In this work, we developed the electrolyte perturbed-chain statistical associating fluids theory (ePC-SAFT) coupled with free volume theory (FVT) using an ion-based approach (i.e., treating IL cation and anion as distinct species) to model the viscosities of 72 ionic liquids (ILs) across various temperatures and pressures. To evaluate the model performance, we compared the ePC-SAFT-FVT model employing a molecular-based approach (i.e., treating IL as a single pure substance) developed in our previous work. The results indicate that the ion-based approach demonstrates desirable performance, achieving an average ARD of 8.73%. This is comparable to the molecular-based approach, which has an average ARD of 6.09%. Importantly, the ion-based approach requires fewer adjustable parameters, reducing the number from 216 to 81 for 72 ILs, and offers enhanced flexibility by allowing the combination of both cation and anion parameters for predictions. Additionally, the ion-specific ePC-SAFT-FVT model was employed to predict the viscosities of IL mixtures, which were then compared to experimental data of 19 IL mixtures. The findings reveal that the model effectively predicts the viscosity of most IL mixtures, achieving an average ARD of 9.1%. Furthermore, the ion-based approach demonstrates superior predictive performance compared to the molecule-specific ePC-SAFT-FVT model. This study indicates that the ePC-SAFT-FVT model, using an ion-based approach, reliably represents the viscosity of pure ILs and IL mixtures, leveraging the flexibility of cation and anion parameter combinations to enhance predictive capabilities.

Ionic liquids (ILs) are promising liquid materials due to their unique physicochemical properties, driving extensive research for diverse applications. Key properties, including thermodynamic properties (e.g., density and solubility) and transport properties (e.g., viscosity and self-diffusion coefficient (SDC)), play a crucial role in developing IL-based technologies. These properties are typically characterized through experiments and theoretical modeling. However, given the time-consuming and costly nature of experiments, developing accurate theoretical models is essential for optimizing IL applications.

Thermodynamic models for various fluids, including ILs, are well-established, with the ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) effectively modeling the thermodynamic properties of ILs. In contrast, transport property models depend largely on experimental data, leading to separate modeling of viscosity and SDC. For viscosity, ePC-SAFT has coupled with free volume theory (ePC-SAFT-FVT), but inconsistencies arise between ion- and molecular-based frameworks in the two models. Meanwhile, theoretical SDC models are scarce and have yet to be applied to ILs. Traditionally, thermodynamic and transport property models use distinct molecular parameters, though, in principle, these should be independent of specific properties. This suggests the feasibility of a universal approach to determining transport properties based on molecular parameters of thermodynamic models. Additionally, as both viscosity and SDC characterize molecular motion, it remains unclear whether they can be simultaneously modeled for ILs.

This thesis aimed to propose a universal approach to modeling thermodynamic and transport properties of ILs, where a predictive SDC model and an ion-specific ePC-SAFT-FVT for viscosity were developed, and the Einstein relation was employed to explore the simultaneous modeling of viscosity and SDC.

1. In the first part, the SDC model for LJ fluids was extended to chain-like fluids using a correction function, with viscosity calculated via the Stokes-Einstein equation. By fitting SDC and viscosity data for 19 n-alkanes using molecular parameters from ePC-SAFT, a universal parameter set was obtained, achieving AARDs of 8.4% for SDC and 7.2% for viscosity. These parameters were used to predict the SDC and viscosity of long n-alkanes, branched alkanes, and cyclic compounds, with higher deviations for the latter two. The model was then extended to ILs, yielding AARDs of 39.4% for SDC and 30.1% for viscosity, with the performance considered acceptable due to using only three universal parameters.

2. In the second part, an ion-specific ePC-SAFT-FVT model was developed to describe the viscosities of 72 ILs. The ion-based approach achieves an AARD of 8.7%, comparable to the molecular-based approach (AARD = 6.1%), while significantly reducing the number of adjustable parameters from 216 to 81. This enhances flexibility by enabling cation-anion parameter combinations for predictions. The model was extended to 19 IL mixtures, yielding an AARD of 9.1%, outperforming the molecular-based approach (AARD = 12.7%). These results show the ion-specific ePC-SAFT-FVT model effectively represents the viscosity of pure ILs and their mixtures.

