Phase separation and self-assembly of colloidal dimers with tunable attractive strength: from symmetrical square-wells to Janus dumbbells (original) (raw)
Related papers
Structure and phase behavior of colloidal dumbbells with tunable attractive interactions
Physical Chemistry Chemical Physics, 2013
We investigate thermodynamic and structural properties of colloidal dumbbells in the framework provided by the Reference Interaction Site Model (RISM) theory of molecular fluids and Monte Carlo simulations. We consider two different models: in the first one we set identical square-well attractions on the two tangent spheres composing the molecule (SW-SW model); in the second scheme, one of square-well interactions is switched off (HS-SW model). Appreciable differences emerge between the physical properties of the two models. Specifically, the k → 0 behavior of SW-SW structure factors S(k) points to the presence of a gas-liquid coexistence, as confirmed by subsequent fluid phase equilibria calculations. Conversely, the HS-SW S(k) develops a low-k peak, signaling the presence of aggregates; such a process destabilizes the gas-liquid phase separation, promoting at low temperatures the formation of a cluster phase, whose structure depends on the system density. We further investigate such differences by studying the phase behavior of a series of intermediate models, obtained from the original SW-SW by progressively reducing the depth of one square-well interaction. RISM structural predictions positively reproduce the simulation data, including the rise of S(k → 0) in the SW-SW model and the low-k peak in the HS-SW structure factor. As for the phase behavior, RISM agrees with Monte Carlo simulations in predicting a gasliquid coexistence for the SW-SW model (though the critical parameters appears overestimated by the theory) and its progressive disappearance moving toward the HS-SW model.
We investigate thermodynamic properties of anisotropic colloidal dumbbells in the frameworks provided by the Reference Interaction Site Model (RISM) theory and an Optimized Perturbation Theory (OPT), this latter based on a fourth-order high-temperature perturbative expansion of the free energy, recently generalized to molecular fluids. Our model is constituted by two identical tangent hard spheres surrounded by square-well attractions with same widths and progressively different depths. Gas-liquid coexistence curves are obtained by predicting pressures, free energies, and chemical potentials. In comparison with previous simulation results, RISM and OPT agree in reproducing the progressive reduction of the gas-liquid phase separation as the anisotropy of the interaction potential becomes more pronounced; in particular, the RISM theory provides reasonable predictions for all coexistence curves, bar the strong anisotropy regime, whereas OPT performs generally less well. Both theories predict a linear dependence of the critical temperature on the interaction strength, reproducing in this way the mean-field behavior observed in simulations; the critical density—that drastically drops as the anisotropy increases—turns to be less accurate. Our results appear as a robust benchmark for further theoretical studies, in support to the simulation approach, of self-assembly in model colloidal systems.
Self-assembly in a model colloidal mixture of dimers and spherical particles
We investigate the structure of a dilute mixture of amphiphilic dimers and spherical particles, a model relevant to the problem of encapsulating globular " guest " molecules in a dispersion. Dimers and spheres are taken to be hard particles, with an additional attraction between spheres and the smaller monomers in a dimer. Using the Monte Carlo simulation, we document the low-temperature formation of aggregates of guests (clusters) held together by dimers, whose typical size and shape depend on the guest concentration χ. For low χ (less than 10%), most guests are isolated and coated with a layer of dimers. As χ progressively increases, clusters grow in size becoming more and more elongated and polydisperse; after reaching a shallow maximum for χ ≈ 50%, the size of clusters again reduces upon increasing χ further. In one case only (χ = 50% and moderately low temperature) the mixture relaxed to a fluid of lamellae, suggesting that in this case clusters are metastable with respect to crystal-vapor separation. On heating, clusters shrink until eventually the system becomes homogeneous on all scales. On the other hand, as the mixture is made denser and denser at low temperature, clusters get increasingly larger until a percolating network is formed. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4976704\]
Self-assembly mechanism in colloids: perspectives from statistical physics
Central European Journal of Physics, 2012
Motivated by recent experimental findings in chemical synthesis of colloidal particles, we draw an analogy between self-assembly processes occurring in biological systems (e.g. protein folding) and a new exciting possibility in the field of material science. We consider a self-assembly process whose elementary building blocks are decorated patchy colloids of various types, that spontaneously drive the system toward a unique and predetermined targeted macroscopic structure.
Complex Self-Assembly from Simple Interaction Rules in Model Colloidal Mixtures
2020
Building structures with hierarchical order through the self-assembly of smaller blocks is not only a prerogative of nature, but also a strategy to design artificial materials with tailored functions. We explore in simulation the spontaneous assembly of colloidal particles into extended structures, using spheres and size-asymmetric dimers as solute particles, while treating the solvent implicitly. Besides rigid cores for all particles, we assume an effective short-range attraction between spheres and small monomers to promote, through elementary rules, dimer-mediated aggregation of spheres. Starting from a completely disordered configuration, we follow the evolution of the system at low temperature and density, as a function of the relative concentration of the two species. When spheres and large monomers are of same size, we observe the onset of elongated aggregates of spheres, either disconnected or cross-linked, and a crystalline bilayer. As spheres grow bigger, the self-assembli...
