The cybotactic region surrounding fluorescent probes dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate: Effects of temperature and added carbon dioxide

We report on the local microenvironment that surrounds three fluorescent solutes (i.e., the cybotactic region) when they are dissolved in a 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) roomtemperature ionic liquid as a function of temperature and added CO₂ (T = 308 K; P = 0-150 bar). In dry [bmim][PF₆] at 293 K, the cybotactic region exhibits a dielectric constant and refractive index of 11.4 ± 1.0 and 1.523 ± 0.025, respectively. The activation energy that describes the [bmim][PF₆] viscous flow is 38.4 ± 0.9 kJ mol^-1. The activation energy for solute rotational reorientation in [bmim][PF₆] is equivalent to the activation energy for [bmim][PF₆] viscous flow, indicating that solute rotational dynamics are correlated entirely with the [bmim][PF₆] dynamics. There is nanosecond dipolar relaxation surrounding a solute dissolved in dry [bmim][PF₆] at 293 K. Even though CO₂ is highly soluble in [bmim][PF₂] (CO₂ mole fraction = 0.6 at 313 K and 68 bar), addition of up to 150 bar CO₂ to [bmim][PF₆] at 308 K causes the solute's cybotactic region dipolarity to decrease by less than 15%. At a fixed temperature (308 K), we observe a 5-fold decrease in the apparent [bmim][PF₆] bulk viscosity between 0 and 150 bar CO₂.