(223f) Interfacial Properties of Realistic Molecular Fluids: A Combined Molecular Simulation and Classical Density Functional Theory Approach

The main objective of our work is to construct a unified platform, in the form of a transferable expression for the Helmholtz energy of fused soft-sphere chains, to simultaneously assess bulk and inhomogeneous properties of realistic molecular fluids. For the bulk phases, the repulsive, dispersion and electrostatic contributions of the Helmholtz free energy are evaluated via a robust comparison with molecular simulations. The Transferable Potentials for Phase Equilibria (TraPPE) at the united-atom level of coarse-graining is used in all simulations. We employ a third-order Weeks Chandler-Anderson (WCA) Thermodynamic Perturbation Theory (TPT) to establish the bridge between the theory and simulation for the repulsive and attractive interactions. For the electrostatic contribution, the long-range interactions are described by an equivalent short-range association potential in the framework of primitive models.

The extension to inhomogeneous phases is carried out by adapting the classical Density Functional Theory (DFT). The equilibrium density profiles of individual united-atom groups are obtained by minimizing the Helmholtz free energy while imposing the equality of chemical potential for variations of density in one direction. To this end, we developed a monomer functional to reproduce the microstructure of soft spheres for a wide temperature and density range. The transition from monomers to chains was carried out using the Statistical Associating Fluid Theory (SAFT). The current state-of-the-art for the inhomogeneous chain functional was further improved to take into account the fused nature of molecules. We have also superseded the widely-used Mean Field approximation and developed a non-local dispersion functional for the attractive interactions. For the bulk fluid properties, our approach shows satisfactory agreement with simulation data of pure fluids and mixtures comprising alkanes, ethers, 1-alkanols, nitriles, water, and carbon dioxide. By average, the deviation of saturated liquid density and vapor pressure of pure fluids from simulation data is about 1% and 5%, respectively. The same level of agreement is also achieved for inhomogeneous properties including surface tension, surface excess and microstructure against walls, in nano-pores, and at vapor-liquid interfaces. For example, by average the surface tension deviates about 5% from simulation data. The detailed agreement with atomistic molecular simulations also holds for the microstructure of more complicated systems such as 1-hexanol, acting as a surface-active compound at the vapor-liquid interface of water. In such cases, our theory provides qualitative agreement with the molecular simulation in showing a clear separation between the density profile of the hydrophilic head and hydrophobic tail groups. This combination of simulation and theory makes it possible to predict the interfacial properties of larger molecules than can be accessed by molecular simulation, while maintaining consistency with the TraPPE model for all molecular interactions.

Cf. Ghobadi and Elliott, JCP, 139:234104 (2013).