3. In the third part, the Einstein relation was combined with the ePC-SAFT-FVT model to describe the SDCs of ILs. Viscosity-derived FVT parameters were used to calculate the sum of ionic SDCs, requiring only one adjustable parameter. This parameter was either fitted for each IL (AARD = 8.1%) or predicted from the van der Waals volume (AARD = 10.3%). The predictive approach was also applied to calculate cationic and anionic SDCs using the total SDC and cationic transference number, yielding AARDs of 10.8% and 10.2%, respectively. These results show that, by utilizing viscosity-derived parameters, the ePC-SAFT-FVT model combined with Einstein relation effectively predicts the SDCs of ILs.

This work focused on how the addition of H₂O influences the properties of eutectic solvents with different anions and hydroxyl group numbers. Choline halides (ChCl, ChBr, and ChI) were chosen as the hydrogen bond acceptors, while ethylene glycol (EG) and glycerol (Gly) acted as the hydrogen bond donors at a 1:2 molar ratio. The density and viscosity measurements were conducted for four out of six systems, specifically those containing Br^– and I^–, across appropriate temperature and concentration ranges. Moreover, electrical conductivities were measured for all six systems. The excess molar volume and viscosity deviation were obtained and combined with those of (ChCl/EG + H₂O) and (ChCl/Gly + H₂O) for further analysis. The excess molar volume and the viscosity deviation both indicate that the contribution of H-bonding interactions is greater than packing effects, and the strengths of the H-bonding interaction are in the orders of Cl^– > Br^– > I^– and Gly > EG. Under the competition of ion concentration, viscosity, and ion interaction, the specific conductivity of the eutectic solvent solution first increases to a maximum and then decreases.

In this work, we developed a new self-diffusion coefficient model for chain-like fluids, which was coupled with the SE equation to simultaneously describe transport properties (i.e., self-diffusion coefficient and viscosity) using the parameters obtained from thermodynamic properties. In modeling, the self-diffusion coefficient model was developed based on the diffusion coefficient of LJ spherical fluids by incorporating a correction function to describe the characteristics of chain-like molecules. Subsequently, the SE equation was used to calculate the viscosity. Based on the molecular parameters in ePC-SAFT (i.e., segment number N, segment diameter σ, and energy parameter ε/kB), one set of universal parameters was determined from the self-diffusion coefficients and viscosities of 19 n-alkanes (C2H4–C20H42) at various temperatures and pressures. The model reproduces the experimental self-diffusion coefficient data (804 data points) with an average ARD of 8.4% and the experimental viscosity data (1539 data points) with an average ARD of 7.2% for 19 n-alkanes over wide ranges of temperature and pressure. Furthermore, the viscosity and self-diffusion coefficient of the other 17 compounds, including long n-alkanes, branched alkanes, and cyclic compounds, were predicted, and among them, the relatively poor prediction results of branched alkanes and cyclic compounds were further discussed. Finally, the proposed model was extended to ionic liquids, generally providing reliable results for these complex fluids. This study suggests that it is possible to describe the thermodynamic and transport properties with one set of molecular parameters based on ePC-SAFT.

The electrolyte perturbed-chain statistical associating fluids theory (ePC-SAFT) coupled with free volume theory (FVT) was combined with Einstein relation, that is, ePC-SAFT-FVT-E, to describe self-diffusion coefficients (SDCs) of ionic liquids (ILs). In modeling, ePC-SAFT was used to calculate density, while FVT parameters, determined from viscosity data, were utilized to calculate the summation of ionic SDCs through the Einstein relation with one parameter. Two strategies were employed to determine this parameter: fitting experimental data for each IL or estimating a universal parameter from van der Waals volume. Comparative analysis reveals good agreement with experimental data, with average absolute relative deviations (ARDs) of 8.14% (strategy 1) and 10.29% (strategy 2). Subsequently, cationic and anionic SDCs were reliably determined from the summation of ionic SDCs, with average ARDs of 10.80% and 10.21%, respectively. This study indicates the ePC-SAFT-FVT-E model, employing viscosity-derived parameters and three universal parameters, reliably predicts SDCs of ILs in wide temperature and pressure ranges.

