(614k) PC-SAFT-DFT Development for the Complex Fluids Confined in Pores

The properties of fluids confined in nanopores are different from those in the bulk phase due to geometrical restrictions and wall-molecule interactions, and understanding and prediction of the confined properties are of considerable importance in various industrial processes and scientific fields. Experimental measurements, molecule simulations and theoretical models have been used to represent the inhomogeneous behavior of confined fluids, in which the classical density functional theory (DFT) in general provides reasonable description of inhomogeneous fluids with moderate computational cost. A variety of DFT models have been developed but it is infeasible to use such models to represent properties of real substances due to the fact that most of these models were applied for model molecules with integer segment numbers. For other DFT models, little effort has been put into incorporating the long-range attraction perturbation and electrostatic correlation that reduces to the SAFT EoS in the bulk limit.

We developed a hybrid PC-SAFT-DFT model, the coupling of density functional theory (DFT) with perturbed-chain statistical associating fluid theory (PC-SAFT), to study the properties of the fluids confined in pores from pure to mixtures. In the developed model, the modified fundamental measure theory was used for the hard sphere contribution; the dispersion free energy functional was represented with weighted density approximation (WDA) by averaging the density in the range of attraction, the chain free energy functional from interfacial SAFT was used to account for the chain connectivity, and the ionic free energy functional was constructed by extending the bulk electrolyte PC-SAFT using a WDA approach.

The developed PC-SAFT-DFT model was used to predict the properties of the confined “model” fluids, in order to compare with those from molecular simulation for model verification. It shows that the model-predicted density distributions of atomic, chain-like and ionic fluids are in good agreement with the results of molecular simulation. The developed model was further used to describe the gas adsorption isotherm, distribution of ionic liquid (IL) and gas/IL mixture in nano-pore. The components of the fluids were modeled as chain molecules, and the parameters were taken from those in the bulk PC-SAFT. The external force field was used to describe the interaction between pore and fluid. In modeling of gas adsorption in porous materials, the parameters were obtained from the fitting of data measured experimentally. In modeling of nano-confined IL, the Lorentz-Berthelot mixing rules was used to obtain the solid-fluid potential parameters. The model prediction was compared with available experimental data and molecular simulation data, which shows that the predictions are reliable for most of cases studied in this work.