The electrical conductivities of 1-hexyl-3-methylimidazolium halides ([Hmim][halide], halide = Cl–, Br–, I–) were measured in methanol (MeOH) and dimethyl sulfoxide (DMSO) at dilute concentrations from 293.15 to 313.15 K, alongside liquid density measurements for parametrization. Molar conductivity (Λ) decreased with increasing IL concentration and decreasing temperature, with solvent effects predominating over those of anion size. Λ was higher in MeOH than in DMSO due to lower viscosity and greater ion dissociation of MeOH. Comparison with a previous study involving H2O, MeOH, DMSO, and isopropanol confirmed that solvent viscosity is the dominant factor influencing Λ at infinite dilution. At higher IL concentrations, Λ in MeOH fell below that in H2O, likely due to a reduced number of free ions and the formation of larger solvated ion complexes.To analyze conductivity behavior, the Debye-Huckel-Onsager model was employed to determine the limiting molar conductivity (Λ0), which was subsequently used in the Shedlovsky equation to calculate the association constant (KA). For comparison, simultaneous regression of Λ0 and KA was also performed. The results indicated that, within the same solvent, Λ0 increased with temperature, while KA exhibited irregular trends. Across different solvents, Λ0 correlated with solvent viscosity, and KA was influenced by dielectric constant and polarity. Solvent effects on both Λ0 and KA were more pronounced than those of anion size, suggesting the dominant role of the solvent environment. Positive Eyring activation enthalpies showed the endothermic ion-pairing process. Additionally, the Walden product suggested stronger ion-solvent interactions and larger solvated ions in MeOH compared to DMSO. These findings provide deeper insight into IL conductivity in diverse solvent environments.

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 fluidic 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-based 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 parameters 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.

• 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.
• 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.
• 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.

Understanding the phase equilibria and physicochemical properties of the CH4–CO2–H2S–H2O–NaCl quinary system and its subsystems is essential for assessing fluid migration and changes in geological formations following CO2 or acid gas injection. Moreover, this system is closely associated with a large percentage of geological fluids responsible for transport, mass transfer, and the formation of critical mineral ore deposits. In this study, a statistical associating fluid theory (SAFT)-based equation of state (EOS) was developed to investigate phase equilibria and thermodynamic properties of the system over temperatures from 298 to 423 K, pressures up to 600 bar, and NaCl concentration up to 6 mol/kgH2O. The model incorporated pure component and cross-interaction parameters from previous studies, along with CH4–H2S cross-interactions derived from experimental data in this work. The SAFT EOS reliably predicted the phase behavior of the CH4–CO2–H2S and CH4–CO2–H2S–H2O systems, as validated against experimental data and other thermodynamic models. It also successfully predicted phase equilibria and densities for the CH4–H2S–H2O–NaCl and CH4–CO2–H2S–H2O–NaCl systems across a NaCl concentration range of 0–6 mol/kgH2O. This study provides the first systematic development of a SAFT-based model for the CH4–CO2–H2S–H2O–NaCl system, demonstrating reliable performance in characterizing their phase behavior and thermodynamic properties.

This study measured densities and viscosities of ionic liquid (IL) mixtures comprising 1-hexyl-3-methylimidazolium halides ([C6mim][Halide], where Halide = Cl–, Br–, and I–) with dimethyl sulfoxide (DMSO) (288.15–323.15 K) and enthalpies of mixing (ΔmixH) at 298.15 and 308.15 K. Excess volumes (VE) and viscosity deviations (Δη) were calculated to investigate nonideal behavior. Negative VE values indicate the formation of dense structures dominated by packing effects. Δη values suggest DMSO disrupts cation–anion interactions, causing deviations from ideality. Consistently negative ΔmixH values confirm strong ion–solvent interactions, with ([C6mim]I + DMSO) showing the most negative ΔmixH values, attributed to stronger iodide–DMSO interactions and weaker cation–anion interactions. The nonrandom two-liquid model and Gibbs–Helmholtz equation successfully modeled ΔmixH, yielding ARDs of 3.9%, 4.5%, and 3.2%, respectively. Qualitative analysis with cosolvents (H2O, MeOH, IPA) reveals the influences of cosolvent and anion on mixture properties. In organic solvents, VE and Δη trends correlate with solvent molar volume and polarity. Specific interactions lead to unique VE variations in DMSO mixtures, whereas extensive hydrogen bonding causes abnormal VE and Δη trends in aqueous IL mixtures. ΔmixH variations reflect the competition between ion–solvent and cation–anion interactions, influenced by solvent dielectric constant, polarity, and anion size.

CO2 separation is critically important for mitigating CO2 emissions, while traditional solvents used in the available absorption technologies suffer from high energy demand and low chemical stability. Deep eutectic solvents (DESs) have gained attention as a promising class of solvents, but their study and performance are still insufficient. To develop DES-based solvents for CO2 capture, in this work, based on one promising DES [ImCl][EDA] identified in our previous work, a systematic study of its aqueous solutions was conducted from the properties (density, viscosity) to absorption capacity, regeneration, and thermal stability, covering experimental measurements and theoretical modeling. In the study, the effects of temperatures (288.15 to 328.15 K), pressures (up to 1.8 MPa), and water contents (60–80 wt%) were considered, and the studied gases include pure CO2, N2, and CH4 as well as two simulated gas mixtures (simulated flue gas, 75 v% N2 + 25 v% CO2; simulated biogas, 60 v% CH4 + 40 v% CO2). The results indicate that the CO2 absorption capacity of aqueous [ImCl][EDA] reaches 3.43 mol-CO2/kg-solvent, which is 21.6 % higher than that of aqueous MEA (2.82 mol-CO2/kg-solvent), its selectivity for CO2 over N2 is up to 842 and that for CO2 over CH4 is up to 601, and the viscosity of the CO2-saturated [ImCl][EDA] solution remains below 5 mPa·s. Further, recyclability tests demonstrated its excellent regeneration performance, and TGA analysis confirmed the thermal stability of the DES. The theoretical model was used to represent the determined gas solubilities reliably. This study highlights the potential of aqueous [ImCl][EDA] and provides theoretical and experimental data to support its industrial CO2 capture applications. 

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.