A transferable potential model for the liquid-vapour equilibria of fluoromethanes

Direct molecular dynamics simulations of the coexisting liquid and vapour phases have been used to calculate the orthobaric curves of the three fluorocarbon refrigerants CH2F2, CHF3 and CF4. By comparison with experiment we have derived a force-field for these molecules consisting of a 12-6 Lennard-Jones potential for each atom with additional point charges to model the Coulombic interactions. The orthobaric densities, vapour pressure and latent enthalpy of vaporization of the model fluids have been determined at a range of temperatures between 200 K and 300 K for CH2F2, between 180 K and 270 K for CHF3 and between 120 K and 200 K for CF4, and compared with experiment. The radial distribution function of the coexisting liquids has been determined for the potential model and compared with the weighted g(r) obtained from coherent neutron scattering. The potential models are adequate for predicting the experimental orthobaric curves. The liquid structure of CHF3 is predicted accurately but the experimentally determined weighted radial distribution for CH2F2 could not be reproduced.