More Information on Room Temperature Ionic Liquids can be found in our powerpoint presentation: RTIL

ROOM TEMPERATURE IONIC LIQUIDS

Room temperature ionic liquids are liquids that are composed entirely of ions and in this sense alone resemble the ionic melts which may be produced by heating normal metallic salts such as sodium chloride to high temperature (e.g., NaCl to over 800oC). In fact, ionic liquids can now be produced which remain liquid at room temperature and below (even as low as - 96oC). A strong case has been made for the investigation of ionic liquids as an alternate reaction medium for catalysis, allowing the controlled production of desired products from reactants with a minimum of waste through the occurrence of side reactions. This has been ascribed to the tendency of ionic liquids to suppress conventional solvation and solvolysis phenomena. Ionic liquids provide a medium which appears to be capable of dissolving a vast range of inorganic (and some organic) molecules to high concentrations. The low temperatures of these systems tends to limit dissociation, disproportionation, and degradation reactions. Stability of otherwise transient species is often extended in the ionic environment.

Ionic liquids are possessed of a number of properties which may be of importance in their application as extractive media in liquid/liquid extraction processes. They are liquid at room temperature, but in fact have an enormous liquid range of 300oC which is larger than that of water and offers the potential for considerable kinetic control of extractive processes. They are good solvents for a wide range of inorganic, organic, and polymeric materials. The composition of ionic liquids may be adjusted enabling control of their acidity or basicity. Several ionic liquids are known which are neither air or water sensitive or miscible with water, thus enabling the concept of liquid/liquid extraction from aqueous media.

Ionic liquids are relatively undemanding and inexpensive to manufacture. A typical example of such an ionic liquid based on methyl imidazole is shown in Figure 1. The anion shown here is hexafluorophosphate, but other suitable anions are known (e.g., tetrafluoroborate). The R group of the cation is also variable (e.g., methy, ethyl, etc. derivatives with suitable behavior are known). The variability of anion and R group may be used to fine-tune the properties of the ionic liquids.

Ionic liquids appear to have a high capacity for organic solutes with up to 50% v/v% solutions of benzene having been reported. Preliminary batch experiments in our own laboratories utilizing ionic liquids we have manufactured in-house, indicate that a wide variety of aryl organic molecules partition favorably to the ionic liquid phase from water. These limited experiments lead to the expectation that applications may be selected in the first instance based on hydrophobicity data (e.g., 1-octanol/water partitioning coefficients). The results of these preliminary experiments are shown in Figure 2, however, a great deal more experience is required before the behavior of ionic liquids in liquid/liquid extraction can be understood and applied.

The constituents of ionic liquids (being ionic) are constrained by high coulombic forces and thus, exert practically no vapor pressure above the liquid surface. This property may allow the development of novel recovery schemes for certain organic species in relation to normal liquid/liquid extraction in which product recovery could be effected by distillation or pervaporation.