Optimization of the anisotropic united atoms intermolecular potential for n-alkanes

Abstract

The parameters of the anisotropic united atoms potential for linear alkanes proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 107, 5197 (1997)] have been optimized on the basis of selected equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities) of ethane, n-pentane, and n-dodecane. The optimized parameters for the CH2 and CH3 groups form a regular sequence with those of methane and the force centers are found between the carbon and hydrogen atoms, as expected. The resulting potential, called AUA4, has been compared with Toxvaerd’s potential (AUA3) by using several molecular simulation methods (Gibbs ensemble Monte Carlo, thermodynamic integration, and molecular dynamics). An investigation performed at temperatures ranging from 140 to 700 K and with various chain lengths up to 20 carbon atoms has shown AUA4 to provide systematic improvements of vapor pressures, vaporization enthalpies, and liquid densities for pure n-alkanes. Significant improvements have been also noticed on the critical temperatures of n-alkanes, estimated from coexistence density curves, and on the equilibrium properties of CO2–n-alkane binary mixtures. Self-diffusion coefficients of n-hexane, however, are slightly improved by the new potential, but still exceed experimental measurements at low temperature. As we have only optimized the intermolecular potential in the present study, it is suggested that further optimization of the intramolecular potentials of the anisotropic united atoms model could allow simultaneous prediction of thermodynamic properties and of transport coefficients, particularly in very dense liquids.

Optimized intermolecular potential for nitriles based on Anisotropic United Atoms model

“…We followed the optimization method used by Ungerer et al [16] to optimize the AUA3 potential parameters obtained by Toxvaerd for nalkanes [27] and applied after by Bourasseau et al for chemical groups specific to branched and cyclic alkanes [16,17].…”

“…The first parameter version of this potential (AUA3 parameters) has shown its limitation and has been optimized for n-alkanes by Ungerer et al in 2000 to obtain a new set of energetic parameters named as AUA4 potential [16] resulting in better predictions of vapor pressures, vaporization enthalpies, liquid densities calculation and the critical temperatures of n-alkanes estimated from coexistence density curves.…”

“…A different approach toward molecular mechanics modeling is the use of a generic force field, meaning that it can be used for a large set of molecules sharing common elementary beads and for the calculation of many properties from transport phenomena coefficients to adsorption coefficients and phase equilibrium [16,29]. Calculation of different properties using macroscopic models requires different unrelated models, an evidence of the intrinsic genericity of molecular based approaches.…”

“…Involved in this idea, the present work focuses on the development of a set of transferable Lennard-Jones (LJ) parameters for the nitrile group (-C≡N) as an addition to the Anisotropic United Atoms (AUA) force field parameter set that already contains transferable parameters for linear, branched and cyclic alkanes, olefins, aromatic, ketone and aldehyde molecular fluids [16][17][18][19][20][21]. Nitriles are industrial solvents and good representative of polar compounds which phase equilibrium is not trivial to model with macroscopic thermodynamic models [22]: popular cubic equation of states are suited for the investigation of any pressure condition but poorly applied to polar molecules.…”

“…Activity coefficient models handle polar compounds but are valid only at low pressures. After the optimization of molecules geometry and the computation of atomic charges by a density functional theory method, a methodology similar to the one used by Ungerer et al [16] is followed. The predictive capacity of the GEMC method coupled with a suitable interaction model is demonstrated for nitriles in the present work.…”