In this work, the excess Gibbs free energy models, i.e., non-random two-liquid (NRTL) model and electrolyte NRTL model, including the original one and those with new strategies (association or hydration), were used to describe the macroscopic properties and interpret the microstructure of ionic liquid (IL) - H2O binary systems, clarifying the role of IL association and ion hydration in model development. To provide systematic data for model development, the enthalpy of mixing of three imidazolium-based IL-H2O systems containing the same cation but different sizes of anions, i.e., Cl−, Br−, and I−, were measured. The models were developed and evaluated based on the newly measured data and the osmotic coefficient from the literature. The results reveal that the model reflecting the intrinsic mechanism of dissociation and hydration gives the best modeling results; and the ionic strength and the degree of IL dissociation as a function of water content can be predicted using the newly established model. The study clarifies the significance of IL association and anion hydration in model development and quantitatively demonstrates how water content influences the microstructure and real species in IL-H2O systems.

To improve the efficiency of electrolyte perturbed-chain statistical associatingfluidtheory–density functional theory (ePC-SAFT-DFT) calculation of the confined system, inthis work,first, the Chebyshev pseudo-spectral collocation method was extended to thespherical pores. Second, it was combined with the Anderson mixing algorithm toaccelerate the iterative process. The results show that the Anderson mixing algorithmcan reduce the computation time significantly. Finally, based on the accelerated ePC-SAFT-DFT program, a systematic study of the effects of the temperature, pressure, poresize, and pore shape on the CO2solubilities in the ionic liquids (ILs) confined inside the silicananopores was conducted. Based on the simulation results, to obtain high CO2solubilitiesin the ILs confined in silica, a better option is to use the silica material with a narrowspherical pore, and the IL-anion should be selected specifically considering that it has amore significant impact on the absorption enhancement effect.

Developing immobilized-ionic liquids (ILs) sorbents is important for CO₂ separation, and prior theoretically screening ILs is desirable considering the huge number of ILs. In this study, the compressibility of ILs was proposed as a new and additional index for screening ILs, and the developed predictive theoretical model, i.e., electrolyte perturbed-chain statistical associating fluid theory, was used to predict the properties for a wide variety of ILs in a wide temperature and pressure range to provide systematic data. In screening, firstly, the isothermal compressibilities of 272 ILs were predicted at pressures ranging from 1 to 6,000 bar and temperatures ranging from 298.15 to 323.15 K, and then 30 ILs were initially screened. Subsequently, the CO₂ absorption capacities in these 30 ILs at temperatures from 298.15 to 323.15 K and pressures up to 50 bar were predicted, and 7 ILs were identified. In addition, the CO₂ desorption enthalpies in these 7 ILs were estimated for further consideration. The performance of one of the screened ILs was verified with the data determined experimentally, evidencing that the screen is reasonable, and the consideration of IL-compressibility is essential when screening ILs for the immobilized-IL sorbents.

For chemical warfare agent removal, the humidity emerges as an unavoidable challenge that significantly affects the performance of metal–organic frameworks. In this work, via density functional theory calculations, ab initio molecular dynamics and classical molecular dynamics simulations, we investigate the structural and diffusion properties of water in the pristine defect‐free UiO‐66, one Zr‐based metal–organic framework. Through the detailed analyses of the distribution probability of water in two different cages of UiO‐66, the binding interaction between water and UiO‐66, the hydrogen bonding networks and resulted localized water clusters, we gain a fundamental understanding of structural and dynamics properties as well as the concentration dependence of water in UiO‐66. We anticipate those theoretical results could provide insight to the competitive adsorption of water and chemical warfare agents, which eventually shows the utmost importance for the design and development of the next generation porous materials with appropriate water properties in real‐life applications.

ePC-SAFT-DFT is a powerful tool for studying the properties of confined ionic liquids for CO2 separation, in which efficient algorithms are required to obtain the calculation efficiently. In this work, the feasibility of accelerating the ePC-SAFT-DFT calculation with the Chebyshev pseudo-spectral collocation method was discussed for the confined ionic liquid (IL)–CO2 systems. In addition, a general scheme was proposed to search the electrical boundary potential. The new algorithm was further combined with the general scheme to model the confined IL–CO2 systems. It was found that the Chebyshev pseudo-spectral collocation method can improve the efficiency of the ePC-SAFT-DFT calculation significantly. Moreover, the new algorithm can be further combined with the general scheme to efficiently describe the density profile of the IL–CO2 system inside the electroneutral nanopores.

