Dr Sharon J. Cooper

(email at sharon.cooper@durham.ac.uk)

Research Interests

Our research group interests concern the general areas of crystallization, colloids and semi-crystalline polymers. A broad range of subjects are available that utilize both experimental (typically X-ray diffraction, FTIR, ESEM, and optical microscopy) and computational modelling techniques. Studies span from fundamental blue skies research,^1-5 through to industrially-sponsored projects.⁶ Examples of current studies are outlined below.

Controlled Crystallisation Strategies

The ability to promote, or inhibit, crystallization of a specific species is beneficial in several diverse biological and industrial processes. For example: the production of a particular polymorph is essential in drug formulations to achieve the correct dosage, deep ocean fish rely on macromolecules to prevent ice crystallizing in their blood, whilst complex biomineralization processes produce bones, shells and teeth which have far superior material properties than can be achieved with current synthetic capabilities. Our research focuses on the use of additives and the confined volumes of emulsions to attain the specific crystallization conditions necessary to produce optimized materials. Emulsion systems are particularly ideal as they can provide control over all the key physical aspects of crystals, i.e. the crystal size, shape and polymorphic form. Recent research highlights include our pioneering work on tunable crystallization rate systems,¹ in which crystallization can be induced on both cooling and freezing, and the production of unique dendritic and macroporous crystal morphologies via the adhesion of emulsion droplets onto growing crystals² (see above). These studies are supplemented by experiments at the planar air-aqueous interface, which enables detailed in situ monitoring by several different techniques.³

Direct measurement of critical nucleus size in confined volumes

For crystallization to occur, a stable nucleus of the new phase must form. This nucleus is termed the critical nucleus. We have recently shown that the critical nucleus size can be determined directly from the volume of confinement,⁴ for the case of ice crystallization in microemulsions. This is a key advance because previously, the critical nucleus size had to be estimated using bulk parameters that are inappropriate for the small critical nucleus size. Our methodology involves determining how the crystallization temperature of a material varies within emulsions and microemulsions. We find that the crystallization temperature becomes reduced at sufficiently small confinements (see above) because there is insufficient material within the droplet to provide a stable crystalline phase. For this droplet size, and all smaller ones, the critical nucleus size can be found simply by measuring the droplet size. We are extending this methodology to solutes, oils and polymers, to establish whether our approach can provide a generic solution to determining critical nucleus sizes.

Development of crystallization theories

The melting and crystallization temperatures of nanoparticles can differ significantly from their bulk values. Hence accurate theories are required to predict how phase transition temperatures are expected to vary with nanocrystal size, particularly given the increasing demand for nanomaterials. We are updating^4,5 classical crystallization theories to account for the highly curved interfaces that occur in nanocrystal systems. A particular aim is to use our findings to develop nanocomposite systems that have higher melting points that their bulk materials.

References

1. C. E. Nicholson, S. J. Cooper, C. Marcellin and M. J. Jamieson, J. Am. Chem. Soc., 2005, 127, 11894.
2. C. E. Nicholson, S. J. Cooper and M. J. Jamieson J. Am. Chem. Soc., 2006, 128, 7718.
3. M. J. Jamieson, S. J. Cooper, A. F. Miller and S. A. Holt, Langmuir, 2004, 20, 3593.
4. J. Lui, C. E. Nicholson and S. J. Cooper, Langmuir, 2007, 23, 7286.
5. M. J. Jamieson, C. E. Nicholson and S. J. Cooper, Cryst. Growth Des., 2005, 5, 451.
6. M. R. Smith, , S. J. Cooper, D J. Winter and N. Everall, Polymer, 2006, 47, 5691.