Solvatochromic absorbance probe behavior within 1-butyl-3-methylimidazolium hexafluorophosphate + propylene carbonate: Preferential solvation or solvent–solvent interaction?

Physicochemical properties of ionic liquids (ILs) can be favorably modified by addition of appropriate cosolvents. Solvatochromic absorbance probes are used to assess physicochemical properties of the mixture of a common IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and propylene carbonate. Parameter ETN obtained from electronic absorbance of Reichardt's betaine dye within the mixture is observed to be higher than that predicted from ideal additive behavior. Absorbance from N,N-diethyl-4-nitroaniline and 4-nitroaniline in concert with absorbance from Reichardt's dye is used to obtain dipolarity/polarizability (π*), hydrogen-bond donating (HBD) acidity (α), and hydrogen-bond accepting (HBA) basicity (β) of the mixture. While π* and α of the mixture are observed to be higher than those expected from ideal additive behavior, β is observed to be lower. Solvatochromic absorbance probe behavior suggests solute-specific preferential solvation; FTIR absorbance of the mixture implies negligible specific solvent–solvent interaction(s) within the mixture. Combined nearly ideal binary solvent/Redlich–Kister (CNIBS/R–K) equation is demonstrated to predict the solvatochromic parameters satisfactorily.