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Molecular Dynamics Simulations of Preferential Adsorption of Tartrate on Calcite Surfaces

Precipitated calcium carbonate (PCC) is a synthetic mineral widely used in various technologies, mainly as multifunctional filler or pigment (paper, plastics, pharmaceuticals, foods). The application of PCC strongly depends on its morphology and size distribution. The control and design of a desired crystal morphology and size can be achieved by employing specific additives during crystal growth processes. The organic- mediated crystal growth control of calcium carbonate is also important in biomineralization. Experimental studies have shown that small organic molecules, such as dicarboxylates or amino acids, can be used as effective regulators of calcite crystal morphology. Previous experimental results indicated specific interactions of tartaric acid with newly stabilized {; ; 1-10}; ; calcite surfaces, thus modulating the crystal morphology.1 However, the atomic-level mechanisms remain unclear. Molecular dynamics (MD) simulations can provide useful information at this level. Recent simulation studies have mainly focused on better understanding of the most stable rhombohedral (104) calcite face/solution interface, seldom including the presence of organics. 2, 3 In this work, the adsorption behavior of tartrate on calcite surfaces was studied employing MD simulations to understand additive-mediated crystal growth. The free energy landscapes for the adsorption and desorption of tartrate are calculated using metadynamics4. By this approach, the adsorption energetics of favourable conformations, orientations and positions of the tartrate near the (104) and (1-10) calcite surfaces was determined. The obtained results provided a molecular-level explanation of the experimentally observed stabilized exposure of {; ; 1- 10}; ; surfaces during calcite growth. Reorientation and conformational change of tartrate as well as interfacial water molecules played a significant role. The simulations showed that tartrate preferentially adsorbed directly to the (1-10) calcite surface, whereas tartrate was more loosely adsorbed on the (104) surface by both direct and solvent-mediated binding. The (1-10) geometry of calcite surface sites closely matches the structure of tartrate, with a specific role of carboxylate and hydroxyl groups in recognising the calcium and carbonate ions, respectively. The results indicated that preorganization of both hydroxyl and carboxylate functional groups plays a major role in the strength of the interactions and hence in the expression of the effects in calcite morphological appearance. Preferential adsorption of tartrate on {; ; 1-10}; ; surfaces could stabilize these, otherwise unstable, faces and thus inhibit crystal growth in {; ; 1-10}; ; directions. Our future work will focus on simulating the growth from supersaturated solutions of specific fast-growing calcite faces, such as (1-10), in the presence of organic additives. References [1] Ukrainczyk, M., Stelling, J., Vučak, M., Neumann T., J. Cryst. Growth 2013, 369, 21–31. [2] Ratieri, P., Gale, J.G., Quigley, D., Rodger, P.M., J. Phys. Chem. C, 2010, 114, 5997–6010. [3] Shen, J., Li, C. van der Vegt, N., Peter, C. J. Phys. Chem.C 2013, 117, 6904–6913. [4] Barducci, A., Bonomi, M., Parrinello, M. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2011, 1, 826– 843.

Molecular Dynamics; Crystal growth; Adsorption; additives; tartrate

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