Room-temperature ionic liquids (RTILs) have shown tremendous promise as replacements for the toxic and volatile organic solvents used in many applications. Depending on a particular application, addition of a "green" cosolvent can very easily modify/alter the physicochemical properties of the RTILs in a favorable fashion. Solvatochromic probe behavior within binary RTIL 1-butyl-3-methyl imidazolium hexafluorophosphate (BMIMPF6) + ethanol solutions are investigated using four popular probes, pyrene, 1-pyrenecarboxaldehyde, 1,3-bis-(1-pyrenyl)propane, and Reichardt's betaine dye. A simplified preferential solvation model based on the weighted mole-fraction probe response shows that the pyrene cybotactic region is rich in BMIMPF6 in comparison to the bulk composition. However, 1-pyrenecarboxaldehyde and 1,3-bis-(1-pyrenyl)propane solvation environments clearly indicate a cybotactic region dominated by ethanol. Reichardt's betaine dye shows absorbance maxima lower than that observed in either of the two neat components, BMIMPF6 or ethanol. This probe is well known to manifest the hydrogen-bond-donating (HBD) ability of the solvent system. HBD ability also depends on the dipolarity/polarizability of the solvent system. Ethanol has a higher HBD ability but lower dipolarity/polarizability than BMIMPF6. As the mole fraction of ethanol is increased, the increase in the HBD ability of the binary BMIMPF6 + ethanol solution is manifested through the unusual Reichardt's dye response.
Kristin A. Fletcher and Siddharth Pandey, "Solvatochromic Probe Behavior Within Binary Room-Temperature Ionic Liquid 1-Butyl-3-Methyl Imidazolium Hexafluorophosphate Plus Ethanol Solutions," Appl. Spectrosc. 56, 1498-1503 (2002)