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Department of Chemistry

Dr Sharon J. Cooper

Associate Professor in the Department of Chemistry
Telephone: +44 (0) 191 33 42098
Member of the Durham X-ray Centre

(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, TEM and optical microscopy) and computational modelling techniques. Studies span from fundamental blue skies research,1-6 through to industrially-sponsored projects.7Examples 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 desired effect, 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 and microemulsions to attain the specific crystallization conditions necessary to produce optimized materials. Emulsion systems can provide control over all the key physical aspects of crystals, i.e. the crystal size, shape and polymorphic form. Research highlights include our pioneering work on the ability of phase-inverting emulsions to act as tunable nucleating agents,1 so that 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 crystals (see above).2 

Crystallization is typically under kinetic control in bulk solution, with metastable polymorphs often crystallizing initially in accordance with Ostwald's rule of stages. Recently, we have shown that microemulsions have the unique capability of bringing crystallization under thermodynamic control.3,4 Thermodynamic control is advantageous because it enables crystallization directly into the most stable crystalline form or polymorph. We are using this methodology to identify the most stable polymorphs of drug compounds, and to crystallize inorganic nanocrystals.

Measurement of critical nucleus size in confined volumes

The critical nucleus size is of pivotal importance for crystallization. Above this size, it is favourable for the new crystalline phase to grow; below this size, nuclei tend to dissolve or melt rather than grow. Furthermore, the size and structure of the critical nucleus typically determines both the crystallization rate and the polymorphic form. We have shown that the critical nucleus size can be determined directly from the volume of confinement,5 for the case of ice crystallization in microemulsions.  This is a key advance because previously, the critical nucleus size was usually 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 updating6 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. 'The use of phase-inverting emulsions to show the phenomenon of interfacial crystallization on both heating and cooling', C. E. Nicholson, S. J. Cooper, C. Marcellin, M. J. Jamieson J. Am. Chem. Soc. 2005, 127, 11894-11895.
  2. 'Unique Crystal Morphologies of Glycine Grown from Octanoic Acid-in-Water Emulsions', C. E. Nicholson, S. J. Cooper, M. J. Jamieson J. Am. Chem. Soc. 2006, 128, 7718-7719.
  3. 'Stable Polymorphs Crystallized Directly under Thermodynamic Control in Three-Dimensional Nanoconfinement: A Generic Methodology'  C. E. Nicholson, C. Chen, B. Mendis, S. J. Cooper  Cryst. Growth Des. 2011, 11, 363-366.
  4. 'Leapfrogging Ostwald's rule of stages: Crystallization of stable g-glycine directly from microemulsions' Methodology'  C. Chen, O. Cook, C. E. Nicholson, S. J. Cooper  Cryst. Growth Des. 2011, 11, 2228-2237.
  5. 'Direct measurement of critical nucleus size in confined volumes', J. Lui, C. E. Nicholson, S. J. Cooper Langmuir 2007, 23, 7286-7292.
  6. 'A simple classical model for predicting onset crystallization temperatures on curved substrates, and its implications for phase transitions in confined volumes', S. J. Cooper, C. E. Nicholson, , J. Lui J. Chem. Phys. 2008, 129, 124715.
  7. 'Detailed mapping of biaxial orientation in polyethylene terephthalate bottles using polarized attenuated total reflection FTIR spectroscopy' M. R. Smith, S. J. Cooper, D J. Winter, N. Everall, Polymer 2006, 47, 5691-5700.

Selected Publications

Journal Article

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