Can Near-Critical Solvents Drive Colloidal Self-Assembly?
In 1978 Fisher and de Gennes predicted the existence of long-ranged solvent-mediated (SM) interactions between two colloidal particles suspended in a near-critical binary solvent. The range of these (universal) SM forces, often referred to as critical Casimir forces, is set by the correlation length of the solvent which diverges on approaching its critical point. The remarkable sensitivity of SM interactions to the temperature and composition of the solvent sparked recent interest, driven by prospects of unparalleled control of colloidal self-assembly in a tunable, reversible, and in-situ fashion. Here we determine both the effective SM pair interactions and the full phase diagram of Brownian discs suspended in an explicit two-dimensional supercritical binary liquid mixture. The SM pair interactions are most attractive at off-critical compositions of the solvent, and combined with the SM many-body interactions they drive colloidal gas-liquid and fluid-solid phase transitions in a surprisingly large regime away from criticality. Our simulation study, supported by a mean-field theory, provides a fresh perspective on colloidal self-assembly mediated by solvent critical fluctuations, and opens new avenues for controlling and manipulating this process.
Self-Assembly of Patchy Colloidal Dumbbells
We employ Monte Carlo simulations to investigate the self-assembly of patchy colloidal dumbbells interacting via a modified Kern-Frenkel potential by probing the system concentration and dumbbell shape. We consider dumbbells consisting of one attractive sphere with diameter sigma1\sigma_1sigma1 and one repulsive sphere with diameter sigma2\sigma_2sigma2 and center-to-center distance ddd between the spheres. For three different size ratios, we study the self-assembled structures for different separations l=2d/(sigma1+sigma2)l = 2d/(\sigma_1+\sigma_2)l=2d/(sigma1+sigma2) between the two spheres. In particular, we focus on structures that can be assembled from the homogeneous fluid, as these might be of interest in experiments. We use cluster order parameters to classify the shape of the formed structures. When the size of the spheres is almost equal, q=sigma2/sigma1=1.035q=\sigma_2/\sigma_1=1.035q=sigma_2/sigma_1=1.035, we find that, upon increasing lll, spherical micelles are transformed to elongated micelles and finally to vesicles and bilayers. For size ratio q=1.25q=1.25q=1.25 we observe a continuously tunable transition from spherical to elongated micelles upon increasing the sphere separation. For size ratio q=0.95q=0.95q=0.95 we find bilayers and vesicles, plus faceted polyhedra and liquid droplets. Our results identify key parameters to create colloidal vesicles with attractive dumbbells in experiments.
Equilibrium phases of one-patch colloids with short-range attractions
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.
Predicting the Phase Diagram of Two-Dimensional Colloidal Systems with Long-Range Interactions †
The Journal of Physical Chemistry B, 2006
The phase diagram of a two-dimensional model system for colloidal particles at the air-water interface was determined using Monte Carlo computer simulations in the isothermic-isobaric ensemble. The micrometerrange binary colloidal interaction has been modeled by hard disklike particles interacting via a secondary minimum followed by a weaker longer-range repulsive maximum, both of the order of k B T. The repulsive part of the potential drives the clustering of particles at low densities and low temperatures. Pinned voids are formed at higher densities and intermediate values of the surface pressure. The analysis of isotherms, translational and orientational correlation functions as well as structure factor gives clear evidence of the presence of a melting first-order transition. However, the melting process can be also followed by a metastable route through a hexatic phase at low surface pressures and low temperatures, before crystalization occurs at higher surface pressure.
Aggregation of colloidal spheres mediated by Janus dimers: A Monte Carlo study
A B S T R A C T We investigate by Monte Carlo simulation the structure and self-assembly of a mixture formed by asymmetric dimers and larger spherical particles. In our model, dimers and spheres interact through a monomer-specific short-range attraction, in addition to hard-core repulsion. The interaction parameters are chosen so as to mimic features of real colloidal mixtures. We find that the dilute mixture at low temperature is characterized by the onset of aggregates of spheres held together by dimers. In particular, when the sphere concentration is sufficiently high, the system stays homogeneous; at intermediate concentrations, very small clusters of spheres co-exist with a few lamellar aggregates; finally, for even lower concentrations a single droplet eventually forms. Upon increasing the density, a sponge-like structure emerges for not too high concentrations. If the attraction between dimers and spheres is switched off, leaving only depletion forces to act, neither phase separation nor demixing are found.