Local intermolecular structure and dynamics in binary supercritical solutions. A molecular dynamics simulation study of methane in carbon dioxide

The binary supercritical mixture methane-carbon dioxide (SC CH4-CO2) with mole fraction of methane X = 0.2 was investigated at temperature T = 323.15 K and density ρ = 0.23814 g/cm3 corresponding to a pressure 9.94 MPa, by using the molecular dynamics (MD) simulation technique in the canonical (NVT) ensemble. The local intermolecular structure of the fluid was investigated in terms of the appropriate pair radial distribution functions, which have been calculated and analyzed. The estimated average local coordination numbers and mole fractions around the species in the mixture have been used to explore the repulsive or attractive nature of the fluid. The results obtained have led to the conclusion that the SC CH4-CO2 mixture at these conditions might be characterized as a repulsive one. This result supports the conclusions of a previously reported experimental work by Baglin et al. [F.G. Baglin, S.K. Murray, J.E. Daugherty, T.E. Palmer, W. Stanbery, Mol. Phys. 98 (2000) 409.]. The single reorientational dynamics of both the solute and solvent species in the mixture were also investigated by calculating the appropriate time correlation functions. The observed time dependence of these functions are discussed in relation with the local environment around the species, which in the case of the solute molecules seems to be strongly affected. © 2005 Elsevier B.V. All rights reserved