engleski

Investigation of vaterite crystal growth mechanism in the presence of poly-L-aspartic and poly-L-glutamic acids

The preparation of new materials with superior mechanical properties has long been a challenge for materials scientists. Increasingly, materials scientists are focusing on emulating biomineralization processes in so-called biomimetic approaches where the principles of controlled crystal growth in living organisms are applied in the laboratory with the aim of synthesising new materials, with enhanced physical-chemical or mechanical properties. Molluscs’ shells are example of natural biocomposites in which calcium carbonate (CaCO3) is the principal mineral component. The most extensively deposited CaCO3 biominerals are calcite and aragonite, of which calcite is thermodynamically the most stable polymorph. Vaterite, the least stable polymorph of CaCO3, is not as wide-spread in nature as the other two polymorphs are. Its occurrence in nature is often associated with biogenic activities as in hard tissues of fish otoliths. It is supposed to be an important precursor in several calcium carbonate forming processes. Therefore, the studies of vaterite nucleation, growth, and stabilization offer an opportunity to understand the mechanisms of biomineralization and also the processes of crystal nucleation and growth in general. In this work, poly-L-aspartic (pAsp) and poly-L-glutamic (pGlu) acids were chosen as simple analogues to the natural acidic proteins. In order to study the specific polyamino acid/mineral surface interactions, the kinetics of vaterite seed crystal growth was determined in the precipitation systems containing different concentrations of polyamino acids. The experiments were initiated by pouring CaCl2 solution into the same volume of Na2CO3 solution containing vaterite seed crystals. An appropriate polyamino acid concentration was also added to the carbonate component. All experiments were carried out at 25 °C. The propagation of the precipitation process was continuously followed by measuring the pH of the solution. Obtained precipitates were analysed by means of PXRD, FT-IR and SEM. Different theoretical models (parabolic and exponential rate law) were tested and indicated that at given conditions vaterite growth in the model system (system without any additives) and systems with the lower concentrations of additives (c(pAsp or pGlu) < 0.55 ppm) were controlled by the addition of constituting ions into the spiral step emerging from the surface dislocation. Addition of pAsp and pGlu also caused the growth rate inhibition, but at the highest concentrations of used polyamino acids (c(pAsp or pGlu) > 0.7 ppm) the rate controlling mechanism changed as well: it changed from the growth controlled by integration of growth units into the spiral step (parabolic law) to the surface nucleation controlled growth (exponential law).

vaterite ; crystal growth ; biomineralization

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