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 so lutes (i.e., the cybotactic region) when they are dissolved in a 1-butyl-3- methylimidazolium hexafluorophosphate ([bmim][PF6]) room-temperature ionic liquid as a function of temperature and added CO2 (T = 308 K; P = 0-150 bar ). Iri dry [bmim] [PF6] at 293 K, the cybotactic region exhibits a dielectr ic constant and refractive index of 11.4 +/- 1.0 and 1.523 +/- 0.025, respe ctively. The activation energy that describes the [bmim][PF6] Viscous flow is 38.4 +/- 0.9 kJ mol(-1). The activation energy for solute rotational reo rientation in [bmim][PF6] is equivalent to the activation energy for [bmim] [PF6] Viscous flow, indicating that solute rotational dynamics are correlat ed entirely with the [bmim][PF6] dynamics. There is nanosecond dipolar rela xation surrounding a solute dissolved in dry [bmim][PF6] at 293 K. Even tho ugh CO2 is highly soluble in [bmim][PF6] (CO2 Mole fraction = 0.6 at 313 K and 68 bar), addition of up to 150 bar CO2 to [bn-dm][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.