SAFT-gamma force field for the simulation of molecular fluids: 3. Coarse-grained models of benzene and hetero-group models of n-decylbenzene

In the first paper of this series [C. Avendano, T. Lafitte, A. Galindo, C. S. Adjiman, G. Jackson, and E. A. Muller, J. Phys. Chem. B 115, 11154 (2011)] our methodology for the development of accurate coarse-grained (CG) SAFT-gamma force fields for the computer simulation of molecular fluids was introduced with carbon dioxide as a particular case study. The procedure involves the use of a molecular-based equation of state to obtain effective intermolecular parameters (from experimental fluid phase equilibrium data) appropriate for molecular simulation over a wide range of fluid conditions. We now extend the methodology to develop coarse-grained models for benzene (C6H6) that can be used in fluid phase simulations. Our SAFT-gamma CG force fields for benzene consist of a simple single-segment spherical model, and a rigid three-segment ring structure of tangent spherical groups interacting via Mie (generalized Lennard-Jones) segment-segment interactions. The description of the fluid phase behaviour of benzene with our simplified CG force fields is found to be comparable to that obtained with the more sophisticated models commonly used in the field; a marked improvement is seen with our SAFT-gamma models for the vapour pressure, particularly at lower temperatures. These models of benzene together with the previously developed SAFT-gamma three-segment chain model of n-decane are used to develop hetero-group force fields for n-decylbenzene, in the spirit of a group contribution methodology. In our approach, the parameters of the phenyl and n-decyl groups are obtained transferably from the individual models of benzene and n-decane, respectively, and the unlike energetic parameters between the phenyl and decyl segments can be obtained from vapour-liquid equilibria data for n-decylbenzene using the SAFT-gamma equation of state. The resulting CG hetero-group models are found to describe the fluid properties of n-decylbenzene over a wide range of conditions, exemplifying how our approach can be used as a group contribution methodology. This is the first example of the development of hetero-group SAFT-gamma force fields for molecules formed from Mie segments of different size, energy, softness/hardness, and range.