As required for all separation techniques, ionic liquids exhibit selectivity towards one or more of the phases of a mixture. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF) is a room-temperature ionic liquid that was identified early on as a viable substitute for volatile organic solvents in liquid-liquid separations. Other [PF]- and [BF]- containing ionic liquids have been studied for their CO absorption properties, as well as 1-ethyl-3-methylimidazolium (EMIM) and unconventional cations like trihexyl(tetradecyl) phosphonium ([P]). Selection of different anion and cation combinations in ionic liquids affects their selectivity and physical properties. Additionally, the organic cations in ionic liquids can be "tuned" by changing chain lengths or by substituting radicals. Finally, ionic liquids can be mixed with other ionic liquids, water, or amines to achieve different properties in terms of absorption capacity and heat of absorption. This tunability has led some to call ionic liquids "designer solvents." 1-butyl-3-propylamineimidazolium tetrafluoroborate was specifically developed for CO capture; it is designed to employ chemisorption to absorb CO and maintain efficiency under repeated absorption/regeneration cycles. Other ionic liquids have been simulated or experimentally tested for potential use as CO absorbents.