A TRANSFERABLE POTENTIAL MODEL FOR THE LIQUID-VAPOR EQUILIBRIA OF FLUOROMETHANES

Direct molecular dynamics simulations of the coexisting liquid and vap our phases have been used to calculate the orthobaric curves of the th ree fluorocarbon refrigerants CH2F2, CHF3 and CF4. By comparison with experiment we have derived a force-field for these molecules consistin g of a 12-6 Lennard-Jones potential for each atom with additional poin t charges to model the Coulombic interactions. The orthobaric densitie s, vapour pressure and latent enthalpy of vaporization of the model fl uids 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 a nd 200 K for CF4, and compared with experiment. The radial distributio n function of the coexisting liquids has been determined for the poten tial model and compared with the weighted g(r) obtained from coherent neutron scattering. The potential models are adequate for predicting t he experimental orthobaric curves. The liquid structure of CHF3 is pre dicted accurately but the experimentally determined weighted radial di stribution for CH2F2 could not be reproduced.