Behaviour of room temperature ionic liquids at gold interfaces: from equilibrium to non-equilibrium and electrified conditions
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Abstract
This thesis investigates the behaviour of room temperature ionic liquids (RTILs), specifically 1-butyl-3-methylimidazolium tetrafluoroborate (\ce{[BMIM][BF4]}), at gold interfaces under varying conditions, including equilibrium, shear flow, and electrified interfaces. The study is motivated by the increasing relevance of RTILs in advanced applications such as energy storage, catalysis, and lubrication, where understanding the molecular interactions between the liquid and the metal at the interfaces is crucial for optimising performance. Through a series of molecular dynamics simulations, this thesis explores the structural, thermodynamic, and dynamical properties of \ce{[BMIM][BF4]} confined between gold surfaces, providing detailed insights into their behaviour under both equilibrium and non-equilibrium conditions.
The thesis begins by establishing the baseline behaviour of \ce{[BMIM][BF4]} at a gold interface under equilibrium conditions. It reveals the formation of distinct layered structures, where the ions organise themselves based on electrostatic and van der Waals interactions. These findings highlight the significant role of the interface in dictating the overall properties of the RTIL, including its diffusion coefficients and interfacial free energy.
Building on this, the study then investigates how external forces, specifically shear flow, alter the RTIL's behaviour. The application of shear flow leads to the formation of a solid-like, glassy layer at the interface, which extends over several nanometres from the gold surface. This non-equilibrium state is characterised by deviations from the ideal linear Couette flow and significant changes in viscosity and friction, which are critical for understanding the RTIL's performance in dynamic applications such as lubrication and energy storage.
The thesis proceeds with introducing a constant metal-surface charge approach to investigating the phenomena at the RTIL-gold interface, away from the potential of zero charge (PZC). In order to ensure the accuracy of these simulations, the thesis validates the use of a polarisable gold model for such off-PZC calculations through a simpler system involving \ce{NaCl} solution at a gold interface. This study confirms the model's ability to capture the nuanced electrostatic interactions at the interface, particularly under varying charge conditions, which are essential for realistic simulations of more complex systems like RTILs.
The final phase of the research explores the behaviour of \ce{[BMIM][BF4]} at a charged gold interface, revealing significant changes in ion distribution, electric double layer formation, and dynamical properties. The study demonstrates how surface charge, coupled with image charge contribution, enhances ion ordering and the differential capacitance of the liquid, and the extent to which it influences the viscosity and mobility of the RTIL, providing critical insights for the design and optimisation of devices such as supercapacitors and batteries.
Overall, this thesis contributes to the fundamental understanding of RTIL-metal interactions, offering detailed insights that are vital for the development of next-generation materials and technologies. By systematically exploring the RTIL's behaviour under different interfacial conditions, the research lays the groundwork for future advancements in the application of RTILs in energy storage, catalysis, and beyond.