This group develops new functionalised surfactants for various applications

Sarah Gold - Surfactants for Green Solvents

PhD, October 2003 -

Sarah joined the group in 2003 from Imperial College where she was working with lyotropic liquid crystals. Now a surfactant chemist, she works on surfactants for use in CO₂ and ionic liquids.

CO₂

Much work on the use of CO₂ as a solvent has been carried out with the intention of finding a 'green' replacement for common VOC's (volatile organic compounds). Unfortunately, CO₂is generally a very poor solvent, particularly for polar and/or high molecular weight solutes, and this places severe limitations on potential applications. A good way to overcome this, and hence enhance solubility in condensed CO₂, would be to incorporate reversed micelles or nano-droplets in a CO₂-continuous phase. Ideally, these systems would be water-in-CO₂ (w/c) microemulsions, stabilized using CO₂-compatible surfactants.

Past research has shown that fluoro-surfactants such as di-HCF4 (figure 1) are the most effective moieties for stabilizing w/c systems.¹ These however, are prohibitively expensive as well as incompatible with current green aims for this area of research .

Figure 1 - Di-HCF4

With this in mind, research is now focusing on understanding what structural features allow for surface activity in CO₂ with a view to using these molecular functionalities to design effective, cheap and ecologically responsible surfactants for use in w/c systems.

Important work in this area by Wallen & Raveedren et al. has shown that the presence of carbonyl groups (figure 2) enhances the solubility of hydrocarbons in CO₂.² Molecular simulations carried out by the same workers has suggested that this increase in surfactant activity is due to lewis acid - lewis base interactions between the carbonyl oxygen on the surfactant, and electron deficient carbon in the CO₂ molecule.³

Figure 2 - a- and b- 1,2,3,4,6 pentaacetyl-D-glucose, both show activity in CO₂

Previous work carried out by A. Paul and S. Nave within this group has uncovered that highly branched chain tips as in AOT3 and AOT 4 (figure 3) can also boost the solubility of surfactants in condensed CO₂⁴and this has been attributed to the lower cohesive energy densities which is a general feature of highly branched alkyl chains.

Figure 3 - a) AOT 3 and b) AOT 4 - CO₂ soluble surfactants incorporating highly branched chain tips

It would seem logical then, that surfactants which incorporate both carbonyl functionality and high branching in chain tips would be an obvious next step in the ongoing search for effective hydrocarbon CO₂ surfactants.

Gallery

Below are some pretty pictures & video clips of some samples I have made during my PhD

W/C microemulsion
W/C below the cloud point
Video clip of water crashing out of a microemulsion then being taken back up as a function of pressure, using a current surfactant of interest. Water loading of w=10 where w= [water]/[surfactant]
Left: Microemulsion of [BMIM][BF₄]/cyclohexane using TX-100 as a surfactant at 45^oC Right: Phase separation of [BMIM][BF₄]/cyclohexane/TX-100 below 40^oC R=[ionic liquid]/[surfactant]=0.5 Surfactant makes up 45% wt. of whole sample

References

1. Eastoe, J.; Cazelles, B.M.H.; Steytler, D.C.; Holmes, J.D.; Pitt, A.R.; Wear, T.J.; Heenan, R.K. Langmuir 1997, 13, 6980

2. Raveendran, P.; Wallen, S.L.; J. Am. Chem. Soc., 2002, 124, 7274

3. Raveendran, P.; Wallen, S.L.; J. Phys. Chem. B., 2003, 107, 1477

4. Eastoe, J.; Paul, A.; Nave, A.; Steytler, D.C.; Robinson, B.H.; Rumsey, E.; Thorpe, M.; Heenan, R.K. J. Am. Chem. Soc. 2001, 123, 988