Most familiar cleaning solvents (e.g., water, ethanol, hexa-ne, etc.) are molecular. That is, regardless of whether they are polar or non-polar, they are basically constituted of molecules. Ionic liquids are different. They are composed of ions, not molecules. Unlike molecular solvents (liquids), regardless of the degree of association, ionic solvents are basically constituted of ions. Molten sodium chloride is an ionic liquid. A water solution of sodium chloride is an ionic solution. Ionic liquids are “designer” solvents in the same sense as are the HFE solvents.

THE STRUCTURE OF IONIC LIQUIDS
Ionic liquids are mixtures of bulky asymmetrical cations with smaller evenly shaped anions. They remain liquid at room temperature because their ionsdon’t pack together well.

While the possible combinations of different anions and cations number in the millions, one which has received considerable interest is shown in Figure 1. It is 1-butyl-3-methylimidazolium (the cation) tetrafluoroborate (the anion),and is known as [bimm][BF4] or BIMM BF4.

WHY IONIC LIQUIDS?
These structures have received considerable global interest from scientists since the mid-1990s because many have extraordinarily low volatility. This characteristicallowed ionic liquids to be called “green solvents.” The adjective “green,” apolitical but not technical term, was used based on the twin beliefs that: (1)every ionic liquid would have low volatility, and (2) a solvent which did notvolatilize couldn’t be harmful.

REALITY BITES
As commonly happens with scientific discovery, initial evaluations are changed as additional research is done. For ionic liquids, both beliefs have been shown to be incomplete.

Vapor pressure data in Figure 2 illustrate the effect of various anions (for the BIMM cation) upon vapor pressure, and refutation of the first belief.

Furthermore, any solvent recipe which includes atoms such as phosphorous, fluorine, arsenic, and boron in addition to structures such as amides, cyanamids, choline, and guandinium should receive substantial priority on anyone’s list of chemicals to be evaluated for hazard potential to humans and the environment. Specific tests have shown for some ionic liquids that: they are not mutagenic by the Ames test; are about as toxic to crustaceans as benzene; are two to four orders of magnitude more toxic to micro algae than IPA; and are considerablyless biodegradable than most surfactants.

PHYSICAL PROPERTIES
Ionic liquids are not a good match for conventional cleaning technologies, despite being generally water-soluble. Surface tensions are 30 to 40 dyne/cm vs. around 15 for HFEs. Evaporation rates are low. Values of density are only slightly above those of water, but heat capacities are half-again above those of water. But total solubility parameters are quite low (similar to HFE solvents), suggesting that the levels of hydrogen-bonding and polar intermolecularforce are low. Flammability and other data are not available yet.