Clusters, complexes and primary particles in the early stages of bioinspired mineral growth
Professor Nico Sommerdijk, Technische Universiteit Eindhoven, The Netherlands
Biominerals possess shapes, structures and properties not found in synthetic minerals. The dream of exploiting the biological principles of controlled mineral formation in materials chemistry inspires a large community to investigate the underlying mechanisms through biomimetic mineralisation experiments. The defining characteristics of biominerals arise from the interplay of the mineral with a macromolecular matrix, which directs crystal nucleation and growth. Within this three dimensional biomolecular assembly, the developing mineral interacts with acidic macromolecules, either dissolved in the crystallisation medium or associated with an insoluble templating structure.
CryoTEM has proven to be a powerful tool to investigate – with great detail – the nucleation and growth of different mineral systems, including calcium carbonate, calcium phosphate, iron oxide and silica. It also allows us to study how organic-inorganic interactions at interfaces affect the crystallisation from solution.[7-9] Interestingly we find that all these pathways involve nanometer sized building blocks that have been termed prenucleation clusters,[7-9] prenucleation complexes and primary particles.[5-6] Using time resolved cryoTEM we now also demonstrate how multistep nucleation pathways are altered through the influence of polypeptide based additives. Controlling the pathways of nucleation and growth may help us to ultimately to control the size, shape and orientation of the crystals and optimise them for specific technological applications.
 Nudelman and Sommerdijk, Angewandte Chemie-International Edition 2012, 51, 6582.
 Chem. Rev. 2008, 108, 4329.
 Pouget, et al., J. Am. Chem. Soc. 2010, 132, 11560.
 Habraken, et al., Nat. Commun. 2013, 4.
 Baumgartner, et al., Nat. Mater. 2013, 12, 310.
 Carcouet, et al., Nano Lett. 2014, 14, 1433.
 Pouget, et al., Science 2009, 323, 1455.
 Dey, et al., Nat. Mater. 2010, 9, 1010.
 Nudelman, et al., Nat. Mater. 2010, 9, 1004.
 Dey, et al., Faraday Discuss. 2015, 179, 215.
 De Yoreo, et al., Science 2015, 349, 498.
Challenges for the simulation of nucleation pathways for biominerals
Professor Julian Gale, Curtin University, Australia
The nucleation of biominerals, such as calcium carbonate and calcium phosphate, has attracted significant attention in the last decade following the proposal of so-called 'non-classical' mechanisms. For the case of calcium carbonate it has been proposed that the pathway to formation under homogeneous conditions can include stable pre-nucleation clusters (PNCs) , liquid-liquid phase separation  and the formation of amorphous calcium carbonate as a thermodynamically stable phase for nanoparticles in a given size range . Given the difficulties of probing such species in situ with experimental techniques, atomistic simulation is a valuable complementary approach. However, simulation faces a number of challenges of it’s own due to the limitations of molecular dynamics with the currently available computing power. For example, the concentrations of ions at saturation for most biominerals are below 1 mM, meaning that unbiased sampling of ion association in aqueous solution at experimental conditions is unfeasible. In this presentation several challenges for simulation of ion association from ion pairing, through pre-nucleation species, to determining critical nucleus size will be examined. This includes issues spanning the underlying accuracy of the free energy landscape through to how to map cluster stability in terms of a manageable set of collective variables that capture the association/dissociation pathways and connect them to identifiable thermodynamic states. Here examples will be drawn from aqueous calcium systems with carbonate, phosphate and oxalate.
 D. Gebauer et al, Science, 322, 1819 (2008)
 A.F. Wallace et al, Science, 341, 885 (2013)
 P. Raiteri & J.D. Gale, J. Am. Chem. Soc., 132, 17623 (2010)
Nucleation and growth processes of gas clathrate hydrates
Professor Carolyn Ann Koh, Colorado School of Mines, USA
Gas clathrate hydrates are crystalline inclusion compounds comprised of a three-dimensional network of hydrogen-bonded water molecules that can trap small gas molecules in the water cavities. The ability to control clathrate hydrate nucleation and growth processes is important in several energy applications, including during the production and transportation of oil/gas in subsea flowlines where gas hydrates can form blockages in the flowline, as well as energy storage of fuels in gas hydrate crystals. The nucleation and growth processes and inter-particle interactions of gas hydrate crystals on gas bubbles and water droplets in water and oil continuous systems are examined at high pressure and low temperature conditions. Addition of polymers and surface-active molecules can be used to modify these processes, e.g. delaying the nucleation and growth processes, or reducing the inter-particle interactions. The clathrate hydrate formation synthesis pathways can be a key strategy to designing higher storage capacity materials, and/or stabilising new stable and metastable crystal structures. Structure metastability has been observed through spectroscopic and computational studies. Examples of the use of different promoter guest molecules, synthesis methods, and pressure conditions are presented for the production of stable and metastable clathrate hydrate phases. These studies can help further our knowledge for developing clathrate materials for storage and other technologies.
Free energy landscape and molecular pathways of ice and gas hydrate nucleation
Professor Tianshu Li, George Washington University, USA
Gas hydrate and ice are similar in many aspects: both appear 'ice-like', and both form in aqueous environments as a result of ordering of water molecules. However the nucleation processes of two solids could proceed with very different pathways. Employing advanced molecular simulation methods, this talk explores the free energy landscape and molecular pathways of both ice and hydrate nucleation. This talk shows that ice nucleation, both homogeneous and heterogeneous, appears to follow a pathway described very well by classical nucleation theory (CNT). The nucleation of hydrate, on the other hand, has been often found to involve multiple steps, thus appearing non-classical. Indeed, structural analysis show that on average, hydrate formation is facilitated by a 'two-step' like mechanism involving a gradual transition from amorphous to crystalline structure. However analysis also shows the existence of direct nucleation pathways where hydrate crystallises without going through the amorphous stage. Interestingly, the calculated free energy profile was also found to fit reasonably well against CNT. The structural diversity and the CNT-like free energy profile imply that hydrate nucleation could be an entropically driven, kinetic process that proceeds via multiple pathways that have similar free energy profiles.