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.

In this work, spot-density gradient theory (DGT), with an approximate density profile in the vapor–liquid interfacial phase, was combined with ePC-soft-statistical associating fluid theory (SAFT) to describe the interfacial properties of binary mixtures. The developed model, which is termed as spot-DGT-ePC-SAFT, was first used for the mixtures containing common substances (e.g., alkane, benzene, CO2) to verify the model and compare the model performance with the rigorous DGT models. It shows that the surface tensions predicted with spot-DGT-ePC-SAFT are almost the same as those with the rigorous DGT, while spot-DGT costs much less calculation time. The developed spot-DGT-ePC-SAFT was further extended to ionic liquid (IL)–IL and IL–CO2 systems. Again, the predicted surface tensions agree well with the experimental data, indicating the reliability of the developed model for the IL-based systems.

Global warming is now widely recognized as being the most significant global issue facing human beings. Mitigating CO₂ emission from fossil-fueled power plants as well as from transports has become an urgent and worldwide research topic, in which CO₂ separation is often needed. Technologies have been developed and commercialized, whereas the cost is still high. Developing new technologies for CO₂ separation is one focus research area. Ionic liquids (ILs) are promising absorbents for CO₂ separation due to their very low vapor pressure, high solubility and selectivity for CO₂ as well as low energy usage for solvent regeneration.  

A drawback of using IL for CO₂ separation is the high viscosities, and using supported ILs has been proposed as a promising solution. This can take advantage of the high selectivity of gas in ILs, and also the high surface area of materials can reduce the impact of viscosity, improve the gas transfer, and hence increase the absorption rate. 

To develop IL-based technologies, thermodynamic properties (density, heat capacity, gas solubility, etc.), viscosity, and surface tension of ILs as well as the thermodynamic properties for confined ILs are the prerequisites. As the number of ILs that can be theoretically synthesized is up to an order of 1018, determining all the properties experimentally is impractical, not to mention the time-consuming with high cost. It is desirable to develop theoretical tools to predict the thermodynamic and transport properties of ILs and IL-containing mixtures in a wide temperature and pressure range.  

In our previous work, the framework of ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) has been developed with reliable results. The developed ePC-SAFT model was further combined with Free Volume Theory (ePC-SAFT-FVT) and Density Gradient Theory (ePC-SAFT-DGT or DGT-ePC-SAFT) to represent the viscosity and surface tension of ILs, respectively.  However, the work is limited to the imidazolium-based ILs, and the model performance for other commonly used ILs is still unclear. It has been pointed out that the model with the parameters fitted to the experimental data may result in pitfalls, and further validation is needed. Meanwhile, DGT-ePC-SAFT for pure ILs needs to be further developed to be consistent with the ion-specific ePC-SAFT model and extended to IL-mixtures. To describe the properties of confined ILs, in our previous work, ePC-SAFT model was combined with DFT (classical Density Functional Theory) (i.e., ePC-SAFT-DFT) to describe the properties of the IL and CO₂/IL confined in nanopores. Still, the algorithm based on equal mesh width leads to intensive computations, making it inefficient.  

In this thesis, these ion-specific ePC-SAFT-based models were further developed and extended to the systems containing the ILs which are composed of the IL-cations ([C_nmim]⁺, [C_npy]⁺, [C_nmpy]⁺, [C_nmpyr^]+, and [THTDP]⁺) and the IL-anions ([Tf₂N]^-,[PF₆]^-, [BF₄]^-, [tfo]^-, [DCA]^-, [SCN]^-, [C₁SO₄]^-, [C₂SO_4]^-, [eFAP]^-, Cl^-, [Ac]^-, and Br^-).

