| 1. |
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| 2. |
- Asencio, Rubén Alvarez, et al.
(author)
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Ionic Liquid Nanotribology : Stiction Suppression and Surface Induced Shear Thinning
- 2012
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 28:26, s. 9967-9976
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Journal article (peer-reviewed)abstract
- The friction and adhesion between pairs of materials (silica, alumina, and polytetrafluoroethylene) have been studied and interpreted in terms of the long-ranged interactions present. In ambient laboratory air, the interactions are dominated by van der Waals attraction and strong adhesion leading to significant frictional forces. In the presence of the ionic liquid (IL) ethylammonium nitrate (EAN) the van der Waals interaction is suppressed and the attractive/adhesive interactions which lead to "stiction" are removed, resulting in an at least a 10-fold reduction in the friction force at large applied loads. The friction coefficient for each system was determined; coefficients obtained in air were significantly larger than those obtained in the presence of EAN (which ranged between 0.1 and 0.25), and variation in the friction coefficients between systems was correlated with changes in surface roughness. As the viscosity of ILs can be relatively high, which has implications for the lubricating properties, the hydrodynamic forces between the surfaces have therefore also been studied. The linear increase in repulsive force with speed, expected from hydrodynamic interactions, is clearly observed, and these forces further inhibit the potential for stiction. Remarkably, the viscosity extracted from the data is dramatically reduced compared to the bulk value, indicative of a surface ordering effect which significantly reduces viscous losses.
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| 3. |
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| 4. |
- Elbourne, Aaron, et al.
(author)
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Adsorbed and near-surface structure of ionic liquids determines nanoscale friction
- 2013
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In: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 49:60, s. 6797-6799
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Journal article (peer-reviewed)abstract
- Surface-adsorbed and near-surface ion layer structure controls nanotribology in the silica-propylammonium nitrate (PAN)-mica system. Atomic Force Microscopy (AFM) imaging and normal force curves reveal that the normal load dictates the number of interfacial ion layers and the lateral layer structure. Shear force measurements show the lubricity of the interface changes with the number, and lateral structure, of the confined ion layer(s).
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| 5. |
- Gebbie, Matthew A., et al.
(author)
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Long range electrostatic forces in ionic liquids
- 2017
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In: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 53:7, s. 1214-1224
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Journal article (peer-reviewed)abstract
- Ionic liquids are pure salts that are liquid under ambient conditions. As liquids composed solely of ions, the scientific consensus has been that ionic liquids have exceedingly high ionic strengths and thus very short Debye screening lengths. However, several recent experiments from laboratories around the world have reported data for the approach of two surfaces separated by ionic liquids which revealed remarkable long range forces that appear to be electrostatic in origin. Evidence has accumulated demonstrating long range surface forces for several different combinations of ionic liquids and electrically charged surfaces, as well as for concentrated mixtures of inorganic salts in solvent. The original interpretation of these forces, that ionic liquids could be envisioned as “dilute electrolytes,” was controversial, and the origin of long range forces in ionic liquids remains the subject of discussion. Here we seek to collate and examine the evidence for long range surface forces in ionic liquids, identify key outstanding questions, and explore possible mechanisms underlying the origin of these long range forces. Long range surface forces in ionic liquids and other highly concentrated electrolytes hold diverse implications from designing ionic liquids for energy storage applications to rationalizing electrostatic correlations in biological self-assembly.
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| 6. |
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| 7. |
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| 8. |
- Hjalmarsson, Nicklas, et al.
(author)
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Effect of Lithium ions on rheology and interfacial forces in Ethylammonium Nitrate and Ethanolammonium Nitrate
- 2016
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:47, s. 26960-26967
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Journal article (peer-reviewed)abstract
- The effect of added Li+ to two ionic liquids (ILs), ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN), has been investigated using rheology and colloidal probe atomic force microscopy (AFM). Rheology data revealed a complex viscosity dependence that can be ascribed to the different bulk nanostructures. AFM force curves revealed steps for the neat ILs, analogous to those in previous studies. The addition of Li+ broadened the steps, which is likely an effect of ion clusters formed. Friction measurements corroborate this data and also showed that the structure of EtAN is much more prone to change as Li+ is added. These results demonstrate the complex behavior of ILs on interfaces and the effect of perturbing such interactions. (Graph Presented).
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| 9. |
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| 10. |
- Hjalmarsson, Nicklas, et al.
