The structure of liquid carbon dioxide and carbon disulfide
Résumé
We present neutron and x-ray scattering data (a 2N+X experiment) of liquid CO2 and CS2 at a density of about 10 molecules/nm3. Because the scattering length contrast of the carbon isotope is very small and, in fact, smaller than anticipated from standard scattering length tables, a direct partial structure factor determination via matrix inversion gives unconvincing results. Instead we search for the best representation of the three independent scattering data sets by a simulation of rigid molecules interacting via a 12-6-1 potential, furthermore restricting the pressure p, the density ρ, and the temperature T to the experimental values. We show that a 12-6-1 potential is completely adequate to describe the structure of CO2; for CS2 we find that the best 12-6-1 potential still slightly overestimates the height of the sulfur-sulfur pair-distribution function gSS. Orientational correlations reflect the similarities much more than the differences of the two molecular systems. The distinct differences in the atom-atom pair distribution functions of CO2 and CS2 do not mean that their structures are radically different and the comparison with the crystalline structures is somewhat deceptive. A linear transformation, wherein all the parameters describing the interaction and the geometry of CS2 are changed to those of CO2, allows us to point out the physical parameters which may be responsible for the differences or similarities in thermodynamic behavior (pressure) and structures (orientations) between the two liquids.