Turning colloidal hard spheres into binary crystals of oppositely charged particles

Sterically stabilized colloidal particles dispersed in apolar solvents are almost always slightly charged ($\sim $100's of charges). However, this does not interfere with crystallization at high volume fractions. At low volume fractions the consequences can be dramatic compared to hard-sphere behavior. We show that dispersions of \textit{oppositely }charged colloids, that readily form equilibrium phases, can be made by using a mixture of sterically stabilized PMMA spheres. In the solvent mixture used both the density and index of the spheres can be matched. Among the binary crystals already observed are large ($\sim $0.5 mm) CsCl-type crystals made up of 2 micron sized spheres of opposite charge. Our findings open up a new direction of investigation for colloidal model systems: particles with (spherically symmetric) long-range attractive interactions. It also provides new avenues for the creation of binary crystals as it enables different crystal structures and easier and faster binary crystal growth.