Evolution with the density of CO2 clustering studied by Raman spectroscopy

We have recently studied by Raman spectroscopy pure supercritical CO2 in the vicinity of the critical domain (307 K) and detected the spectral signatures associated with the formation of transient dimers in a large domain of density and supported by ab initio calculations. Moreover, we put in evidence, in this pure fluid, local density enhancement (LDE) effects due to large density fluctuations through the analysis of the shifts and widths of the Fermi dyad. In the current study we present new Raman measurements along a higher isotherm (323 K, T⁎ = T/TC = 1.06) in the reduced density range 0.04 < ρ⁎ = ρ/ρC < 2.0 (ρC ~ 467.6 kg m− 3, ρC is the critical density). We intend to show the variation of the CO2 clustering as the temperature departs from the critical one. Specific spectral signatures have been observed in three domains. In the region of the ν2 bending mode a new band has been detected. This observation, predicted from ab initio calculations, is interpreted as the activation of the bending mode after degeneracy removal due to the formation of (CO2)2 parallel slipped dimers. A broad polarised band has been observed between the Fermi dyad peaks and assigned on the basis of the symmetry arguments to the complex formation. A detailed analysis of the Fermi dyad showed that two Lorentzian profiles are needed to describe the band shape of each component of the dyad. The ensemble of these results confirms that CO2 molecules can be detected in two kinds of environments during the Raman time scale of observation. The molecules can form transient dimers and also can interact with themselves non-specifically. The evolution with the density of the band centre positions and bandwidths exhibits a non linear behaviour for ρ⁎ ranging from 0.4 to 1.7. These departures, less pronounced than those observed along the near critical isotherm, have been interpreted as due to the enhancement of the local density of the fluid compared to its bulk density and the reduced local density excess Δρ⁎ = ρ⁎loc − ρ⁎bulk has been evaluated. The results here presented are consistent with those observed along the near critical isotherm.