Organisers

• Paddy Royall (University of Bristol, United Kingdom)
• Richard Sear (Department of Physics, University of Surrey, United Kingdom)

Supports

   CECAM

Description

The cornerstone of our understanding of nucleation lies in classical nucleation theory. This theory does an excellent job of showing why nucleation is an activated process, and why it typically occurs at a surface. However, in particular for nucleation at surfaces it can be highly inaccurate or even qualitatively wrong [1,2]. This problem can be overcome via computations using specialised algorithms for the study of activated processes. Computational algorithms such as umbrella sampling [3,4], and recent developments such as forward-flux sampling (FFS) [5] can calculate nucleation rates exactly and relatively efficiently.  However even FFS cannot calculate rates in the presence of slow dynamics [6] (as found for example in glasses and gels). Modern sophisticated algorithms for dealing with the rough energy landscapes are available but have not yet been applied to crystallisation. Examples that we believe are particularly relevant to nucleation are the methods for locating local minima, their connectivity and transition states that have been pioneered by Wales and coworkers [7], and methods that have been developed for activated processes such as the “string” method [8]. 

The only experimental systems in which we have the techniques to study the microscopic details of crystallisation are model colloidal dispersions [9,10,11], where the interactions can also be carefully controlled. Model experimental systems have progressed beyond hard spheres to include: mixtures of oppositely charged ionic particles [12]; colloid-polymer mixtures [13] the study of field-induced martensitic phase transitions between exotic crystal structures [14]; controlled frustration due to impurities [15] and the competition between crystallisation and vitrification [16]. Moving to somewhat more complex situations, sedimentation can compete with crystallisation [17] or be controlled : colloidal epitaxy [18],  in fact experiments in microgravity showed a novel  dendritic growth morphology, suggesting that even weak gravitational fields may play an important role in influencing the mechanism of crystal growth [19].

Like the experiments on model systems, recent simulation work has also moved beyond simple one-step homogeneous nucleation to look at more complex systems. For example, systems with multi-step nucleation [20], poisoning [21], and competing polymorphs [22]. All these studies are for simple models of colloids or of simple molecules (e.g., noble gases).

So, we are studying more complex systems in both simulation and colloid experiments. However, even in colloids we are only starting to understand complex phenomena such as poisoning and in more complex molecular systems our understanding is much less developed. 

The time is now right, and we have many of the tools required, to model and hence try to understand systems that are quite complex but are key to, for example, large industries and biological processes. An example is calcium carbonate which is important both inapplications such as the paper industry, and in which we wish to understand the many examples of its controlled mineralisation in biology [23] (biomineralisation). Recent experimental work [24,25] on this system has found small long-lived nanoclusters that appear to be precursors to crystallisation. It appears that crystallisation in this highly important system is very different from the classical picture, but there have not yet been simulations of this phenomenon. By contrast simulation of crystal growth during chemical vapour deposition as identified the importance of molecular pathways in controlling the growth rate [26].

Other outstanding problems include the two-step nucleation process of the protein haemoglobin ? the microscopic phenomenon that underlies sickle-cell anaemia [27] ? and the anti-AIDS drug Ritonavir [28]. Ritonavir, like almost all substances, has several polymorphs. A highly stable second polymorph appeared during production of the drug and caused an estimated $250 million of losses due to lost production. The second polymorph appeared only after full-scale production had begun because nucleation of this form is extremely slow.

Scientific Objectives

(1) To discuss future directions in studies of nucleation and crystallisation.

(2) To bring together researchers from different disciplines and fields, who seek to understand/control crystallisation in different systems.

(3) To identify where we will need new simulation algorithms to calculate crystallisation rates in more complex systems, and how this can be done.

(4) To identify ways to develop complimentary simulation and experimental techniques, such as simulation with biased ensembles and single-particle level imaging using confocal microscopy.

(5) To identify the key physical phenomena that determine the rate of crystallisation in key systems, such as calcium carbonate. Also to determine the roles in crystallisation of hydrodynamics, and of the free volume in slowly crystallising highly viscous materials.

kinetics? What is the role of hydrodynamics? What is the role of free volume in slowly crystallising highly viscous materials?

References

[1] Caccuito A, Auer S. & Frenkel, D., Nature 428, 404, (2004).
[2] Page A.J. & Sear R.P., Phys. Rev E, submitted.
[3] ten Wolde, P. R et. al. Phys. Rev. Lett., 75, 2714-2717, (1995).
[4] Auer S. & Frenkel, Nature, 409, 1020-1023, (2001).
[5] Allen R.J. et al Phys. Rev. Lett., 94, 018104, (2005).
[6] Sear R.P., J. Chem. Phys., 128, 214513, (2008).
[7] Wales D.J., Energy Landscapes (Cambridge University Press, 2004).
[8] Maragliano L., Fischer A. & Vanden-Eijnden, E., 125, 024106 (2006).
[9] Schope H.J. et al, J. Chem. Phys, 127, 084505, (2007).
[10] Palberg T.J. Phys. Condens. matter, 11, R323-R360 (1999).
[11] Gasser U. et al Science, 292, 258, (2001).
[12] Leunissen M.E.; et. al. Nature, 437, 235, (2005).
[13] Palberg T. et.al. Phys. Rev. Lett. 102, 038302, (2009).
[14] Yethiraj A. et. al. Phys. Rev. Lett., 92, 058301, (2004).
[15] Villeneuve V.W.A. et al Science, 309, 1231-1233, (2005).
[16] Royall C.P. et. al., J. Phys:Cond. Matter., 20, 404225, (2008).
[17] Dullens R. P. A. et. al. Phys. Rev. Lett., 97, 228301, (2006).
[18] Hoogenboom J. et. al. Phys. Rev. Lett. 90, 138301, (2003).
[19] Zhu J. et. al. Nature, 387, 883-885, (1997).
[20] van Meel J.A. et. al. J. Chem. Phys., 129, 204505, (2008).
[21] Schilling T. & Frenkel, D. Phys. Rev. Lett., 92, 088505, (2004).
[22] Desgranges C. and Delhommelle J., J. Chem. Phys. 126, 054501 (2007).
[23] Chem. Rev. 2008, 108, issue 11 is devoted to reviews of biomineralisation.
[24] Gebauer D., Volkel, A., & Colfen, H., Science, 322, 1819, (2008).
[25] Meldrum F.C. & Sear R.P., Science 322, 1802, (2008).
[26] May P.W. Science 2008, 319 1490, Cheeseman A., Harvey J.N. and Ashfold M.N.R., J. Phys. Chem. A 2008, 112 11436
[27] Galkin O. et al. Biophysical J. 93, 902-913 (2007).
[28] Bauer J. et al., Pharm. Res. 18, 859 (2001); S. L. Morissette et al., PNAS 100, 2180 (2003).