Porous crystals in charged sphere suspensions by aggregate-driven phase separation

Abstract

The kinetics of phase transition processes often governs the resulting material microstruc-ture. Using optical microscopy, we here investigate the formation and stabilization of a po-rous crystalline microstructure forming in low-salt suspensions of charged colloidal spheres containing aggregates comprising some 5-10 of these colloids. We observe the transformation of an initially crystalline colloidal solid with homogeneously incorporated aggregates to indi-vidual, compositionally refined crystallites of perforated morphology coexisting with an ag-gregate-enriched fluid phase filling the holes and separating individual crystallites. A prelimi-nary kinetic characterization suggests that the involved processes follow power laws. We show that this route to porous materials is neither restricted to nominally single component systems nor to a particular microstructure to start from. However, it necessitates an early rapid solidification stage during which the aggregates become trapped in the bulk of the host-crystals. The thermodynamic stability of the reconstructed crystalline scaffold against melt-ing under increased salinity was found comparable to that of pure phase crystallites grown very slowly from a melt. Future implications of this novel route to porous colloidal crystals are discussed.