(author)
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Electro-Responsive Surface Composition and Kinetics of an Ionic Liquid in a Polar Oil
- 2019
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 35:48, s. 15692-15700
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Journal article (peer-reviewed)abstract
- The quartz crystal microbalance (QCM) has been used to study how the interfacial layer of an ionic liquid dissolved in a polar oil at low weight percentages responds to changes in applied potential. The changes in surface composition at the QCM gold surface depend on both the magnitude and sign of the applied potential. The time-resolved response indicates that the relaxation kinetics are limited by the diffusion of ions in the interfacial region and not in the bulk, since there is no concentration dependence. The measured mass changes cannot be explained only in terms of simple ion exchange; the relative molecular volumes of the ions and the density changes in response to ion exclusion must be considered. The relaxation behavior of the potential between the electrodes upon disconnecting the applied potential is more complex than that observed for pure ionic liquids, but a measure of the surface charge can be extracted from the exponential decay when the rapid initial potential drop is accounted for. The adsorbed film at the gold surface consists predominantly of ionic liquid despite the low concentration, which is unsurprising given the surtactant-like structures of (some of) the ionic liquid ions. Changes in response to potential correspond to changes in the relative numbers of cations and anions, rather than a change in the oil composition. No evidence for an electric field induced change in viscosity is observed. This work shows conclusively that electric potentials can be used to control the surface composition, even in an oil-based system, and paves the way for other ion solvent studies.
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| 11. |
- Hjalmarsson, Nicklas
(author)
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Ionic liquids : The solid-liquid interface and surface forces
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Ionic liquids (ILs) present new approaches for controlling interactions at the solid-liquid interface. ILs are defined as liquids consisting of bulky and asymmetric ions, with a melting point below 373 K. Owing to their amphiphilic character they are powerful solvents but also possess other interesting properties. For example, ILs can self-assemble and are attracted to surfaces due to their charged nature. As a result, they are capable of forming nanostructures both in bulk and at interfaces. This thesis describes how the solid-IL interface responds to external influences such as elevated temperatures, the addition of salt and polarisation. An improved understanding of how these factors govern the surface composition can provide tools for tuning systems to specific applications such as friction.Normal and friction forces are measured for ethylammonium nitrate (EAN) immersed between a mica surface and a silica probe, at different temperatures or salt concentrations. The results demonstrate that an increase in temperature or low concentrations of added salt only induce small changes in the interfacial structure and that the boundary layer properties remain intact. In contrast, at sufficiently large salt concentrations the smaller lithium ion prevails and the surface composition changes. The interfacial layer of a similar IL is also investigated upon the addition of salt and the results reveal that lithium ions affect the surface composition differently depending on the ion structure of the IL. This demonstrates that the surface selectivity strongly depends on the ion chemistry.Remarkably, a repulsive double layer force manifests itself for EAN at 393 K, which is not observed for lower temperatures. This indicates a temperature dependent change in EAN’s microscopic association behaviour and has general implications for how ILs are perceived.A new method is developed based on a quartz crystal microbalance to investigate how the surface compositions of ILs respond to polarisation. The approach demonstrates that interfacial layers of both a neat IL and an IL dissolved in oil can be controlled using potentials of different magnitudes and signs. Furthermore, the method enables two independent approaches for monitoring the charges during polarisation which can be used to quantify the surface composition. The technique also provides information on ion kinetics and surface selectivity.This work contributes to the fundamental understanding of the solid-IL interface and demonstrates that the surface composition of ILs can be controlled and monitored using different approaches.
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| 12. |
- Hjalmarsson, Nicklas, et al.
(author)
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Is the boundary layer of an ionic liquid equally lubricating at higher temperature?
- 2016
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 18:13, s. 9232-9239
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Journal article (peer-reviewed)abstract
- Atomic force microscopy has been used to study the effect of temperature on normal forces and friction for the room temperature ionic liquid (IL) ethylammonium nitrate (EAN), confined between mica and a silica colloid probe at 25 °C, 50 °C, and 80 °C. Force curves revealed a strong fluid dynamic influence at room temperature, which was greatly reduced at elevated temperatures due to the reduced liquid viscosity. A fluid dynamic analysis reveals that bulk viscosity is manifested at large separation but that EAN displays a nonzero slip, indicating a region of different viscosity near the surface. At high temperatures, the reduction in fluid dynamic force reveals step-like force curves, similar to those found at room temperature using much lower scan rates. The ionic liquid boundary layer remains adsorbed to the solid surface even at high temperature, which provides a mechanism for lubrication when fluid dynamic lubrication is strongly reduced. The friction data reveals a decrease in absolute friction force with increasing temperature, which is associated with increased thermal motion and reduced viscosity of the near surface layers but, consistent with the normal force data, boundary layer lubrication was unaffected. The implications for ILs as lubricants are discussed in terms of the behaviour of this well characterised system.
