Local Density Enhancement in Supercritical Carbon Dioxide Studied by Raman

Dezembro 2007

The polarized IVV and depolarized IVH Raman profiles of the Fermi dyad (1285 cm-1 and 1388 cm-1) of supercritical (SC) CO2 have been measured along the isotherms 307 K, 309 K, 313 K and 323 K in the reduced density range 0.04 < &#961;* = &#961;/&#961;C < 2.04 (&#961;C is the critical density). A band shape analysis of the dyad component shows that each one can be decomposed in two well-defined Lorentzian profiles in all the temperature and density range investigated. These profiles have been assigned with the transitions of CO2 probing two kinds of environments. In each dyad peak, the Lorentzian profiles centred at higher frequency is associated with CO2 interacting through usual Van der Waals interactions with its nearest neighbours. The Lorentzian profiles centred at lower frequency in each dyad peaks have been related to the transition of CO2 involved in a transient (CO2)2 dimer. The evolution with the density of the band centre positions and bandwidths of the Lorentzian profiles shaping the lower and upper dyad components exhibits a non linear behaviour along the near critical isotherm (307 K) for &#61554;* ranging from 0.4 to 1.7. This behaviour, although less pronounced is still detected at higher temperatures. The deviation from linearity was interpreted as due to an enhancement of the density which leads to a reduced local density excess &#916;&#961;* = &#961;*loc - &#961;*bulk. Even if the spectroscopic observables involved probe differently the interactions in SC CO2, we emphasize that the scaled spectral features are straightforwardly related with the Local Density Enhancement (LDE) phenomenon taking place in SC fluids (SCF). We show that the LDE effect can be also put in evidence from the band shape analysis of the weak satellite band situated at 1370 cm-1 associated with the upper Fermi dyad transition of the 13C16O2 molecule (1% isotopic natural abundance). The asymmetric shape of the evolution with density of the &#916;&#961;* found from our spectroscopic observables shows some similitude with that obtained in a recent MD simulation [Samios et al. 2007] and the possible inter-connection and difference with these calculations are discussed.

Autores da comunidade :

• 

  M

  Maria Isabel Fialho Cabaço

  ist10559

Journal of Physical Chemistry A

12966

12971

111

50

physical-sciences - Física

eng - Inglês