- Title
- Nanoscale friction mechanisms in ionic liquid systems
- Creator
- Sweeney, James Thomas
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis relates the confined structure of several ionic liquids (ILs) and mixtures of ILs with various n-alkanols and the polymer poly(ethylene oxide) (PEO) to their nanoscale friction behaviours. The influence of various parameters including IL molecular structure, applied electric potential, bulk liquid viscosity and interfacial liquid viscosity on nanoscale friction were studied. Friction force microscopy (FFM) was used to investigate the friction behaviours of a series of protic ionic liquids (PILs) confined between a silica colloidal probe and a mica surface. The effect of solvophobic interactions on friction was investigated by varying cation alkyl chain length (propylammonium cation vs. ethylammonium cation). The effect of the hydrogen bonding capacity of both the cation (ethanolammonium cation vs. ethylammonium cation) and anion (nitrate vs. formate) was also studied. The effect of steric hindrance was probed using an IL (dimethylethylammonium formate) with weak interfacial structure. The friction coefficient changed at a critical normal load equal to the final push-through force in normal force-distance profiles in each IL. At this critical normal load all but a single layer of ions was removed from between the surfaces. Soft contact atomic force microscopy (AFM) imaging showed a change in the morphology of the propylammonium nitrate (PAN)-mica system from a structure with an undulating appearance to one with a worm-like appearance as the imaging load crossed the critical normal load. This indicated a structural rearrangement, consistent with the number of confined ion layers changing as load increased. Addition of n-alkanols to PAN resulted in a decrease in friction. The reduction was dependent on the concentration of n-alkanol, but independent of n-alkanol alkyl chain length. The friction reduction was not correlated with the bulk viscosity of the system, and thin-film drainage experiments showed that localised viscosity reductions did not occur in the system. The decrease in the friction force was attributed to weakening of intermolecular interactions between ions adsorbed to the mica surface and those in near surface layers. Friction was reduced when PEO was adsorbed to silica surfaces and immersed in pure PAN and in the aprotic ionic liquid (AIL) 1-butyl-3-methylimidazolium tetrafluoroborate. A range of PEO molecular weights were investigated. Theoretical fits to AFM force-distance profiles showed that PEO adsorbed onto silica from both ILs in a mushroom conformation. FFM experiments showed that friction was lower when PEO was present compared to either pure IL, and that friction decreased as polymer molecular weight increased. Friction was reduced due to the high osmotic pressures generated as polymer layers were compressed; this provided an effective barrier to direct surface-surface contact. The ability of an IL to lubricate an electrically charged contact was investigated by applying a surface potential to a gold surface lubricated by the IL 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate. Friction varied as a function of both the sign and magnitude of the bias. The composition of the adsorbed layer of ions varied as surface polarity changed from positive to negative or vice versa. The adsorbed layer composition influenced ion packing arrangements and both had a significant effect on friction; the relatively long alkyl chains of the cations lowered friction more effectively than the bulky anions.
- Subject
- ionic liquids; atomic force microscopy; surface morphology; nanoscale friction; nanotribology; nanofriction; thesis by publication; nanostructure; AFM; ammonium cations; protic ionic liquids; aprotic ionic liquids; atomic force microscope; cation alkyl chains; interfacial structure
- Identifier
- http://hdl.handle.net/1959.13/1315720
- Identifier
- uon:22993
- Rights
- Copyright 2016 James Thomas Sweeney
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Abstract | 3 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Thesis | 10 MB | Adobe Acrobat PDF | View Details Download |