Understanding the phase equilibria and physical-chemical characteristics of the CH4–CO2–H2O–NaCl quaternary system is important for evaluating costs and risks for the storage of CO2 in depleted natural gas reservoirs as well as fluid inclusion studies. In this study, phase equilibria and thermodynamic properties of this system were investigated through the utilization of a statistical association fluid theory-based (SAFT) equation of state (EOS) at temperatures from 298 to 513 K (25–240 °C), pressures up to 600 bar (60 MPa) and concentration of NaCl up to 6 mol/kgH2O. The model parameters were obtained from the fitting of available experimental data of subsystems (i.e., CH4–H2O, CH4–CO2, and CH4–H2O–NaCl) that were judged to be reliable and incorporation of available parameters for the subsystems (i.e., pure component, CO2–H2O, and CO2–H2O–NaCl). Using the SAFT EOS developed in this study, we predicted the solubility of (CH4 + CO2) gas mixtures in pure H2O and compared it with the available experimental data and the predicted values from four popular numerical simulators. The results indicate that our model can provide reliable predictions for the CH4–CO2–H2O ternary system. Subsequently, we further predicted the phase equilibria and density of the CH4–CO2–H2O–NaCl system with NaCl varying from 0 to 6 mol/kgH2O. We also employed the SAFT EOS to predict the solubility of CO2 and CH4 in the water-alternating-gas process for CO2-enhanced oil recovery, demonstrating good agreement with the simulation results obtained through the Peng-Robinson EOS for predicting the CO2 and CH4 solubility. These predicted thermodynamic properties and phase behaviors in the CH4–CO2–H2O–NaCl system provide quantitative insights into the implications of CO2 storage in depleted oil and gas reservoirs.

In this work, densities and viscosities of the mixtures of ionic liquids (ILs) 1-hexyl-3-methylimidazolium halides ([C6mim]X, where X = Cl–, Br–, and I–) and isopropanol (IPA) were measured over the temperature ranging from 288.15 to 323.15 K to investigate the influence of anions, IL concentration, and temperature on the physical properties of the mixtures of ([C6mim]X + IPA). Excess volumes (VE) and viscosity deviations (Δη) were also calculated to study the nonideal behavior of ([C6mim]X + IPA). The VE and Δη values of ([C6mim]X + IPA) are negative over the whole compositional range at all temperatures, indicating that IPA molecules preferentially coordinate with IL ions to form more densely packed structures. In addition, the molar enthalpies of mixing (ΔmixH) for the mixtures were determined under 298.15 and 308.15 K, and the nonrandom two-liquid model along with the Gibbs–Helmholtz equation was introduced to describe ΔmixH for the studied systems. The mixtures of ([C6mim]Br + IPA) and ([C6mim]I + IPA) showed endothermic behavior within the full range of compositions, while the mixtures of ([C6mim]Cl + IPA) showed endothermic first and then changed to exothermic behavior with increasing IL mole fraction.

The thermodynamic properties of four binary mixtures of deep eutectic solvent (DES), i.e., ChCl/Gly (choline chloride + glycerol at a molar ratio of 1:2) and ChCl/EG (choline chloride + ethylene glycol at a molar ratio of 1:2) with two common co-solvents, i.e., dimethyl sulfoxide (DMSO) and isopropanol (IPA), were studied. Density and viscosity were determined at temperatures from 288.15 to 323.15 K, while measuring the enthalpy of mixing was performed at 308.15 and 318.15 K. The volumetric properties (i.e., excess molar volume and excess partial molar volume) and excess viscosities (i.e., viscosity deviation and logarithmic excess viscosity) were further calculated to investigate the effects of temperature, types of co-solvent and DES, and their contents on the non-ideal behavior of these systems, where the influence of treating the DES as a mixture of two components or a pseudo-component was discussed. The results of volumetric properties indicate that the combined influence of packing effects and H-bonding interactions between the DES and co-solvents led to the contraction of mixture volume, and the results of excess viscosities show H-bonding interactions play an important role in their variations. The results of enthalpy of mixing show that the mixing of DES with IPA is endothermic, while the mixing of DES with DMSO is exothermic. Furthermore, the nonrandom two-liquid (NRTL) model and Gibbs-Helmholtz equation were combined to represent the experimental results of the enthalpy of mixing, and the total average relative deviation (ARD) of all studied systems is 5.43%.