Control of inorganic morphologies by organic templates
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Control of inorganic morphologies by organic templates John H. Harding and Dorothy M. Duffy, Department of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, U.K. ABSTRACT Experiments have shown that ordered monolayers of large organic molecules can control crystal nucleation and growth. The work of nucleation controls which morphology and phase will appear. We have used molecular dynamics to simulate the interfaces between stearic acid and a number of calcium carbonate surfaces. An important issue is the ionisation state of the acid which depends on the pH of the solution at the surface. We estimate this by solving the nonlinear Grahame equation for a calcium carbonate solution. The species in solution are chosen from geochemical models. The calculated interfacial binding energy between monolayer and material is often of the order of 1 Jm-2, suggesting that the nucleation rate is strongly enhanced. INTRODUCTION Many organisms can control the nucleation and growth of minerals, using this to obtain complex morphologies and unusual phases. The structures of bones, teeth, shells and scales are the most obvious examples [1]. A major emerging theme is the use of organic ‘scaffolding’ to act as a template for the mineral growth. A number of workers have studied this using simpler systems as the template; for example Langmuir monolayers [2,3], self-assembled monolayers [4,5] or complex polymers [6]. Complete control has not been achieved, although some useful trends are emerging. For example, Langmuir monolayers with carboxylate heads favour growth of the (10.0) calcite face whereas phosphate and sulphate groups favour (00.1). This can be understood using simple matching arguments for the local geometries of the functional groups. However, this does not explain cases where one face is strongly favoured over another even though the quality of matching is apparently comparable. We have used molecular modelling methods to study the interfaces between calcium carbonate crystals and monolayers of stearic (octadecanoic) acid. These make good model systems because the density and charge state of the monolayer can easily be controlled by varying the surface pressure and the sub-phase pH [3]. Molecular modelling enables us to study the behaviour of interfaces in detail. Moreover, it should enable us to calculate the critical work of nucleation for a given phase or morphology and to predict, within the limits of classical nucleation theory, which phase should be observed. The presence of a substrate will alter the morphology of a crystal by lowering the interfacial energy of the face in contact with the substrate. The Wulff-Kaishew theorem [7] states that the distance from the centre to the crystal face (hk) on the substrate is reduced by the ratio of the interfacial binding energy ( β k ) to the corresponding surface energy ( σ k ). hk (substrate present ) β = 1− k hk (homogeneou s ) σk
(1)
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This paper thus proceeds in three parts. We first consider the state of ionisation of the monolaye
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