MOLECULAR-DYNAMICS SIMULATIONS OF FLUORINATED ETHANES

Differences in thermodynamic properties are investigated by means of c onstant pressure-constant temperature molecular dynamics simulations f or three isomeric pairs of fluorinated ethanes: CHF2CHF2 (HFC-134) and CF3CH2F (HFC-134a); CF3CH3 (HFC-143) and CHF2CH2F (HFC-143a); and CH2 FCH2F (HFC-152) and CHF2CH3 (HFC-152a). These semi-rigid molecules hav e interaction centres at the atomic sites and internal rotation about the C-C bond. Intermolecular interactions consist of repulsion-dispers ion and Coulombic parts. Molecular dynamics simulations were performed at the normal boiling points and the accord between simulated and exp erimental values of the potential energy and molar volume is very good . A great difference was found between the repulsion-dispersion and Co ulombic contributions to the potential energy for every isomeric pair. A detailed analysis of the repulsion-dispersion and Coulombic interac tions was carried out and was discussed in relation to the thermodynam ic properties. A comparison of the radial distribution functions, auto correlation functions and self-diffusion coefficients for these substa nces is also presented.