Liquid-vapor isotopic fractionation factors of diatomic fluids: A direct comparison between molecular simulation and experiment

Liquid-vapor fractionation factors of molecular fluids are studied by molecular-based simulation, Gibbs ensemble Monte Carlo, and isothermal-isochoric molecular dynamics of realistic models for N ₂, O ₂, and CO. The temperature dependence of the fractionation factors for ¹⁵N ¹⁴N/ ¹⁴N ₂, ¹⁵N ₂/ ¹⁴N ₂, ¹⁸O ¹⁶O/ ¹⁶O ₂, ¹⁸O ₂/ ¹⁶O/ ¹⁶O ₂, ¹³C ¹⁶O/ ¹²C ¹⁶O, and ¹²C ¹⁸O/ ¹²C ¹⁶O along the vapor-liquid coexistence curves as predicted by simulation is compared with the existing experimental data to assess the accuracy of ℏ ²-order Kirkwood-Wigner free energy expansion for specific model parametrizations. Predictions of the fractionation factors for other isotopologue pairs, including ¹⁸O ¹⁷O/ ¹⁶O ₂, ¹⁶O ¹⁷O/ ¹⁶O ₂, and ¹⁷O ₂/ ¹⁶O ₂, as well as tests of some approximations behind the microscopic interpretation of the fractionation factors are also given.