Super-critical carbon dioxide has long been regarded as a potential ‘green’ replacement for volatile organic solvents. However, the truth is that sc-CO2 is generally a poor solvent. For 15 years now groups in the USA and UK have been trying to create CO2-compatible surfactants to achieve improvements in solvent properties of CO2. Ideally these surfactants would stabilise nanodroplets of water in the CO2. Small angle neutron scattering (SANS), which measures the size and interaction of particles, has provided key evidence for generation of nanoscale droplets in CO2 using these stabilizers. Initial successes were with fluorocarbon molecules such as that shown in the figure, but unfortunately these are expensive, limiting a vast potential for industrial applications. Recently it was found that the surfactant tail tips are especially important, triggering the design of functionalised fluorine-free surfactants. The latest SANS results from these are the best yet, showing good levels of water solubilization and moderate stability. The next stage is optimisation of chemical structure to generate environmentally-acceptable and commercially-viable additives for this green solvent.

J Eastoe, S Gold (University of Bristol), DC Steytler (University of East Anglia)

Research date: December 2006