Dissolution of Trihexyltetradecylphosphonium Chloride in Supercritical CO2

Abstract

We present steady-state and time-resolved fluorescence spectroscopic data derived from coumarin 153 (C153) in a binary solution comprised of trihexyltetradecylphosphonium chloride ([P6,6,6,14]+Cl−) and supercritical CO2 (scCO2). Steady-state fluorescence of C153 was measured in neat scCO2 and ionic liquid (IL)-modified scCO2 solutions. The steady-state excitation and emission peak frequency data in neat scCO2 and IL/scCO2 diverge at low fluid density (ρr = ρ/ρc < 1). The prominent spectral differences at low fluid density provided clear evidence that C153 reports different microenvironments, and suggested that the IL is solubilized in the bulk scCO2 and heterogeneity of the C153 microenvironment is readily controlled by scCO2 density. C153 dimers have been reported in the literature, and this formed the basis of the hypothesis that dimerization is occurring in scCO2. Time-dependent density functional theory (TD-DFT) electronic structure calculations yielded transition energies that were consistent with excitation spectra and provided supporting evidence for the dimer hypothesis. Time-resolved fluorescence measurements yielded triple exponential decays with time constants that further supported dimer formation. The associated fractional contributions showed that the dominant contribution to the intensity decay was from C153 monomers, and that in high density scCO2 there was minimal contribution from C153 dimers.

Keywords

• ionic liquid
• trihexyltetradecylphosphonium chloride
• supercritical fluid
• carbon dioxide
• time-resolved fluorescence
• electronic structure calculations
• physical chemistry
• characterization
• chemical engineering applications