Molecular dynamics investigation on the infinite dilute diffusion coefficients of organic compounds in supercritical carbon dioxide

Abstract

A new approach to represent the infinite dilute diffusion coefficients D 12 ∞ of organic compounds in supercritical carbon dioxide by molecular dynamics simulation from the molecular specific constants was proposed. From the experimental critical constants of organic compounds or critical constants estimated by Joback–Lydersen methods and the equation of state of Lennard–Jones fluid, new Lennard–Jones parameters of 38 organic compounds were obtained according to the corresponding state principle. With these parameters, D 12 ∞ for some high-value-added biochemical and pharmaceutical compounds in supercritical carbon dioxide at 313.15 K and 16.0 MPa were predicted for the first time by molecular dynamics simulation with simple combining rules. Without any adjustable parameters, the accuracy of prediction reached the qualitative or semi-quantitative level. The effects of combining rules on the simulation results were also reported. It was shown that the empirical combining rule proposed by the authors (ZLWS) σ 12=(1/4)σ 11+(3/4)σ 22, ε 12= ε 11ε 22 predicted more precisely than the Lorentz–Berthelot combining rule did. It was also found that D 12 ∞ of organic compounds in supercritical carbon dioxide were highly related with the critical constants. A new empirical equation D 12 ∞=6.58×10 −8( T c/ P c) −0.3471 for D 12 ∞ of organic compounds in supercritical carbon dioxide at 313.15 K and 16.0 MPa was proposed. This equation has a simple form and good predictive function.