Molecular Thermodynamic Models for Nano-Micro Fluid/Solid Interfaces

Abstract: Introducing materials with large interfaces to enhance process performance has become a feature of advanced chemical engineering, where the research focus has been changed from the traditional ideal isotropic fluid in the bulk phase to the highly non-ideal anisotropic confined fluid on the nano-micro interfaces, owing to the strong and asymmetric interactions between the complex fluids (supported metal nanoparticles, ionic liquids, proteins, etc.) and the sophisticated solid-surface (roughness, electrostatic effects, chemical heterogeneities, etc.). The traditional theories cannot be used to accurately describe the properties of the fluids at the complex solid-surfaces, due to the lack of considering molecular interactions between the fluid and solid-surface, and establishing new models is essential.In this thesis, a generalized interfacial molecular thermodynamic model was proposed with the consideration of molecular interactions between the fluid and solid-surface. Firstly, the original and surface-energy modified Gibbs-Thomson equations were analyzed to calculate the melting points of mono noble metals and compared with the literature data, highlighting the importance of developing new models with the consideration of the interfacial effect. An empirical model was proposed to represent the interfacial effect for calculating the melting points of mono noble metals. Then, the mono noble metal nanoparticle supported at the flat solid-surface was chosen as the “model” system to develop a generalized model, and the developed model was extended to the supported alloy systems. The CO2 absorption capacity (or solubility) of the ionic liquids immobilized on the porous solid materials (substrates) was further investigated with the developed model. The main results were summarized as follows:To develop models for representing the melting points of mono noble metal nanoparticles, the original and surface-energy modified Gibbs-Thomson equations were analyzed and then further modified empirically considering the effect of substrate. The results revealed that the original Gibbs–Thomson equation is invalid for the particles with radii smaller than 10 nm, and the performance of the surface-energy modified equation was improved but further modification by considering the interfacial effect is necessary for the particles smller than 5 nm in radius. The empirical model with the interfacial effect further improved the model performance, and the adjustable parameters can be predicted quantitatively from the thermodynamic properties of the metal and substrate. Additionally, the micro-wetting parameter αw can be used to qualitatively study the overall impact of the substrate on the melting point depression.Combined with the analysis of the corresponding state theory, a generalized molecular thermodynamic model was developed. It was found that, the developed generalized model can provide accurate results of melting points with deviations within ± 15 K. The developed model was used to predict the melting point of Pt nanoparticles on the substrates of TiO2 and carbon (C), and the results showed that Pt on TiO2 was more stable than that on C, being consistent with the newly measured experimental results.The generalized model was further parameterized based on the analysis of the interfacial tensions and molar volumes of Al-Si3N4, Pb-Si, Bi-C, and In-C, and the model showed the deviation was within ± 36 K. The model with fully generalized parameters was extended to the supported alloy nanoparticles to illustrate their stabilities, where the common catalysts, Pd-Au alloy nanoparticles supported on different substrates, developed for H2O2 reaction, were chosen as the examples. The model prediction displayed that the Pd-Au alloy nanoparticles supported on C/TiO2 (molar ratio: 0.01) with the mass proportion Pd5Au1 (i.e., mass ratio of 5:1) is more stable than the mono noble metals. Furthermore, the model prediction indicated that the supported alloy nanoparticles are more stable than the supported Pd.The generalized model was also successfully extended to study the CO2 absorption capacity in the immobilized ionic liquids, where the Gibbs free energy of CO2 in the immobilized ionic liquids was modeled from both macro- and micro-analyses. The theoretical investigations revealed that the substrate has a crucial effect on the gas solubility in the ionic liquid immobilized on the substrates, and the performance of the model with the consideration of surface-energy and interfacial effects was further verified with the newly determined experimental data.