Leicester Ionic Liquids Group
Green Solutions
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Research

The main research interests of the group are firmly based around green chemistry with particular emphasis on electrochemical processes. It is active in developing novel solvent systems with industrial applications such as metal deposition and dissolution. It collaborates strongly with industry and much of the work to date has been in the development of novel processes using ionic liquids.

Deep Eutectic Solvents: Eutectic Based Ionic Liquids

The main consensus seems to be that the first major studies of ambient temperature salts (the term ionic liquid has been used to describe salts that melt below 100°C) were prepared in the 1940s by a group at Rice University who mixed pyridinium halides with aluminium chloride. [1] These mixtures contain complex chloroaluminate anions such as AlCl4- and Al2Cl7- that are large and hence reduce the charge density. Similar ionic liquids have been made using a wide variety of quaternary ammonium salts and most notably imidazolium cations. Aluminium chloride is very reactive with water and this has limited the use of these types of ionic liquids to the electrodeposition of aluminium metal and some synthetic reactions requiring very strong Lewis acid catalysts. Work by our group [2,3] and Sun et al. [4,5] showed that eutectic mixtures of zinc halides and quaternary ammonium halides also have melting points close to ambient conditions. This has been further extended to a wide range of other salts and organic compounds that form eutectic mixtures with quaternary ammonium salts. This area has received comparatively little attention compared with the chloroaluminate and discrete anions but the principle is simple in that the complexing agent just needs to be able to complex the simple anion to effectively delocalise the charge and decrease the interaction with the cation.

Deep Eutectic Solvents comprise bulky cations and smaller anions which are bound to a hydrogen bond donor.

The systems so far described can be expressed in terms of the general formula Cat+X- · z Y. where Cat+ is in principle any ammonium, phosphonium or sulphonium cation, X is generally a halide anion (usually Cl-). They are based on equilibria set up between X- and a Lewis or Brønsted acid Y, z refers to the number of Y molecules which complex X-. The ionic liquids described can be subdivided into three types depending on the nature of the complexing agent used.

Eutectic Type 1Y = MClxM = Zn,[2-5] Sn,[3] Fe,[3] Al,[1] Ga,[6] In[7]
Eutectic Type 2Y = MClx·yH2OM = Cr,[8] Co, Cu, Ni, Fe
Eutectic Type 3Y = RZZ = CONH2,[9] COOH,[10] OH [11]

To date the only Cat+ species studied have been based on pyridinium, imidazolium and quaternary ammonium moieties. In general, as with the chloroaluminate and discrete anion systems, the imidazolium based liquids have the lowest freezing points and viscosities and higher conductivities. One of the main differences between ionic liquids and aqueous solutions is the comparatively high viscosity of the former. The viscosities are typically in the range 10 - 500 cP (0.01 - 0.50 Pa s) and this affects the diffusion coefficients of species in solution. We have fitted the viscosity of ionic liquids using hole theory.12 The theory was developed for molten salts but has been shown to be very useful for ionic liquids. It was shown that the value of Eη is related to the size of the ions and the size of the voids present in the liquid.[10] The viscosity of ionic liquids is several orders of magnitude higher than high temperature molten salts due partially to the difference in size of the ions, but also due to the increased void volume in the latter. It has been shown [13] that hole theory can be applied to both ionic and molecular fluids to account for viscosity and can aid with the design of new ionic liquids. [14] This work has also allowed us to predict the amount of molecular component that can be added before the properties become those of an ionic solution.[15] For a full review of ionic liquids and their effects on metal processing see some recent reviews. [16,17,18]


[1]F. H. Hurley and T. P. Weir
J. Electrochem. Soc. 98 207 (1951).
[2]A. P. Abbott, G. Capper, D. L. Davies, H. Munro, R. Rasheed and V. Tambyrajah
Chem. Commun. 2001 2010 (2001).
[3]A. P. Abbott, G. Capper, D. L. Davies, H. Munro and R. Rasheed
Inorg. Chem. 43 3447 (2004).
[4]S-I. Hsiu, J-F. Huang, I-W. Sun, C-H. Yuan, J. Shiea
Electrochim. Acta 47 4367 (2002 ).
[5]Y-F Lin and I-W Sun
Electrochim. Acta 44 2771 (1999 ).
[6]W. G. Xu, X-M. Lu, J. Z. Yang, J-S. Gui, J. Z. Yang
Chinese J. Chem. 24 331 (2006 ).
[7]J. Z. Yang, W. G. Xu, P. Tian and L-L. He
Fluid Phase Equil. 204 295 (2003 ).
[8]A. P. Abbott, G. Capper, D. L. Davies and R. Rasheed
Chem. Eur. J. 10 3769 (2004).
[9]A. P. Abbott, G. Capper, D. L. Davies, R. Rasheed and V. Tambyrajah
Chem. Commun 2003 70 (2003).
[10]A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and R. Rasheed
J. Am. Chem. Soc. 126 9142 (2004).
[11]A. P. Abbott, G. Capper, B. G. Swain, D. A. Wheeler
Trans. Inst. Met. Fin. 83 51 (2005).
[12]A. P. Abbott
Chem. Phys. Chem. 5 1242 (2004 ).
[13]A. P. Abbott
Chem. Phys. Chem. 6 2404 (2005 ).
[14]A. P. Abbott, G. Capper and S. Gray
Chem. Phys. Chem. 7 803 (2006).
[15]A. P. Abbott, R. C. Harris and K. S. Ryder
J. Phys. Chem. B 111 4910 (2007).
[16]A. P. Abbott, K. S. Ryder and U. Koenig
Trans. I. M. F. 86 196 (2008).
[17]A. P. Abbott, and K. J. McKenzie
Phys. Chem. Chem. Phys. 8 4265 (2006).
[18]F. Endres A. P. Abbott, and D. MacFarlane (Eds.)
Electrodeposition of Metals from Ionic Liquids Wiley VCH (2007).