Fluid Phase Equilibria, Vol.263, No.2, 182-189, 2008
Transferable step potentials for perfluorinated hydrocarbons
The step potential equilibria and discontinuous molecular dynamics (SPEADMD) model is adapted for characterizing the interaction potentials of perfluorocarbons and their mixtures with n-alkanes. We seek to explain the peculiar behavior of these systems, especially with regard to the unfavorable mixing behavior. The methodology is based on discontinuous molecular dynamics (DMD) and second order thermodynamic perturbation theory (TPT). DMD simulation is applied to the repulsive part of the potential. The effects of disperse attractions and hydrogen bonding are treated by TPT, discretizing the attractive potential into four distinct wells of variable depth. This approach accelerates the molecular simulations in general and the parameterization of the transferable potentials in particular. C-3-C-8 straight chain perfluorocarbons are characterized along with perfluorobenzene, perfluorocyclobutane, and heptafluoropropane applying explicit atom models for all fluorine atoms. Each compound is simulated at 21 densities. Interpolation with density combines with TPT to give a complete equation of state. The depths of the attractive wells are optimized by iterating on their values until the vapor pressures computed by the resulting equation of state provide the minimum deviation from experimental data. Step potentials offer the prospect of setting the step depths without preconceived notions regarding the shape of the potential. In the case of perfluorocarbons, it is found that the minimum of the potential is reached at roughly 1.8 sigma, with the range from sigma to 1.8 sigma acting as a soft "shoulder" region which is weakly attractive, where or designates the site diameter. This shoulder region can explain the unusually low heat of vaporization of perfluorocarbons and high vapor pressures relative to compounds with similar molecular volume. Application of this approach results in average absolute deviations of 8% in vapor pressure and 3% in liquid density. Coincidentally, the soft shoulder also explains the weak mixture interactions between perfluorocarbons and n-alkanes. The Lorentz-Berthelot combining rule assumed to describe site-site interactions between corresponding steps leads to epsilon(ijk)similar to (epsilon(ijk)epsilon(jjk))(1/2) where epsilon(ijk) is the depth of the potential well between sites i and j in the kth step. For steps at short range, the alkane steps are at their deepest, but the perfluorocarbon interactions are weak, making the mixed interaction weak. For steps at long range, the perfluorocarbon steps are deeper, but the alkane interactions are weak, again making the mixed interaction weak. (c) 2007 Elsevier B.V. All rights reserved.
Keywords:physical properties;vapor pressure;density;phase equilibria;molecular simulation;perfluorinated;SPEADMD