Calcite mesocrystals:: Morphing crystals by a polyelectrolyte

Crystallization of calcite from different concentrated calcium chloride solutions by the CO2 vapor diffusion technique in the presence of polystyrenesulfonate (PSS) yields defined assemblies of nanoparticles with unusual morphology. From the typical calcite rhombohedra, the morphology can be systematically varied via rounded edges and truncated triangles to concavely bent lens-like shapes. Although these crystals are apparently well facetted as observed in light microscopy, electron microscopy analysis and BET isotherms reveal that the structures are highly porous and are composed of almost perfectly three-dimensionally aligned calcite nanocrystals, scaffolded to the final, partly curved structures. The formation of all mesostructures is discussed within the framework of a polymer-mediated structure-formation process, in which the polymer is acting in four different ways. The present model case also provides evidence for the importance of nonclassical mesoscopic processes in polymer-controlled crystallization in general.