the small micelles and supra-micellar aggregates. Interestingly, at the concentration of c = 0.005 g cm⁻³, both the size and micelle aggregation number (N_agg) were found to increase on increasing the temperature in the range of 10 ≤ T ≤ 55 °C. This phenomenon was characterised by the dehydration process of the ethylene oxide (EO) segments, as evidenced by the calculation of excess water in the hydrophilic shell of the micelles. The more compact (less hydrated) structure of the hydrophilic PEO chains, which strongly influenced the spontaneous curvature of the amphiphile hydrophilic region, turned out to be the driving factor that favoured the increase in the micelle aggregation number with the increase in temperature. The obtained results evidence that the self-assembly process of PDMS–PEO copolymer amphiphiles is a gradual process that is already present at the very low concentration region (far below the macroscopically determined c.m.c.); moreover, it is characterised by a multi-stage organization process, where the primary building blocks self-assemble into more complex secondary structures that encompass multiple length scales.

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.