Colloidal suspensions are widely used to study processes such as melting, freezing and glass transitions. This is because they display the same phase behaviour as atoms or molecules, with the nano- to micrometre size of the colloidal particles making it possible to observe them directly in real space. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive, short-range attractive, hard-sphere-like and dipolar, can be realized and give rise to equilibrium phases. However, spherically symmetric, long-range attractions (that is, ionic interactions) have so far always resulted in irreversible colloidal aggregation. Here we show that the electrostatic interaction between oppositely charged particles can be tuned such that large ionic colloidal crystals form readily, with our theory and simulations confirming the stability of these structures. We find that in contrast to atomic systems, the stoichiometry of our colloidal crystals is not dictated by charge neutrality; this allows us to obtain a remarkable diversity of new binary structures. An external electric field melts the crystals, confirming that the constituent particles are indeed oppositely charged. Colloidal model systems can thus be used to study the phase behaviour of ionic species. We also expect that our approach to controlling opposite-charge interactions will facilitate the production of binary crystals of micrometre-sized particles, which could find use as advanced materials for photonic applications.

We report the growth of binary colloidal crystals with control over the crystal orientation through a simple layer-by-layer process. Well-ordered single binary colloidal crystals with a stoichiometry of large (L) and small (S) particles of LS2 and LS were generated. In addition, we observed the formation of an LS3 superstructure. The structures formed as a result of the templating effect of the first layer and the forces exerted by the surface tension of the drying liquid. By using spheres of different composition, one component can be selectively removed, as is demonstrated in the growth of a hexagonal non-close-packed colloidal crystal.

We studied crystal structures in mixtures of large and small oppositely charged spherical colloids with size ratio 0.31 using Monte Carlo simulations and confocal microscopy. We developed an interactive method based on simulated annealing to predict new binary crystal structures with stoichiometries from 1 to 8. Employing these structures in Madelung energy calculations using a screened Coulomb potential, we constructed a ground-state phase diagram, which shows a remarkably rich variety of crystals. Our phase diagram displays colloidal analogs of simple-salt structures and of the doped fullerene C 60 structures, but also novel structures that do not have an atomic or molecular analog. We found three of the predicted structures experimentally, which provides confidence that our method yields reliable results. DOI: 10.1103/PhysRevLett.96.138308 PACS numbers: 82.70.Dd, 61.50.Ah Colloidal dispersions consist of nanometer to micrometer sized particles suspended in a solvent. Colloids are important model systems for atoms and molecules, as they exhibit the same phase behavior, but are easier to investigate and to manipulate. The possibility to tune colloidal interactions chemically or by an external field has led to a great variety of model systems. Recently, our group and others presented a new model system, consisting of oppositely charged colloids that form equilibrium crystals [1,2]. A profound difference with atomic systems is that ionic colloidal crystal structures are not dictated by charge neutrality as the charge balance is covered by the presence of counterions. This severs the link between charge ratio and stoichiometry and enlarges the number of possible crystal structures. Predicting these is a computational challenge, not only because of the overwhelming number of possible structures and system parameters (charge, size, solvent, salinity, composition, etc.) but also because of the intricate interplay between attractive and repulsive interactions, entropy, and packing effects. In this Letter, we develop a novel interactive simulation method to predict binary crystal structures of oppositely charged colloids, based on simulated annealing [3]. Employing this method, we are able to predict a whole variety of new binary crystal structures with different stoichiometries, which we used in Madelung energy calculations to map out the ground-state phase diagram. Results are presented for binary mixtures of small and large oppositely charged colloids with size ratio 0.31, corresponding to one of our experimental systems that triggered our theoretical interest [1]. The calculated phase diagram exhibits a plethora of different crystal structures, some of which have atomic and molecular analogs, while others do not. Using the theoretical predictions, we experimentally confirmed the stability of three of the crystal structures.Our experimental system consists of spherical, sterically stabilized polymethylmethacrylate (PMMA) particles [4], dispersed in a less polar solvent mixture. The negatively charged larger part...

A major problem in the practical application of antifoams (substances used to avoid undesirable foam) is the gradual loss of their activity in the course of foam destruction. Several experimental methods are combined in the present study to reveal the origin of this phenomenon, usually termed as the antifoam “exhaustion” or “deactivation”. A typical mixed antifoam, comprising silicone oil and hydrophobized silica aggregates of fractal shape and micrometer size, has been studied in solutions of the anionic surfactant sodium dioctylsulfosuccinate (AOT). The results unambiguously show that the exhaustion in this system is caused by two interrelated processes:  (1) segregation of oil and silica into two distinct populations of antifoam globules (silica-free and silica-enriched), both of them being rather inactive; (2) disappearance of the spread oil layer from the solution surface. The oil droplets deprived of silica, which appear in process 1, are unable to enter the air−water interface and to destroy the foam lamellae. On the other side, the antifoam globules enriched in silica trap some oil, which is not readily available for spreading on the solution surface. As a result, the spread layer of silicone oil gradually disappears from the solution surface (process 2) due to oil emulsification in the moment of foam film rupture. Ultimately, both types of globules, silica-enriched and silica-free, become unable to destroy the foam films, and the antifoam transforms into an inactive (exhausted) state. The introduction of a new portion of oil (without any silica) on the surface of an exhausted solution results in a perfect restoration of the antifoam activityreactivation of the antifoam. The experiments show that the reactivation process is due to restoration of the spread oil layer and to rearrangement of the solid particles from the exhausted antifoam with freshly added oil into new antifoam globules having optimal silica concentration. The results provide deeper insight into the mechanisms of antifoam action and suggest ways for improving the antifoam efficiency and durability.

Solution-phase pyridine treatment displaced oleic acid capping ligands from the surface of PbSe nanocrystals. During ligand displacement the nanostructure morphology dramatically changed from well-stabilized, individual nanocrystals to form crystallographically connected nanocrystal networks. The network morphology was governed by the diameter of the constituent nanocrystals. Larger nanocrystals showed dipolar alignment but maintained individual nanocrystal character, while smaller nanocrystals crystallographically fused along the <100> axis. Optical studies of nanocrystal thin films showed that pyridine ligand displacement quenches the nanocrystal photoluminescence. Blends of nanocrystals and conjugated polymer showed photoluminescence quenching of the polymer with increasing nanocrystal content. The extent of photoluminescence quenching was only weakly dependent on the nanocrystal size or surface chemistry, suggesting that the interaction between nanocrystal and polymer is mostly in the form of energy transfer rather than charge transfer.