Before modeling the properties, a method and scheme were developed to investigate the pitfall when modeling IL and IL-gas systems with ePC-SAFT. All 96 ILs considered in the thesis and their binary mixtures with CO₂, H₂S, CO, O₂, CH₄, N₂, and H₂ were covered. The results show that ePC-SAFT with the density-fitted parameters is virtually free of the undesired pitfall for almost all ILs with only one exemption ([C₈mpy][BF₄]) and the parameters for [Cnmpy]+ may need to be modified in future work.

Afterward, ePC-SAFT was used to predict gas solubilities and second-order thermodynamic properties, such as heat capacities, isothermal and isentropic compressibilities, speeds of sound, thermal expansion coefficients, and internal pressures. The model predictions were evaluated by comparing with the experimental data, showing reliable results.  

ePC-SAFT-FVT was used to model the viscosities of ILs and IL-mixtures and compared with the available experimental data. It shows the model can represent the viscosity of pure ILs in a wide temperature and pressure range, and the parameters obtained from pure ILs can be used to predict the viscosity of IL-mixtures reliably. The model performance of Cl-based ILs at low temperatures is poor, and the temperature-dependent FVT parameter may be used to improve the model results.  

DGT-ePC-SAFT can provide reliable results for pure ILs in a wide temperature range, and it can be further used to describe the density profile in the interface. Furthermore, spot-DGT-ePC-SAFT based on the approximate density profile was proposed to predict the surface tension of IL-IL and IL-CO₂ systems, and the reliable predictions imply the promising of spot-DGT-ePC-SAFT. 

To calculate the properties of confined ILs efficiently, Chebyshev pseudo-spectral collocation method was applied to accelerate the ePC-SAFT-DFT calculation. The feasibility of accelerating the ePC-SAFT-DFT calculation with the Chebyshev pseudo-spectral collocation method was discussed for the confined IL-CO₂ systems. It was found that the Chebyshev pseudo-spectral collocation method can improve the efficiency of ePC-SAFT-DFT calculation significantly.  

In this work, ion partitioning and selectivity in cylindrical nanopores with heterogeneous surface charges at equilibrium with reservoirs are investigated by a two-dimensional (2D) classical density functional theory (DFT). We present an efficient numerical method for the large 2D system in which the fast Hankel transform and fast Fourier transform are used to calculate convolution integrals, and a hybrid method of Picard iteration and Anderson mixing is used to solve the Euler-Lagrange equations. The performance of the 2D DFT is tested by calculating the profiles of a model electrolyte in long homogeneous cylindrical nanopores. The profiles from the 2D DFT model matches well with those from a 1D DFT, and the computing time of the hybrid iteration algorithm is six times shorter than that of pure Picard iteration. We apply the model to electrolytes in cylindrical nanopores with heterogeneous surface charges. It is found that the ion adsorption and selectivity are strongly affected by the surface charge pattern, the magnitude of the surface charge, the size of charged domains on the surface, and the pore size.

In this work, the nonrandom two-liquid model (NRTL) was coupled with the Eyring's absolute rate theory (the Eyring-NRTL model), to study thermodynamic properties and viscosities simultaneously with one set of parameters, and ten non-aqueous mixtures containing ionic liquids (ILs)/deep eutectic solvents (DESs) were chosen in the study as the first step. In model parameterizing, three strategies were investigated, which were viscosity as the input only, enthalpy with/without vapor pressure as the inputs, and enthalpy and viscosity as inputs, respectively. The investigation shows that it is possible to represent thermodynamic and kinetic properties simultaneously. Furthermore, the viscosity can be predicted using the model with reasonable parameters determined from the excess enthalpy, and incorporating vapor pressures and limited viscosity data in parameterizing can further improve the model performance on the viscosity. For the first time, the thermodynamic model was coupled with the Eyring's theory to study the systems of (IL/DES + molecular solvent), how to obtain the model parameters was clarified, and the possibility and reliability of the used model in representing thermodynamic and kinetic properties were illustrated and discussed.