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 (BMIMPF<sub>6</sub>) + ethanol solutions are investigated using four popular probes, pyrene, 1-pyrenecarboxaldehyde, 1,3-<i>bis</i>-(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 BMIMPF<sub>6</sub> in comparison to the bulk composition. However, 1-pyrenecarboxaldehyde and 1,3-<i>bis</i>-(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, BMIMPF<sub>6</sub> 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 BMIMPF<sub>6</sub>. As the mole fraction of ethanol is increased, the increase in the HBD ability of the binary BMIMPF<sub>6</sub> + 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)