Combined Influence of Organic Additives on Growth Morphology of Calcium Carbonate

INTRODUCTION

The aesthetic control over structures and properties of biominerals produced in nature inspired chemists, biologists and materials scientists since a long time^1-3. Mimicking the design and synthesis of biominerals by living organisms is the current challenge for scientists^4-10.Calcium carbonate is among the most studied biominerals because of its high abundance and ubiquity in nature. It is well established now that in case of calcium carbonate biominerals, templates with carboxylate groups are likely to direct structure through carboxylate moieties occupying carbonate sites during crystal growth and thereby directing the structure to produce the desired shape and functionality^11,13. Our previous works have shown that by varying the concentration, it is possible to synthesize different polymorphs of calcium carbonate using dicarboxylic acids such as malonic acid, succinic acid etc as templating agents^13-14. Another polycarboxylate was employed to synthesize calcite-micro trumpets^{14, 15-16} which closely mimic the coccoliths of an unicellular algae, Discosphaera tubifera. In nature, living organisms use a organic matrix containing mixed functionalities for controlling the growth, size and morphology of biominerals.  Hence we have attempted to investigate the influence of mixed additives such as β-cyclodextrin (β-CD), N-(2-hydroxyethyl) ethylene diamine-N,N’,N’-triaceticacid  (HEDTA), L-aspartic acid (L-ASP) and L-glutamic acid (L-GLU) (Scheme 1) on growth morphology of calcium carbonate and reported in this paper.

 

Scheme 1: (a) β-cyclodextrin(b) N-(2-hydroxyethyl) ethylene diamine-N,N’,N’-triaceticacid (c) L-aspartic acid(d) L-glutamic acid. Click here to View Scheme

 

EXPERIMENTAL

Materials

All chemicals were of analytical grade and used without any further purification. CaCl2.2H2O, (NH4)2CO3, L-aspartic acid, L-glutamic acid and NaOH were obtained from Merck, N-(2-Hydroxyethyl) ethylenediamine-N,N’,N’,-triaceticacid (HEDTA) and β- cyclodextrin (β-CD) were obtained from Sigma-Aldrich and Himedia chemicals, respectively.

Synthesis

The mineralization of calcium carbonate was carried out by a slow CO2 gas diffusion method. 0.1136g β-CD (0.1 mmol) taken along with the 0.147g CaCl2.2H2O (1.0 mmol) in a glass beaker containing 10ml distilled water and dissolved the solids by stirring thoroughly with the help of magnetic stirrer. Then the pH of the solution was adjusted to 7.0with the help of dil. NaOH. The final solution was transferred into the cell culture dish and covered with parafilm. 2 to 3 holes were made on parafilm and placed in the top chamber of dessicator.Carbon dioxide released from ammonium carbonate, placed in the bottom chamber of dessicator, entered into reaction mixture. After 24 hours at 25±1 oC, crystals were filtered, washed with distilled water and dried in dessicator. Same experimental procedure was followed for the influence of HEDTA, L-asparticacid and L-glutamicacid. In place of 0.1 mmol β-CD, 0.0278g HEDTA (0.1 mmol) or 0.0133g L-ASP (0.1 mmol) or 0.0147g L-GLU (0.1 mmol) was taken for the crystallization of calcium carbonate in presence of these additives. To examine the mineralization of calcium carbonate in presence of combined additives, 0.1 mmol β-CD) with 0.1 mmol HEDTA/0.1 mmol L-ASP /0.1 mmol L-GLU were mixed in place of single additive.

Characterization methods

Perkin-Elmer Spectrum Two FT-IR Spectrometer and PANALYTICAL X’Pert pro powder diffractometer system operating with monochromated Cu Kα₁ radiation were used for phase characterization. FEI Quanta 200 FEG with EDS scanning electron microscope was used to obtain the SEM images.

RESULTS AND DISCUSSIONS

 Morphology

 It was observed that the thermodynamically most stable calcite rhombohedral phase formed exclusively in control experiments i.e., without using any additive (Fig. 1). The shape and phase controlling influence of the additive on the formation of CaCO₃has been investigated at different molar ratios of [Ca²⁺]/[additive] at 50, 20, 10 at  room temperature 21.0±1 ^oC at solution pH 7.0. But best resulted obtained at [Ca²⁺]/[additive] = 10 were presented in this paper

 

Fig. 1. SEM Image of calcite and vaterite phases of calcium carbonate produced in control Click here to View Figure

 

Polysaccharides are known to control the growth of calcite in coccoliths (unicellular algae) by preferential adsorption onto particular surfaces of the calcite crystal15. Hence, the influence of cyclic polysaccharide, β-CD on the crystallisation of calcium carbonate was studied. β-CD is a truncated conical molecule with a hollow cavity. Chemically, the cavity and the inside of the cone are hydrophobic and the outside of the cone is hydrophilic. 25-50µmsized regular rhombohedral crystals transformed to truncated rhombohedral morphology by the influence of β-CD (Fig. 2a). Similar effect was observed in case of HEDTA (Fig. 2b). 15µm to 30µm sized rhombohedral sub-crystals on aggregate faces were resulted with the influence of L-ASP(Fig. 2c). It was observed that vaterite also formed in small quantity beside the major calcite phase.Truncated rhombohedral calcite crystals (25-50µmsize) along with more amorphous vaterite resulted with the influence of L-GLU (Fig. 2d).

