So you might be surprised to hear that this chemical compound has a hidden talent – as a liquid, CO2 can act as an efficient, cheap, non-toxic, and non-flammable replacement for petrochemical solvents. What’s the catch? Well, liquid CO2 needs a helping hand in the form of chemical additives in order to efficiently operate as a solvent. The problem is that finding an environmentally friendly additive is challenging and the best additives contain fluorine, which is environmentally damaging.

A group of scientists from Bristol, Malaysia and Japan who have been tackling this problem for some time have recently taken a new approach. They are using neutrons to study a new superefficient, yet fluorinated additive, in the hope of unlocking how its beneficial properties are related to its molecular structure. The information generated will then be used as a blueprint to chemically design better, greener additives.

Supercritical Carbon Dioxide shows huge promise as a green solvent with major applications in extraction, dry cleaning, polymerisation oil recovery and much more, however it has a low solubility and low viscosity which limit its use in these applications. A promising approach to increase the solubility and thicken the solvent is to form micro emulsions from soap like surfactants or additives which encapsulate water within the solvent. Some success has been found already with the generation of hydrocarbon surfactants for enhanced oil extraction, however to be a viable green technology the amount of surfactant needs to be as small as possible.

Professor Julian Eastoe, University of Bristol, explains, “CO2 is currently a very topical molecule, but it is little known that CO2 in the liquid form has uses in industrial processes. This solvent dense liquid supercritical CO2 is an unusual solvent - you normally think of CO₂ as a gas but actually, it doesn’t take much to condense this into a liquid. In fact, the CO2 that is transported industrially is nearly always liquid. So it’s desirable to be able to modify this fluid to maximise its potential in industrial processes.”

“Our research is focused on how you go about designing molecules that will jump to attention in CO2, We design molecules and synthesise them in the lab and we use ISIS to understand the relationship between the structure of the molecules and how they function. We then take what we’ve learnt from the neutron experiments and we use it to resynthesize better and better molecules - it’s the chemistry done here in Bristol and from my colleagues in Japan that helps us to do that.”

The team developed short tail fluorocarbon surfactants that have a high solubilising power and act to thicken the fluid. As the amount of surfactant used needs to be as small as possible in order for the solvent to be a viable green technology, finding a fluorine light surfactant with a high solubilising power is exciting. The team used small-angle neutron scattering at ISIS to characterise this surfactant to uncover the origins of this superefficiency by looking at the aggregation behaviour and the nanostructures of these short tail fluorocarbon surfactants.

The neutron data showed the formation of a lamellar or sheet-like structures in CO2 and the relationships between the nanostructure and the surfactant chain length were revealed, however more studies on this efficient additive, 4FG(EO)₂, will continue. Julian Eastoe explains, “What’s interesting about this research is the formation of a certain kind of dispersion in CO2 which is very efficient for increasing the viscosity of the CO2. We can see from the neutron data is this kind of “house of cards” structure is formed, which will enhance the viscosity. So the neutron scattering proves we have materials, nanostructures, which are thickening agents. This is an important observation, as here we have shown for the first time that we can make these kinds of lamellar structures in CO2”

Self-assembly of surfactants in liquid carbon dioxide forming the 'house of cards' structure.

“Further structure performance studies are needed to develop liquid CO2 as an environmentally benign and energy saving solvent. We want to design molecules that are fluorine free in order to be environmentally friendly. In this current paper the additives are still containing fluorine, however, by studying fluorine it’s a means to an end. We can learn more quickly by doing the research that actually works! Here we have learnt how to make the sheet-like lamella or house of cards structures now we can go back into the lab and fine tune the chemistry for non-fluorine components.”

                                                                                              Felice Laake

Julian Eastoe et al.

Research date: December 2013