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Title:Ionic liquid behavior on rough surfaces
Author(s):Sheehan, Alexis J
Advisor(s):Espinosa-Marzal, Rosa M
Department / Program:Civil & Environmental Eng
Discipline:Environ Engr in Civil Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Ionic Liquids
atomic force microscopy (AFM)
Abstract:This study focuses on the effects of surface heterogeneities on the behaviour of ionic liquids. Ionic liquids have shown great promise as both electrolyte replacements and as lubricants in industrial applications. Previous studies have focused on ionic liquid behaviour on flat substrates. Understanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. This work is comprised of three sets of experiments to determine the effects of nanoscale roughness and chemical surface properties on IL static and dynamic behavior. The first is an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM] Ntf2) by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL-solid interface. The second was a study of the frictional forces in [HMIM] Ntf2 on surfaces of varying roughnesses, both with and without confinement. These experiments focused on speed and load dependence of the friction force. The third investigated the effect of plasma treatment vs. UV ozone treatment of silica surfaces, in both [HMIM] Ntf2 and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM] Ntf2), on friction forces and adhesion. These experiments were completed without confinement. This work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contact. Most broadly an increase in layering is seen with confinement, as expected, but the layer size and push out forces are largely dependent on surface roughness. It is observed that the flat substrate shows a distinct layering pattern from the rough substrates; however notably layering is present on even the roughest surfaces. This work also showed that the decrease in ordering due to the presence of roughness leads to a decrease in friction forces, both unconfined and confined. The presence of roughness also appears to decrease the speed dependence of the friction response. This decrease in friction is likely due to an increase in liquid-like behavior of the IL on the rough surface. Last this work showed an increase in adhesion and friction force due to plasma treatment for [EMIM] Ntf2 with time but not for [HMIM] Ntf2. This reveals that while both ILs are hydrophobic they have different time-dependent responses to the more hydrophilic surface rich in silanol groups. Overall this work illuminates the importance of studying systems closer to real applications as it clear that roughness and surface chemical modifications play an important role in IL surface behavior. It also suggests that IL may become more liquid-like than previously thought in rough pores, or between rough surfaces increasing their viability as both electrolyte replacements and lubricants.
Issue Date:2016-04-25
Rights Information:Copyright 2016 Alexis Sheehan
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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