Microscopic particles dispersed in solvents — known as colloidal suspensions — are attractive models for studying a wide range of phase transitions and nucleation phenomena. The suspended particles can be observed directly in three-dimensional space, and the interactions between them are easily modified. However, processes involving that seemingly most simple and ubiquitous of inorganic solids — ionic crystals formed from oppositely charged atoms — cannot be mimicked by colloids. Charged colloidal particles don't ‘do ionic’; they prefer to aggregate instead.
But Mirjam E. Leunissen and colleagues, writing in this issue (Nature 437, 235–240; 2005), show that colloids can be coaxed into forming ionic crystals after all. The authors observed that if salt is added to polymer spheres dispersed in an organic solvent mixture, the charge on the spheres can be controlled and the electrostatic interactions between them can be regulated. This enabled the preparation of binary mixtures of colloids that carried small, opposite charges and readily formed ionic crystals. When an electric field was applied, the crystal melted, and particles of opposite charge moved towards opposite electrodes.
The charged colloidal particles therefore clearly resemble ionic species. But there are differences. In particular, a diffuse layer of ‘counter-ions’ surrounds each particle, forming an overall charge-neutral unit that participates in the growth of the crystal. So the structure of the colloidal crystals is not dictated by charge neutrality, as in atomic systems, leaving the authors free to create remarkable new binary structures. One example, a crystal comprising particles of positive (green, radius 0.36 μm) and negative (red, radius 1.16 μm) charge in the ratio 6:1, is shown in the image.
Colloidal crystals can also form from charged spheres made of different materials, such as a polymer and silica. It is then straightforward to burn the polymer spheres away to give all-silica structures. Given the ease with which these structures grow into large crystals, ionic colloids should prove an alluring proposition for those creating advanced materials such as photonic crystals.
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Helmer, M. Isn't it ionic. Nature 437, 206 (2005). https://doi.org/10.1038/437206a