Additive-induced morphological tuning of self-assembled silica-barium carbonate crystal aggregates

Abstract

Crystn. of barium carbonate from alk. silica solns. results in the formation of extraordinary micron-scale architectures exhibiting non-crystallog. curved shapes, such as helical filaments and worm-like braids. These so-called "silica biomorphs" consist of a textured assembly of uniform elongated witherite nanocrystallites, which is occasionally sheathed by a skin of amorphous silica. Although great efforts have been devoted to clarifying the phys. origin of these fascinating materials, to date little is known about the processes underlying the obsd. self-organization. Herein, we describe the effect of two selected additives, a cationic surfactant and a cationic polymer, on the morphol. of the forming crystal aggregates, and relate changes to expts. conducted in the absence of additives. Minor amts. of both substances are shown to exert a significant influence on the growth process, leading to the formation of predominantly flower-like spherulitic aggregates. The obsd. effects are discussed in terms of feasible morphogenesis pathways. Based on the assumption of a template membrane steering biomorph formation, it is proposed that the two additives are capable of performing specific bridging functions promoting the aggregation of colloidal silica which constitutes the membrane. Morphol. changes are tentatively ascribed to varying colloid coordination effecting distinct membrane curvatures.