Solution structures of 1-butyl-3-methylimidazolium hexanuorophosphate ionic liquid saturated with CO2:: Experimental evidence of specific anion-CO2 interaction

X-ray diffraction measurements for 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid ([BMIM]-[PF6])-CO2 systems were carried out at high pressures with a newly developed polymer cell. The intermolecular distribution functions (g(inter)(r)) were obtained at 25 degrees C for neat [BMIM][PF6] and its solutions saturated with CO2 at 4 and 15 MPa, where the mole fractions (x) Of CO2 correspond to 0.5 and 0.7, respectively. In g(inter)(r) for x = 0.5, two peaks appeared at around 2.8 and 3.2 angstrom. These two peaks in g(inter)(r) appreciably increased for x = 0.7; moreover, there was another peak observed at similar to 3.8 angstrom. Only assuming the correlations between CO2 and [PF6](-), it is reasonably determined that the nearest-neighbor P([PF6](-))center dot center dot center dot C(CO2) distances are 3.57 and 3.59 angstrom with the coordination numbers being 1.8 and 4.0 for x = 0.5 and 0.7, respectively. It is concluded that CO2 molecules are preferentially solvated to the [PF6](-) anion.