Nature has demonstrated how it is possible to use heterogeneous nucleation and growth to achieve complete control over crystallisation -from orientation, size, and morphology to choice of polymorph. Coccolithophores, for example, grow intricate microcrystals of calcite (coccoliths), with a remarkable range of shape and size. We want to understand the processes involved, and to use this understanding to gain total control over crystal nucleation and growth. The initial part of the project will focus on calcium carbonate, because of its relevance to biomineralisation, industry and natural reservoirs for oil and drinking water, but the methods developed will be applied to other industrially, technologically and medically important materials (calcium phosphate and iron oxide for example).
Crystallisation in "real" situations almost invariably occurs in direct association with a surface - which may be rough rather than smooth, curved rather than planar, mobile rather than static - and frequently occurs in confinement rather than in bulk solution. The substrate surface influences the orientation of the crystal nucleus, and even which polymorph is favoured, but these effects have never been studied in detail before. The influence of the surface is even stronger when crystallisation occurs in confinement, such as inside a cell membrane or vesicle. In this project we will study crystallisation at surfaces and in confinement with a combined modelling and experimental programme. The results will enable us to come closer to understanding, and replicating, biomineralisation than ever before.
The major challenges in this theme are the identification and understanding of the effects of the substrate properties and confinement on crystallisation and application of this knowledge to give control over crystallisation orientation and polymorph. Crystallisation generally occurs on surfaces and the surface properties (structure, charge density, curvature and flexibility) are known to influence which crystal face forms. This surface control is often referred to as templating, which implies expitaxial or superstructure matching of the surface structure to crystal face at some level, but it is clear from experiment that simple matching rules generally fail. Polar crystal faces tend to nucleate on charged substrates but the charge density of the crystal surface does not generally match that of the substrate. It has been suggested that stereochemical matching plays a more significant role than epitaxy in nucleation face selection1and that nucleation is enhanced by substrate flexibility, or adaptability, as it permits substrate rearrangement to adjust to the growing crystal. Such complementary crystallisation has been supported by recent modelling results2(Fig.1). Confinement enhances surface effects as a large proportion of the nucleating particle is in contact with a surface rather than the solution, which affects the thermodynamics of the process. Limited reagent supply affects growth and confinement volumes close to the critical nucleus size stabilise the nucleus and increase nucleation rates. The challenge is in the separation, identification and understanding of the different effects and the application of these effects to control morphology and polymorph.
Achievements so far, June 2013
- Crystallisation on substrates.
- Modelling investigation of calcite crystallization on carboxylate terminated SAMs with a range with a range of lengths.
- Models of benzoic acid terminated SAMs and their interactions with calcite surfaces for comparison with experimental measurements at LLNL.
- An ab initio modelling investigation of the effect of pH on the protonation of water and bicarbonate on calcite surfaces.
- Crystallisation in confinement.
- The characterization of the rotation of the lattice vectors in calcite nanorods crystallised in confinement
- The identification of the origin of the rotation as combination of intrinsic stress and crystalline anisotropy
- The development of a methodology for calculating local stress fields due to defects and inhomogeneities using atomistic modelling.
- Models of high temperature crystallization in nanocylinders and the calculation of relative growth rates of different surfaces.
- The calculation of intrinsic stress field of the nanorods crystallized in confinement.
Figure 1. Metadynamics simulation of calcite crystallisation on a self assembled monolayer
Figure 2Glass beads were exposed to slightly supersaturated solutions of CaCl2and NaHCO3. Vaterite transformation to calcite was favoured in the bead contact areas,where the volume was smaller (Stipp et al, in preparation)