 

Fig. 2. SEM Image calcite and vaterite phases of calcium carbonate produced in control (a) and with the influence of β-CD (b), HEDTA (c), L-ASP (d), and L-GLU (e). Click here to View Figure

 

Further, cooperative influence of β-CD with other acidic carboxylate organic moleculeslike HEDTA, L-ASP, L-GLU on the growth morphology of calcium carbonatewas investigated.  In general, low molecular organic additives make achoice for biomimetic production of CaCO₃minerals. These additives are selectively adsorb on growing faces of minerals and change the morphology. It is important to mention that, the nature and number of acidic functional groups on additive will play a key role incontrolling the crystal growth.It was assumed that, mixed component of β-CD and HEDTA are adsorbed on to the {1-10} faces of calcite and inhibited the growth of thesefaces, leading to the formation of elongated hexagonal calcite crystals (Fig. 3a and 3b).Similarly β-CD with L-ASP directed the formation of truncated rhombohedral calcite with complete agglomeration morphology (Fig. 3c). Again,calcite with rhombohedral sub crystals on aggregate faces resultedwith the cooperative influence of   β-CD and L-GLU on calcium carbonate precipitation (Fig. 3d).

 

Fig. 3. SEM images calcite produced with the cooperative influence of (a) β-CD and HEDA, (b) magnified image of calcite crystal (c) β-CD and L-ASP, (d) β-CD and L-GLU. Click here to View Figure

 

X-ray diffraction analysis

The structure and composition of the obtained products wascharacterized through Powder XRD analysis. XRD patterns of CaCO3 crystals obtained in the absence of additive (control) and in the presence of L-GLU are presented here (Fig. 4). The characteristic diffraction peaks at 23.09, 29.40, 36.00, 39.43, 43.18and 48.52 in XRD spectrum indicate the formation of calcite phase (Fig. 4a). As shown in Fig. 4b additional diffraction peaks at 21.0, 24.82, 27.0, 32.68 in XRD spectrum indicate the formation of vaterite phase in addition to calcite phase with influence of L-GLU.

 

Fig.4.Powder XRD patterns of (a) CaCO3 produced in control experiments (b) CaCO3 produced in the presence of β-CD and L-GLU (c = calcite, v = vaterite). Click here to View Figure

 

 IR analysis

IR analysis of CaCO₃ crystals nucleated in the absence of additives is shown in Fig. 5a. The spectrum confirmed the formation of calcite phase with characteristic absorption bands observed at 711.7, 874 .5 and 1414.1 cm^-1, corresponding to the n₄, n₂ and n₃ carbonate absorption bands of calcite. Both calcite and vaterite phases were obtained with the influence of L-GLU on crystallization of CaCO₃. As shown inFig. 5b, characteristic peaks at 744, 1087 corresponding to the n₄ andn₁ carbonate absorption bands of vaterite phase of CaCO₃.

 

Fig. 5. FT-IR spectrum of (a) CaCO3 produced in control experiments (b) CaCO3 produced in the presence of β-CD and Aspartic acid. Click here to View Figure

 

Probable mechanism of calcium carbonate crystal growth

A probable mechanism is being proposed for the formation of new calcite crystal morphology under the influence of organic additives. In the initial stage of mineralization, organic additive in the reaction mixture binds with Ca²⁺ions on the growing CaCO₃particles and subsequently inhibit the regular array of Ca²⁺and CO₃^2-required for the formation of mineral.In the next step, additives can influence the growth along {104} face and promote the formation of steps and result in truncation and sometimes in the aggregation of crystals.

CONCLUSIONS

 In conclusion, it is noteworthy to mention that the organic additives such as β-cyclodextrin, N-(2-hydroxyethyl) ethylene diamine-N,N’,N’-triaceticacid, L-aspartic acid and L-glutamic acid  can preferentially attached on the surface of growing phase and morphology of CaCO₃.  Calcium carbonate with truncated, agglomerated, hexagonal shapes was observed with the influence of the above said additives. Further, the cooperative influence of two additives such as β-CD with either HEDTA or L-ASP or L-GLU was investigated.

ACKNOWLEDGEMENTS

We sincerely acknowledge the SAIF, IIT, Madras for assistance with SEM measurements.

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