Molecular Simulations of Microstructure, Thermodynamics, and Dynamics of Complex Liquid Mixtures at Interfaces and Confined Spaces

Abstract: In modern chemical engineering, process intensification is realised by introducing a nano- or micro-interface. The introduction of such an interface breaks the homogeneous nature of the fluid and forms a unique nano- or micro-interfacial structure, which has a significant impact on the properties of fluids. However, the existing traditional chemical engineering theories cannot be used to describe this inhomogeneous behaviour and clarify the underlying intrinsic mechanisms, making it difficult to find control factors which enhance chemical processes. It is necessary to establish theoretical models suitable for liquid–solid systems which can describe the fluid behaviour at interfaces. The key is to accurately recognise the structural as well as thermodynamic and dynamic properties of the complex fluid mixtures at these nano- or micro-interfaces. Previous studies that have been conducted to study the behaviour of simple fluids at interfaces found that, because of the strongly asymmetric interactions, the fluid tends to form layered structures at or close to the interface, which further affects the behaviour of the fluid molecules in close vicinity. However, for complex fluids and/or fluid mixtures, the effects of the interface and the interactions of fluid molecules on the formation of the layered structure and the fluid behaviour above the formed layer have not been elucidated.In this thesis, to conduct a systematic study, several typical liquid mixtures, which are also important in the modern chemical industry, i.e. immiscible dimethyl carbonate (DMC)/water mixtures with relatively weak van der Waals interactions, miscible aqueous alcohol solutions with strong interactions due to hydrogen bonding (H-bonding), and deep eutectic solvents (DESs) with strong electrostatic interactions, were selected as representatives to construct a complex fluid–solid interface. Molecular dynamics simulation was used as the main tool to quantitatively describe the formation and influence of the adsorption layer on the structures and properties of the fluids at the interface at the molecular level. Additionally, in the future, key parameters will be provided for establishing theoretical nano- or micro-interface models. The main results obtained are summarised as follows:(1) The local composition and microstructure of DMC/water mixtures in carbon nanotubes (CNTs) were studied. It is found that DMC molecules preferentially get adsorbed on the inner surface of CNTs and form a “DMC tube”-like structure, i.e. a “secondary confinement” for the water molecules. Because of the formed DMC adsorption layer, the water molecules in the “DMC tube” are more widely dispersed owing to the disordered orientation structure and the destroyed H-bonding structure.(2) The molecular behaviour of aqueous solutions containing alcohol (i.e. methanol, ethanol, n-propanol, and n-butanol, respectively), confined in a two-dimensional graphene slit, was studied. A distinctly layered structure is formed at the interface, and the alcohol molecules are preferentially adsorbed on the graphene wall. Among the four studied systems, for the n-propanol system, the water molecules on the interfacial adsorption layer have the longest residence time because of the least distortion to the H-bonding network of the water molecules, restricting their motion. The nanophase separation (e.g. separation from water at the interface) of aqueous methanol solutions with stronger intermolecular interactions is less prominent compared to that of DMC/water and other aqueous alcohol solutions.(3) The wetting behaviour of the deep eutectic choline chloride (ChCl)/urea (1:2) droplets on the ionic model substrate was studied, in which the substrate continuously and linearly changed its state from hydrophobic to hydrophilic. Due to the strong electrostatic interactions between the anions and cations, the neutral urea molecules are stripped out and adsorbed on the interface, forming a stable “new interface”. The orientation and H-bonding structure between the urea molecules in the adsorption layer lead to a difference in hydrophilicity of the “new interface”, further affecting the wetting behaviour of the upper molecules.A further comparison of the results for the different systems that were studied shows that for a weaker intermolecular interaction in the bulk phase, a clearer separation at the interface of the nanophase is observed. Therefore, for more complex systems (such as aqueous ionic liquids/DESs), it is essential to study their microscopic mechanism in the bulk phase before investigating their interfacial behaviour. Subsequently, the microstructure of ChCl/urea/water (i.e. one type of aqueous DES) was studied. The investigation shows that the hydration strength of chloride ions is higher than that of choline ions, indicating that the anions have a greater impact on the non-ideal behaviour of the mixture, which was further proven by analysing the interaction energy. In addition, the competition between the ion pairs and ionic hydration was suggested as the underlying mechanism for the non-ideal changes in the thermodynamic properties of complex systems with strong electrostatic interactions.