Temperature-dependent microscopic solvent properties of ‘dry’ and ‘wet’ 1-butyl-3-methylimidazolium hexafluorophosphate: correlation with ET(30) and Kamlet–Taft polarity scales

Abstract

As a result of heightened awareness of a wealth of potential in clean manufacturing processes, room temperature ionic liquids (RTILs) have been the target of increased investigation. As an integral part of the green chemistry movement, RTILs have found application in synthesis, catalysis, polymerization, industrial cleaning, liquid/liquid extraction, and separations. While some groundwork has been laid, the optimal utilization and tailoring of these solvents has been hobbled by an incomplete understanding of their solvent properties, particularly at the molecular level. In this work, we use solvatochromic measurements to determine ‘energy of transition’ E_T(30) values and Kamlet–Taft solvent parameters (α, β, π*) for the relatively hydrophobic RTIL 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]⁺[PF₆]⁻, as a function of temperature (10–70 °C) and water content (50 ppm or less water, ‘dry’ or 2% water (v/v), ‘wet’). The results of these experiments demonstrate that dry [bmim]⁺[PF₆]⁻ exhibits a hydrogen bond donor strength on the order of short chain alcohols with a linear temperature dependence. Dry and wet [bmim]⁺[PF₆]⁻ exhibit hydrogen bond acceptor abilities, which are weak functions of temperature, intermediate between that of water and acetonitrile. The π* parameter for wet and dry [bmim]⁺[PF₆]⁻ is higher than short chain alcohols, but lower than water or dimethylsulfoxide at ambient conditions, and it exhibits a strong linear temperature dependence. Finally, the addition of water to [bmim]⁺[PF₆]⁻ does not affect the β and π* values significantly.