Modeling interfacial properties of ionic liquids with ePC-SAFT combined with density gradient theory

In this work, ePC-SAFT-DGT (i.e., the coupling of ePC-SAFT with the density gradient theory (DGT)) was further developed by modifying the expression for estimating the chemical potential of IL ion-pair and extended to study the interfacial properties of 82 ionic liquids (ILs) containing one of the IL-cations ([Cnmim]+, [Cnpy]+, [Cnmpy]+, [Cnmpyr]+, and [THTDP]+) and one of the IL-anions ([Tf2N]−, [PF6]−, [BF4]−, [tfo]−, [DCA]−, [SCN]−, [C1SO4]−, [C2SO4]−, [eFAP]−, Cl−, [Ac]−, and Br−). The available experimental surface tensions for these 82 ILs from the literature have been surveyed and evaluated for adjusting the model parameters before the investigation. It shows that the modification results in more reasonable magnitude of influence parameters and ePC-SAFT-DGT can be used to represent the surface tension of ILs reliably compared with the experimental data. Furthermore, using the anion-specific influence parameters that are linearized with the molecular weight of the IL-cations for a homologous series of ILs allows semi-predicting (i.e., parameters obtained by interpolation and extrapolation) the surface tension for the ILs in the same homologous series. ePC-SAFT-DGT can be further used to predict other interfacial properties, for example, the density profile and interfacial thickness in the vapor-liquid interface.