Colloidal suspensions are widely used to study processes such as melting, freezing1,2,3 and glass transitions4,5. 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 space3,4. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive1,3, short-range attractive5, hard-sphere-like2,3,4 and dipolar3, 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 aggregation6. 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 applications7.
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We thank R. P. A. Dullens, D. Derks, N. A. M. Verhaegh, P. Vergeer and C. M. van Kats for particle synthesis, A. D. Hollingsworth for solvent characterization, J. H. J. Thijssen for help with the pictures of the Bragg reflections and both P.N. Pusey and A.B. Schofield for pointing out the resemblance between our LS6-structure and certain fullerene compounds. This work is part of the research program of the ‘Stichting voor Fundamenteel Onderzoek der Materie’ (FOM), which is financially supported by the ‘Nederlandse organisatie voor Wetenschappelijk Onderzoek’ (NWO). Author contributions M.E.L. and C.G.C. investigated the phase behaviour of the experimental binary systems, A.-P.H. and M.D. performed the computer simulations, A.-P.H. and R.v.R. calculated the Madelung energies, C.P.R. worked on the lane formation, A.I. on the Bragg scattering, A.I.C. made different plus/minus systems and A.v.B. initiated the work and co-wrote the paper together with M.E.L.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Experimental details of the laser light powder diffraction measurements that we performed to determine the lattice parameter of the CsCl-type PMMA crystals. (DOC 19 kb)
Listing of the laser light powder diffraction data of the CsCl-type PMMA crystals, including the diffraction angles, the peak assignments and the calculated lattice parameter. (DOC 26 kb)
A photograph of the Bragg reflections from CsCl-type binary crystals under white light illumination, revealing complete freezing of the sample into large, differently oriented crystallites. (DOC 666 kb)
Plot of the laser light powder diffraction data of Supplementary Table 1, confirming the peak assignments and giving 2.364±0.010 m for the lattice parameter. (DOC 39 kb)
Confocal images of the CsCl-type crystal growth showing the strongly ordered initial liquid phase and the rapid, homogeneous nucleation of differently oriented crystallites. (DOC 918 kb)
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Leunissen, M., Christova, C., Hynninen, AP. et al. Ionic colloidal crystals of oppositely charged particles. Nature 437, 235–240 (2005). https://doi.org/10.1038/nature03946
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