The Cybotactic Region Surrounding Fluorescent Probes Dissolved in 1-Butyl-3-methylimidazolium Hexafluorophosphate: Effects of Temperature and Added Carbon Dioxide

Abstract

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]) room-temperature 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][PF6] (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.