Ionic colloidal crystals of oppositely charged particles

Abstract

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.

Colloids with valence and specific directional bonding

“…Because the self-assembly of these particles is largely controlled by their geometry, only a few relatively simple crystals have been made: face-centered and body-centered cubic crystals and variants 9 . Colloidal alloys increase the diversity of structures [10][11][12] , but many structures remain difficult or impossible to make. For example, the diamond lattice, predicted more than 20 years ago to have a full 3-dimensional photonic band gap 13 , still cannot be made by colloidal self-assembly because it requires 4-fold coordination.…”

Lyotropic Smectic B Phase Formed in Suspensions of Charged Colloidal Platelets

“…3−5 Subsequently, these studies were extended to colloids with anisotropic shapes, such as rods and plates, 6,7 and also to a variety of colloidal interactions. 8 Attraction (depletion, 9−12 van der Waals, 13,14 or Coulomb 13,15 ) and/or repulsion (steric 16 or Coulomb 17 ) were applied to govern the colloidal self-assembly process. Additionally, recognition mechanisms based on particles with complementary shapes 18 or on Watson−Crick attraction between DNA strands 19−22 have led to an extended control over the self-organization process.…”

Dispersion polymerization in non-polar solvent: Evolution toward emerging applications

“…In a particular case, the crosslinked core was subsequently used as a seed to grow the particles further in the absence of the dye to obtain a fluorescent core-nonfluorescent shell structure allowing for better particle visualization using confocal microscopy. [43,44] For instance, such particles have been used for efficiently tracking single particles in colloidal crystals ( Figure 4) [41][42][43][44]46], and more recently in clusters of controlled number of particles [45] and in structures assembled upon applying an electric field.…”