Translational diffusion coefficients of diphenylcyclopropenone (DPCP), diphenylacetylene (DPA), and carbon monoxide (CO) in carbon dioxide (CO2) mixtures of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIm][NTf2]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), respectively, were determined by transient grating (TG) spectroscopy under various pressures of CO2. With increasing CO2 pressure up to 15 MPa, the diffusion coefficients of DPCP and DPA increased by an order of magnitude, while the increase of the diffusion coefficient of CO was relatively small. Sound velocities of the mixtures were also determined by TG spectroscopy, which decreased by ca. 20% with increasing pressure up to 10 MPa, and then turned to increase with pressure. The solvation structure around DPCP was also investigated by Raman spectroscopy. It was found that the vibrational spectra of solute and solvent molecules did not show remarkable changes. Small-angle X-ray scattering (SAXS) profiles for CO2 mixture of 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIm][BF4]) were measured under various pressures of CO2. The peak observed for the SAXS profile at 3 nm−1 for [OMIm][BF4] did not show meaningful shift with increasing CO2 pressure, while the peak width showed a small increase.

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Effects of dissolved carbon dioxide (CO 2) on the solvation dynamics of coumarin 153 were studied in 1-butyl-3-methylimidazolium hexafluorophosphate, and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide by the time resolved fluorescence spectroscopy. The solvation dynamics showed an ultrafast response less than 1 ps and a slower response extended to a half of ns under ambient condition. With increasing the CO 2 pressure, the slower component became fast, although no significant change was observed for the faster component. The CO 2 effect on the average solvation time was compared with that on the translational diffusion of the solute molecule dissolved in the mixture.

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