Effect of Bicarbonate Ions on the Crystallization of Calcite on Self-Assembled Monolayers Dorothy M. Duffy, † A. Markus Travaille, ‡ Herman van Kempen, ‡ and John H. Harding* ,†,§ Department of Physics and Astronomy, UniVersity College, London, Gower Street, London WC1E 6BT, United Kingdom, Institute for Molecules and Materials, Radboud UniVersity Nijmegen, ToernooiVeld 1, 6525 ED Nijmegen, The Netherlands, and Department of Engineering Materials, Sir Robert Hadfield Building, UniVersity of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom ReceiVed: NoVember 28, 2004; In Final Form: January 17, 2005 We use molecular dynamics simulations to investigate the nucleation of calcite crystals on self-assembled monolayers. We show how the presence of bicarbonate ions adsorbed on the monolayer surface can both aid nucleation and control the orientation of the growth of the crystal. Using a simple model of the nucleation process and calculated interfacial energies, we calculate the enhancement (with respect to the homogeneous nucleation rate) of the nucleation of calcite on the (011 h2) and (0001) faces. The calculations show clearly that the (011 h2) face is favored over the (0001) face and that the nucleation rate is enhanced for self-assembled monolayers made from molecules containing an even number of carbon atoms in the alkyl chain over those containing an odd number. Introduction Self-assembled monolayers (SAMs) of alkanethiols, with functionalized headgroups, have been widely used as substrates for the crystallization of minerals. Such experiments attempt to reproduce the remarkable control over crystal orientation exhibited by organisms in biomineralization. While complete control of crystal orientation has not been achieved in the laboratory, limited control is feasible. One notable result is the behavior of calcium carbonate growing on self-assembled monolayers (SAMs) of alkanethiols terminated by carboxylic acid groups, themselves on a gold substrate. Here, calcium carbonate exhibits a strong tendency to nucleate as calcite, growing on the (011 h2) plane. Moreover, this effect depends on the length of the alkyl chain. Chains with an even number of carbon atoms produce (011 h2)-oriented growth; chains with an odd number of carbon atoms produce either no oriented growth or growth on a different crystal face. 1 The prevalence of the (011 h2) calcite orientation on SAMs has been attributed to templating and matching of the headgroup orientation with the carbonate angle orientation. This explanation is not without problems. The (011 h2) surface of calcite is com- mensurate with the SAM in one direction only, and there is a significant lattice mismatch in the perpendicular direction. The (0001) orientation, however, is completely commensurate with the SAM, and thus we might expect the SAM to nucleate calcite with this orientation. Although both (0001) and (011 h2) are polar surfaces in calcite (and therefore can only be present if the inter- facial structure can cancel the macroscopic bulk dipole 2 ), the (011 h2) interface with the SAM must also accommodate the lat- tice mismatch. We have previously shown 2 that both conditions can be satisfied by the introduction of line defects at the calcite/ SAM interface. These defects can accommodate the lattice mismatch, but their formation energy is rather high, such that the energy of the (011 h2) calcite/SAM interface is higher than that of the (0001) calcite/SAM interface. We would, therefore, expect nucleation in the (0001) orientation. In this paper, we present an alternative model for the (011 h2) calcite/SAM interface in which the mismatch is accommodated using bicarbonate ions. Since the experimental solutions are supersaturated with these ions, it is reasonable to expect them to play a role in the nucleation and growth of a carbonate mineral. In the model we shall discuss, the bicarbonate ions are incorporated into the plane of the SAM along lines parallel to the commensurate direction. The ions compress the monolayer molecules, resulting in both a good lattice match and zero net dipole moment. In this paper, we investigate both the odd-even effect and the preferential nucleation of the (011 h2) orientation over the (0001) orientation. We model thin films of calcite in the (011 h2) and (0001) orientations on both 16-mercaptohexadecanoic acid (MHA) and 15-mercaptopentadecanoic (MPA) monolayers, with both carbonate and bicarbonate terminations. We calculate angu- lar order parameters for the thin film for the four interfaces con- sidered. With the help of a simple model of heterogeneous nucle- ation theory and calculations of the relevant interfacial energies, we show the influence of interfacial energy on the nucleation barrier. The results demonstrate that bicarbonate ions play a very significant role in determining the orientation of calcite growth on SAMs. Methods and Procedures We model interfaces between the flat polar calcite surfaces, (0001) and (011 h2), and the 3 ×3 R30° configuration of MHA and MPA on gold, to study both the odd-even effect and the prevalence of (011 h2) calcite growth on SAMs. The (0001)/SAM interface is coherent because the calcite surface structure closely matches the hexagonal SAM structure, but the (011 h2)/SAM interface is coherent in one direction only. The surface unit cell of the (011 h2) face of calcite is 4.97 Å × 6.63 Å, and that of the monolayer is 4.97 Å × 8.61 Å. In order for crystal growth to occur in a polar crystal direction, it is necessary to cancel the macroscopic dipole moment induced by crystal planes of alternating charge. This is normally achieved in simulations by terminating the crystal with a layer with a charge density half that of the crystal planes. In the case of the (0001) surface, the dipole is canceled by the fully ionized mono- * Author to whom correspondence should be addressed. E-mail: j.harding@ sheffield.ac.uk. † University College, London. ‡ Radboud University, Nijmegen. § University of Sheffield. 5713 J. Phys. Chem. B 2005, 109, 5713-5718 10.1021/jp044594u CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005