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Title:Flow of ionic liquids in nanoconfinement
Author(s):Han, Mengwei
Director of Research:Espinosa-Marzal, Rosa M.
Doctoral Committee Chair(s):Espinosa-Marzal, Rosa M.
Doctoral Committee Member(s):Gewirth, Andrew A.; Leal, Cecilia; Rogers, Simon A.
Department / Program:Civil & Environmental Eng
Discipline:Environ Engr in Civil Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Ionic liquids
electrical double layer
Abstract:Ionic liquids (ILs) are considered well suited alternatives for conventional electrolytes in next-generation electrochemical devices to grant enhanced stability, safer handling and improved performance. Key to ILs’ application as electrolytes are the structure and transport properties near the solid-liquid interface and in nanopores, as most electrochemical processes take place at the electrolyte-electrode interface and mesoporous electrode materials are often adopted to maximize specific area. Unlike dilute electrolyte, which is well characterized by the Debye–Hückel equation and electrical double layer (EDL) model, the understanding of ILs (and concentrated electrolytes in general) near the solid-liquid interface and under confinement is still primitive and sometimes self-conflicting, especially in regards of the unequilibrated processes. The dissertation summarizes my attempts to elucidate on ionic liquids (ILs)’ interfacial structures, the kinetics in the formation of such structures, the flow properties and relaxation kinetics, through nanomechanical measurements. Experimental evidence is contributed to answering to three key questions pivoted around the theme: 1) how do ions arrange themselves within in the last tens of nm from a charged solid-IL interface; 2) how do ILs flow or relax in confined space only a few times the dimension of ions; and 3) how will water affect the flow of nanoconfined ILs. Specifically, the interfacial region between IL and a charged surface is investigated with surface forces apparatus (SFA) using the classic mica sensors. The nanostructures of and the interactions within the interfacial region are revealed by the surface force profile as a function of separation between the two opposing mica surfaces. Time-dependent measurements revealed an ultra-slow kinetics in the formation of the structure, which is concerted with a transition in the bulk nanostructure studied with X-ray. Rheological properties of the nanoconfined ILs are measured by either squeezing out the ILs as the opposing mica surfaces approach each in the normal direction, or laterally oscillating one of the mica surfaces to create shear motions. Finally, water is introduced through gas phase by modulating the relative humidity in the sealed chamber to probe its influence. It is discovered that the long-ranged repulsive surface force across a range of a few tens of nanometers could be due to the presence of multi-ionic structures intrinsic to each IL, while the step-like features at below 10 nm region suggest the presence of multiple layers immediately on the solid surface. Such layers, upon adequate normal force, are squeezed out sequentially with increasing force thresholds. The squeeze-out processes are modelled with Reynold’s law of lubrication to extract the effective viscosity for each layer, which is found to increase by orders of magnitude in the next resolvable layer compared to the previous one. The effective viscosity obtained from squeeze-out is further studied by taking into account the influence of multiple parameters including the long-ranged screening length, the bulk viscosity and the degree of confinement. Further study through oscillatory shear motion asserted on the confined ILs deconvolutes the viscous and elastic moduli of the ILs in each layer. Through modeling the storage and loss moduli with Einstein Stokes law and the effective shear viscosity as a function of shear rate by Eyring’s theory, the collective motion of ions under confinement is unveiled and discussed regarding to the chemical structures of the ions. The compressibility of the ions is found to be the key. When water is introduced, the interfacial structures, the effective viscosity, and the moduli are clearly altered, despite its low abundance. The intrusion of water into the nanoconfined ILs is investigated separately in static measurements. The dissertation extends the scope of IL nanorheology to address the application of IL as a lubricant and comments on the link between the nanorheology and nanotribology of ILs, and the influence of water.
Issue Date:2021-07-14
Rights Information:Copyright 2021 Mengwei Han
Date Available in IDEALS:2022-01-12
Date Deposited:2021-08

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