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 ([C_nmim]⁺, [C_npy]⁺, [C_nmpy]⁺, [C_nmpyr]⁺, 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.

An ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) has been developed to describe the thermodynamic and transport properties of ionic liquids (ILs) for developing IL-based technologies to separate CO₂ from gas mixtures. However, as it has been pointed out previously, SAFT-based models can lead to a pitfall, manifesting as an additional fictitious liquid-liquid critical line connected to an additional mechanically stable critical point when modeling pure substances. In this work, a method for detecting a pitfall in ILs was developed, where an expression with ionic term for calculating the critical point with ePC-SAFT was derived and an algorithm for detecting the additional fictitious phase equilibrium for ILs was proposed. The pitfall occurrence for an extended set ILs taken from our previous work was investigated. It shows that a pitfall occurs only for one single IL among all 96 ILs in the temperature and pressure range of interest. For [C₈mpy][BF₄], at 290.27–291.78 K, a pitfall may occur at the pressures of interest; at 273.15–290.20 K, it may occur at any given pressure. The ion-specific ePC-SAFT parameters for [C₈mpy]⁺ may therefore need to be modified in the future when more reliable experimental results are available for parameter fitting.

Viscosity is one of the most important physical properties when developing ionic liquids (ILs) for industrial applications such as CO₂ separation. The viscosities of ILs have been measured experimentally, while the modeling work is still limited. In this work, the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) was combined with the free volume theory (FVT) to model the viscosities of pure ILs and IL mixtures up to high pressures and temperatures, in which the ePC-SAFT was used to calculate the density as inputs for modeling the viscosity of ILs with FVT. The ILs under consideration contain one of the IL cations [C_nmim]⁺, [C_npy]⁺, [C_nmpy]⁺, [C_nmpyr]⁺, or [THTDP]⁺ and one of the IL anions [Tf₂N]⁻, [PF₆]⁻, [BF₄]⁻, [tfo]⁻, [DCA]⁻, [SCN]⁻, [C₁SO₄]⁻, [C₂SO₄]⁻, [eFAP]⁻, Cl^–, [Ac]⁻, or Br^–. In total, 89 ILs were considered combined with a thorough literature survey of the available experimental viscosity data and evaluation. The comparison with the available experimental viscosities shows that the model can provide reliable representation and prediction for most of the pure ILs in a wide temperature and pressure range, and it can be further used to predict and describe the viscosity of IL mixtures reliably.

In this work, the ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) was extended to predict the second-order thermodynamic derivative properties and gas solubility of the ionic liquids (ILs) containing one of the IL cations ([C_nmim]⁺, [C_npy]⁺, [C_nmpy]⁺, [C_nmpyr]⁺, and [THTDP]⁺) and one of the IL anions ([Tf₂N]⁻, [PF₆]⁻, [BF₄]⁻, [tfo]⁻, [DCA]⁻, [SCN]⁻, [C₁SO₄]⁻, [C₂SO₄]⁻, [eFAP]⁻, Cl^–, [Ac]⁻, and Br^–). The ideal-gas isobaric heat capacities of ILs were estimated by the group contribution method for obtaining the heat capacity. The model prediction results were compared with the available experimental data, and the comparison shows that the ePC-SAFT prediction is reliable for most ILs. Furthermore, one adjustable ion-specific binary interaction parameter between the IL ion and CO₂ can be used to further improve the model prediction performance for the CO₂ solubility in ILs.

The occurrence of numerical pitfalls has been pointed out for the models of equation of state, including the statistical associating fluid theory (SAFT)-based. Following our previous work on pure ILs when modeling with ePC-SAFT, this work extended the research to the IL–gas mixtures, in which the associating term was derived for predicting the critical point with PC-SAFT, and a scheme for detecting a pitfall in IL–gas mixtures was developed. Then, the scheme was applied to detect the pitfall occurrence in IL–gas (gas = CO₂, H₂S, CO, O₂, CH₄, N₂, and H₂) systems, which are commonly encountered in developing IL-based CO₂ separation technologies. It shows that a pitfall occurs only in [C₈mpy][BF₄]–gas (CO₂, H₂S, CO, O₂, CH₄, N₂, and H₂) among all ILs considered in the temperature and pressure range of interest.