New insights into the early stages of silica-controlled barium carbonate crystallisation

Abstract

Recent work has demonstrated that the dynamic interplay between silica and carbonate during co-precipitation can result in the self-assembly of unusual, highly complex crystal architectures with morphologies and textures resembling those typically displayed by biogenic minerals. These so-called biomorphs were shown to be composed of uniform elongated carbonate nanoparticles that are arranged according to a specific order over mesoscopic scales. In the present study, we have investigated the circumstances leading to the continuous formation and stabilisation of such well-defined nanometric building units in these inorganic systems. For this purpose, in situ potentiometric titration measurements were carried out in order to monitor and quantify the influence of silica on both the nucleation and early growth stages of barium carbonate crystallisation in alkaline media at constant pH. Complementarily, the nature and composition of particles occurring at different times in samples under various conditions were characterised ex situ by means of high-resolution electron microscopy and elemental analysis. The collected data clearly evidence that added silica affects carbonate crystallisation from the very beginning (i.e. already prior to, during, and shortly after nucleation), eventually arresting growth on the nanoscale by cementation of BaCO3 particles within a siliceous matrix. Our findings thus shed light on the fundamental processes driving bottom-up self-organisation in silica-carbonate materials and, for the first time, provide direct experimental proof that silicate species are responsible for the miniaturisation of carbonate crystals during growth of biomorphs, hence confirming previously discussed theoretical models for their formation mechanism.

Biomorph Oscillations Self-organize Micrometer-Scale Patterns and Nanorod Alignment Waves

“…In each iteration of this cyclic process, the deposition of amorphous silica limits the barium carbonate structures to nanorods, thus preventing them from growing into larger euhedral witherite crystals. 32 Our results show that the topographic patterns form in the immediate wake of the crystallization front which is in agreement with the pH oscillation model. In addition, the stripes are parallel to the growing front suggesting a spatial synchronization of these oscillations.…”

Exploring the effect of poly(acrylic acid) on pre- and post-nucleation BaSO4species: new insights into the mechanisms of crystallization control by polyelectrolytes

“…4 Our results add to these previous findings and suggest that Ba 2+ and SO 4 2− ions undergo association to form stable complexes prior to liquid-liquid separation and solid barium sulphate nucleation, as reported previously for other sparingly soluble minerals such as CaCO 3 (ref. 22 The formation of ion associates (pairs and/or larger clusters) prior to nucleation thus seems to be a widespread feature of many mineral precipitation processes. 22 The formation of ion associates (pairs and/or larger clusters) prior to nucleation thus seems to be a widespread feature of many mineral precipitation processes.…”

“…In these calibration runs, Ba 2+ potential was measured as a function of solution concentration, using identical settings as in the precipitation runs. 22)) and to quantitatively assess the multiple effects of additives on the early stages of nucleation and growth of such minerals. This results in a sustainable approximation.…”

Probing local pH-based precipitation processes in self-assembled silica-carbonate hybrid materials

“…1,23 Most probably, this behaviour is caused by the presence of silica, which may act as an efficient inhibitor for carbonate growth (thus restricting the size of the crystallites to the nanoscale) 26 and/or accelerate nucleation by lowering interfacial energies through epitaxial matching. In our opinion, this is a plausible scenario as it was shown that the bulk mother solution of biomorphs is sufficiently supersaturated throughout the entire growth process 18 and hence BaCO 3 crystallisation does actually not need a local increase in supersaturation, while this is not necessarily true for silica.…”