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 1||Y = MClx||M = Zn,[2-5] Sn, Fe, Al, Ga, In|
|Eutectic Type 2||Y = MClx·yH2O||M = Cr, Co, Cu, Ni, Fe|
|Eutectic Type 3||Y = RZ||Z = CONH2, COOH, OH |
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. 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  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.  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.
For a full review of ionic liquids and their effects on metal processing see some recent reviews. [16,17,18]
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