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| 13. |
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| 14. |
- Hjalmarsson, Nicklas, et al.
(author)
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Weighing the surface charge of an ionic liquid
- 2015
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In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 7:38, s. 16039-16045
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Journal article (peer-reviewed)abstract
- Electrochemical quartz crystal microbalance has been used to measure changes in the composition of the capacitive electrical double layer for 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate, an ionic liquid, in contact with a gold electrode surface as a function of potential. The mass difference between the cation and anion means that the technique can effectively "weigh" the surface charge accurately with high temporal resolution. This reveals quantitatively how changing the potential alters the ratio of cations and anions associated with the electrode surface, and thus the charge per unit area, as well as the kinetics associated with these interfacial processes. The measurements reveal that it is diffusion of co-ions into the interfacial region rather than expulsion of counterions that controls the relaxation. The measured potential dependent double layer capacitance experimentally validates recent theoretical predictions for counterion overscreening (low potentials) and crowding (high potentials) at electrode surfaces. This new capacity to quantitatively measure ion composition is critical for ionic liquid applications ranging from batteries, capacitors and electrodeposition through to boundary layer structure in tribology, and more broadly provides new insight into interfacial processes in concentrated electrolyte solutions.
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| 15. |
- Li, Hua, et al.
(author)
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Addition of low concentrations of an ionic liquid to a base oil reduces friction over multiple length scales : a combined nano- and macrotribology investigation
- 2016
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 18:9, s. 6541-6547
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Journal article (peer-reviewed)abstract
- The efficacy of ionic liquids (ILs) as lubricant additives to a model base oil has been probed at the nanoscale and macroscale as a function of IL concentration using the same materials. Silica surfaces lubricated with mixtures of the IL trihexyl(tetradecyl) phosphonium bis(2,4,4-trimethylpentyl)phosphinate and hexadecane are probed using atomic force microscopy (AFM) (nanoscale) and ball-on-disc tribometer (macroscale). At both length scales the pure IL is a much more effective lubricant than hexadecane. At the nanoscale, 2.0 mol% IL (and above) in hexadecane lubricates the silica as well as the pure IL due to the formation of a robust IL boundary layer that separates the sliding surfaces. At the macroscale the lubrication is highly load dependent; at low loads all the mixtures lubricate as effectively as the pure IL, whereas at higher loads rather high concentrations are required to provide IL like lubrication. Wear is also pronounced at high loads, for all cases except the pure IL, and a tribofilm is formed. Together, the nano- and macroscales results reveal that the IL is an effective lubricant additive - it reduces friction - in both the boundary regime at the nanoscale and mixed regime at the macroscale.
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| 16. |
- Li, Hua, et al.
(author)
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An ionic liquid lubricant enables superlubricity to be "switched on" in situ using an electrical potential
- 2014
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In: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 50:33, s. 4368-4370
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Journal article (peer-reviewed)abstract
- Atomic force microscopy measurements reveal that superlubricity can be "switched'' on and off in situ when an ionic liquid is used to lubricate the silica-graphite interface. Applying a potential to the graphite surface changes the ion composition of the boundary layer and thus the lubricity. At positive potentials, when the interfacial ion layer is anion rich, friction falls to ultra-low levels.
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| 17. |
- Li, Hua, et al.
(author)
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Boundary layer friction of solvate ionic liquids as a function of potential
- 2017
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In: Faraday discussions. - : Royal Society of Chemistry. - 1359-6640 .- 1364-5498. ; 199, s. 311-322
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Journal article (peer-reviewed)abstract
- Atomic force microscopy (AFM) has been used to investigate the potential dependent boundary layer friction at solvate ionic liquid (SIL)-highly ordered pyrolytic graphite (HOPG) and SIL-Au(111) interfaces. Friction trace and retrace loops of lithium tetraglyme bis(trifluoromethylsulfonyl) amide (Li(G4) TFSI) at HOPG present clearer stick-slip events at negative potentials than at positive potentials, indicating that a Li+ cation layer adsorbed to the HOPG lattice at negative potentials which enhances stick-slip events. The boundary layer friction data for Li(G4) TFSI shows that at HOPG, friction forces at all potentials are low. The TFSI- anion rich boundary layer at positive potentials is more lubricating than the Li+ cation rich boundary layer at negative potentials. These results suggest that boundary layers at all potentials are smooth and energy is predominantly dissipated via stick-slip events. In contrast, friction at Au(111) for Li(G4) TFSI is significantly higher at positive potentials than at negative potentials, which is comparable to that at HOPG at the same potential. The similarity of boundary layer friction at negatively charged HOPG and Au(111) surfaces indicates that the boundary layer compositions are similar and rich in Li+ cations for both surfaces at negative potentials. However, at Au(111), the TFSI- rich boundary layer is less lubricating than the Li+ rich boundary layer, which implies that anion reorientations rather than stick-slip events are the predominant energy dissipation pathways. This is confirmed by the boundary friction of Li(G4) NO3 at Au(111), which shows similar friction to Li(G4) TFSI at negative potentials due to the same cation rich boundary layer composition, but even higher friction at positive potentials, due to higher energy dissipation in the NO3- rich boundary layer.
