(166c) Colloidal Assembly By Capacitive Deionization

The response of colloids to changes in temperature, mechanical stress, and electric and magnetic fields make it possible to assemble them into ordered materials capable of magnifying the optical, magnetic, electric, and dielectric properties of their constituent particles. Similarly, because of the large surface potentials produced by the spontaneous dissociation of ionizable surface moieties, changes in salt concentration can dramatically affect the behavior of colloidal particles that are dispersed in aqueous or polar solvents. The strength, reliability, and ubiquity of this response has made it a staple of the pharmaceutical and food industries, but it has been difficult to tame this strength enough to mediate the delicate inter-particle interactions required to self-assemble ordered structures -- rather than the strong gelation that characterizes amorphous flocs and curds -- and to provide the kind of fine spatial control that may be required to form precise assemblies. Such control, moreover, would make it possible to use electrokinetic effects, which can propel ordinary colloidal particles over large distances, to drive self-assembly instead of fretting over electrokinetic instabilities that may sweep away and destroy assembled structures. Here we demonstrate a simple and scalable approach to control the salt concentration in a colloid using the supercapacitance of mesoporous electrodes. First, we show that these electrodes can push charged particles apart by pulling nearly all the ions out of the surrounding fluid: thus enabling electrostatic screening lengths several microns long in fluids with large dielectric constants. We also show how these electrodes can controllably and reversibly pull particles together by modulating the salt concentration in a dispersion of oppositely charged particles, dramatically reducing the strength of an interaction that would ordinarily cause irreversible flocculation.