engleski

The precipitation of calcium carbonate polymorphs under the influence of polyamino acids

Calcium carbonate can precipitate from electrolyte solutions as six distinct polymorphs and hydrates: calcite, aragonite, vaterite, calcium carbonate hexahydrate, calcium carbonate monohydrate or amorphous calcium carbonate. Formation of a certain modification, its morphological characteristics and crystal habit, is governed by different parameters among which the influence of the initial reactant concentration, temperature and the presence of additives are the most relevant [1]. At standard conditions of temperature, calcite is the most stable phase. In the calcified biological systems, calcium carbonate has been found predominantly as calcite and aragonite or less commonly as vaterite and amorphous calcium carbonate. During the process of calcium carbonate formation in biological systems (biomineralization), the organic matrix plays a crucial role in determining precipitation of the particular calcium carbonate polymorph [2]. Organic matrix consists of a structural framework of insoluble macromolecules (hydrophobic proteins, chitin) onto which hydrophilic macromolecules, the so-called acidic macromolecules, are anchored. The hydrophilic macromolecules are proteins rich in aspartic acid and glutamic acid residues, usually adopting β -pleated sheet conformation, which acts as a surface on which the nucleation of mineral phase occurs. Negatively charged carboxylic groups from the side chains of these acidic macromolecules are assumed responsible for nucleation of the particular calcium carbonate polymorph. In this work the influence of poly-L-aspartic acid (pAsp), poly-L-glutamic acid (pGlu) and poly-L-lysine (pLys), the synthetic molecules of known composition and structure, on the crystal growth of calcite is investigated. The experiments were initiated by introduction of well defined calcite seed crystals into supersaturated calcium carbonate solution (γ calc = 1.0 g dm-3, c0(Na2CO3) = c0(CaCl2) = 2.5×10-3 mol dm-3, Vtot=200 cm3, t = 25 &ordm ; ; C). An appropriate amount of additive (pAsp, pGlu, pLys) was previously added into the solution. The advance of calcite crystal growth was followed by recording pH as a function of time (Fig. 1). In the control system (cad = 0 &micro ; ; mol dm-3) pH drops from the initial value pH = 10.5 to pH = 9.3 (quasi equilibrium) after approximately 30 minutes. In the cases when different concentrations of acidic polyamino acids were added, the corresponding retardation of pH drop was observed. At higher concentrations of pAsp (cad > 50 &micro ; ; mol dm-3), the crystal growth of calcite was completely inhibited, as indicated by apparently constant value of pH. Relatively fast pH change, observed in these systems after approximately 100 minutes, corresponds to the subsequent nucleation and crystal growth of vaterite. In contrast to the inhibition effect of acidic macromolecules, pLys had apparently no influence on calcium carbonate crystal growth kinetics, as was seen from the corresponding progress curves. Scanning electron micrographs of the calcite crystal seeds, isolated from the precipitation system containing different amount of pAsp, are shown in Fig. 2. By the addition of pAsp the rhombohedral calcite crystals became rounded. At the highest concentration of pAsp used, the spherulites of subsequently nucleated vaterite were observed. Decrease of the inhibition efficiency observed (InhpAsp> InhpGlu >> InhpLys) could be explained by the decreasing adsorption affinity of the respective polyamino acids that could be, again, correlated with their secondary structure: pAsp adopts predominant β -sheet, pGlu adopts partial β -sheet while pLys adopts predominant random coil conformation. a) b) c) d) Fig 1. Change of pH during the crystal growth of calcite initiated by introduction of seed crystals into the metastable calcium carbonate solution. The initial concentrations of respective polyamino acids are indicated. Fig 2. Scanning electron micrographs of calcite seed crystals isolated from the precipitation system into which pAsp was added. Concentrations of added pAsp are a) 0 &micro ; ; mol dm-3, b) 6 &micro ; ; mol dm-3, c) 43 &micro ; ; mol dm-3 and d) 70 &micro ; ; mol dm-3pAsp9.29.49.69.810.010.210.410.60501001502000 &micro ; ; mol dm-36 &micro ; ; mol dm-310 &micro ; ; mol dm-313 &micro ; ; mol dm-370 &micro ; ; mol dm-3pGlu9.29.49.69.810.010.210.410.6050100150200pH9 &micro ; ; mol dm-30 &micro ; ; mol dm-315 &micro ; ; mol dm-362 &micro ; ; mol dm-3pLys9.29.49.69.810.010.210.410.6050100150200t/min0 &micro ; ; mol dm-355 &micro ; ; mol dm-34 &micro ; ; mol dm-316 &micro ; ; mol dm-3REFERENCES [1] Lj. Brečević and D. Kralj, Kinetics and Mechanisms of Crystal Growth in Aqueous Systems (chapter 8), from: Interfacial Dynamics (N. Kallay, editor) Marcel Dekker, New York, 2000. [2] A. Bigi, G. Falini, M. Gazzano, N. Roveri and A. Ripamonti, Chim. Ind. 80 (1998) 615-621

calcium carbonate; poly-L-aspartic acid; poly-L-glutamic Acid; poly-L-lysine; precipitation

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