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| 18. |
- Li, Hua, et al.
(author)
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Combined nano- and macrotribology studies of titania lubrication using the oil-ionic liquid mixtures
- 2016
-
In: ACS Sustainable Chemistry and Engineering. - : American Chemical Society (ACS). - 2168-0485. ; 4:9, s. 5005-5012
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Journal article (peer-reviewed)abstract
- The lubrication of titania surfaces using a series of ionic liquid (IL)-hexadecane mixtures has been probed using nanoscale atomic force microscopy (AFM) and macroscale ball-on-disk tribometer measurements. The IL investigated is trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, which is miscible with hexadecane in all proportions. At both length scales, the pure IL is a much more effective lubricant than pure hexadecane. At low loads, which are comparable to common industrial applications, the pure IL reduces the friction by 80% compared to pure hexadecane; while the IL-hexadecane mixtures lubricate the titania surface as effectively as the pure IL and wear decreases with increasing IL concentration. At high test loads the adsorbed ion boundary layer is displaced leading to surface contact and high friction, and wear is pronounced for all IL concentrations. Nonetheless, the IL performs better than a traditional zinc-dialkyl-dithophosphate (ZDDP) antiwear additive at the same concentration.
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| 19. |
- Li, Hua, et al.
(author)
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Ionic Liquid Adsorption and Nanotribology at the Silica-Oil Interface : Hundred-Fold Dilution in Oil Lubricates as Effectively as the Pure Ionic Liquid
- 2014
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 5:23, s. 4095-4099
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Journal article (peer-reviewed)abstract
- The remarkable physical properties of ionic liquids (ILs) make them potentially excellent lubricants. One of the challenges for using ILs as lubricants is their high cost. In this article, atomic force microscopy (AFM) nanotribology measurements reveal that a 1 mol % solution of IL dissolved in an oil lubricates the silica surface as effectively as the pure IL. The adsorption isotherm shows that the IL surface excess need only be approximately half of the saturation value to prevent surface contact and effectively lubricate the sliding surfaces. Using Its in this way makes them viable for large-scale applications.
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| 20. |
- Li, Hua, et al.
(author)
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Ionic liquid lubrication : influence of ion structure, surface potential and sliding velocity
- 2013
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 15:35, s. 14616-14623
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Journal article (peer-reviewed)abstract
- Colloid probe atomic force microscopy (AFM) has been employed to investigate the nanotribology of the ionic liquid (IL)-Au(111) interface. Data is presented for four ILs, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([EMIM] FAP), 1-butyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate ([BMIM] FAP), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([HMIM] FAP) and 1-butyl-3-methylimidazolium iodide ([BMIM] I), at different Au(111) surface potentials. Lateral forces vary as a function of applied surface potential and ion structure because the composition of the confined ion layer changes from cation-enriched (at negative potentials) to mixed (at 0 V), and to anion-enriched (at positive potentials). ILs with FAP(-) anions all exhibit similar nanotribology: low friction at negative potentials and higher friction at positive potentials. [BMIM] I displays the opposite behaviour, as an I- anion-enriched layer is more lubricating than either the [BMIM](+) or FAP(-) layers. The effect of cation charged group (charge-delocalised versus charged-localised) was investigated by comparing [BMIM] FAP with 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([Py-1,Py-4] FAP). [BMIM] FAP is less lubricating at negative potentials, but more lubricating at positive potentials. This indicated that even at positive potentials the cation concentration in the boundary layer is sufficiently high to influence lubricity. The influence of sliding velocity on lateral force was investigated for the [EMIM] FAP-Au(111) system. At neutral potentials the behaviour is consistent with a discontinuous sliding process. When a positive or negative potential bias is applied, this effect is less pronounced as the colloid probe slides along a better defined ion plane.
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| 21. |
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| 22. |
- Marquet, Philip, et al.
