Models of a thermodynamic potential for molecular interactions in the fluid phase

Abstract

A thermodynamic potential accounting for molecular interactions is used to gain insights into fluid phase molecular interactions over a wide range of temperature and density. It is a lattice based mean field approximation incorporated into a Helmholtz equation of state (EOS) for pure fluids. Excess molar Helmholtz energy is fe=ε−Tsc, where ε is thermodynamic interaction potential energy and sc is configurational entropy. (This is not the usual definition of excess over ideal; here P=−(∂fe/∂v)T.) The work shows that at low temperatures ε has the characteristics of a mechanical potential such as the Lennard-Jones potential but with a temperature dependence. At low temperatures there is a definite potential energy well that is washed away by rising temperature. At high temperatures net interactions are strictly repulsive. In the liquid state ε can be calculated using Lennard-Jones Devonshire cell theory for spherically symmetric molecules. This is demonstrated with neon, argon, krypton, and xenon. Van der Waals EOS, when recast as a lattice model, exhibits similar interaction potential behavior as found in this work. A method is proposed for separating potential energy arising from intramolecular bonds from that arising from intermolecular interactions, allowing for models of mixtures with different size molecules. This is demonstrated in a series of n-alkanes.