(author)
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Molecular Scale Characterization of the Titania-Dye-Solvent Interface in Dye-Sensitized Solar Cells
- 2010
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:12, s. 9612-9616
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Journal article (peer-reviewed)abstract
- Charge separation at the dye/titania interface in dye sensitized solar cells is strongly influenced by the thickness and homogeneity of the sensitizing dye layer, as this controls the potential drop across the interface, and the probability of an excited electron being transferred from the dye to the titania. In this study we use atomic force microscopy and the depth profiling method neutral impact collision ion scattering spectroscopy (NICISS) to investigate the thickness and homogeneity of N719 dye adsorbed to titania before and after rinsing with pure acetonitrile. Both experimental methods show that the dye layers are closed but inhomogencous. Inhomogeneity is more pronounced for unrinsed samples.
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| 23. |
- Niga, Petru, et al.
(author)
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Structure of the Ethylammonium Nitrate Surface : An X-ray Reflectivity and Vibrational Sum Frequency Spectroscopy Study
- 2010
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:11, s. 8282-8288
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Journal article (peer-reviewed)abstract
- X-ray reflectivity and vibrational sum frequency spectroscopy are used to probe the structure of the ethylammonium nitrate (EAN)-air interface. X-ray reflectivity reveals that the EAN-air interface is structured and consists of alternating nonpolar and charged layers that extend 31 angstrom into the bulk. Vibrational sum frequency spectroscopy reveals interfacial cations have their ethyl moieties oriented toward air, with the CH3C3 axis positioned similar to 36.5 degrees from interface normal. This structure is invariant between 15 and 51 degrees C. On account of its molecular symmetry, the orientation of the nitrate anion cannot be determined with certainty.
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| 24. |
- Sweeney, James, et al.
(author)
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Control of Nanoscale Friction on Gold in an Ionic Liquid by a Potential-Dependent Ionic Lubricant Layer
- 2012
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 109:15, s. 155502-
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Journal article (peer-reviewed)abstract
- The lubricating properties of an ionic liquid on gold surfaces can be controlled through application of an electric potential to the sliding contact. A nanotribology approach has been used to study the frictional behavior of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py-1,Py-4]FAP) confined between silica colloid probes or sharp silica tips and a Au(111) substrate using atomic force microscopy. Friction forces vary with potential because the composition of a confined ion layer between the two surfaces changes from cation-enriched (at negative potentials) to anion-enriched (at positive potentials). This offers a new approach to tuning frictional forces reversibly at the molecular level without changing the substrates, employing a self-replenishing boundary lubricant of low vapor pressure.
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| 25. |
- Sweeney, James, et al.
(author)
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Effect of ion structure on nanoscale friction in protic ionic liquids
- 2014
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 16:31, s. 16651-16658
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Journal article (peer-reviewed)abstract
- The effect of ionic liquid (IL) molecular structure on nanoscale friction has been investigated using colloidal probe Friction Force Microscopy (FFM). The ILs studied were ethylammonium formate (EAF), ethylammonium nitrate (EAN), propylammonium formate (PAF), propylammonium nitrate (PAN), dimethylethylammonium formate (DMEAF), and ethanolammonium nitrate (EtAN). ILs were confined between a silica colloid probe and a mica surface, and the friction force was measured as a function of normal load for sliding velocities between 10 and 40 mu m s(-1). At low normal forces, multiple IL layers are found between the probe and the surface, but at higher force, in the boundary layer regime, a single ion layer separates the probe and the surface. In the boundary layer regime energy is dissipated by two main pathways. Firstly, the ionic liquid near the surface, with the exception of the boundary layer, is expelled from the advancing contact made by the probe on the surface. This disruption in the interactions between the boundary layer and the near surface multilayers, leads to energy dissipation and depends on the strength of the attraction between the boundary and near surface layers. The second pathway is via rotations and twists of ions in the boundary layer, primarily associated with the cation terminal methyl group. The friction coefficient did not vary over the limited range of sliding speeds investigated.
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| 26. |
- Wakeham, Deborah, et al.
(author)
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Nonionic Surfactant Adsorption at the Ethylammonium Nitrate Surface : A Neutron Reflectivity and Vibrational Sum Frequency Spectroscopy Study
- 2010
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827.
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Journal article (peer-reviewed)abstract
- The adsorbed layers of polyoxyethylene n-alkyl ether surfactants C12E4, C14E4, and C16E4 at the LEAN surface have a headgroup layer that is thin and compact (only similar to 30 vol % EAN). The headgroups do not adopt a preferred orientation and are disordered within the ethylene oxide layer. Alkyl tails contain a significant number of gauche defects indicating a high degree of conformational disorder. The thickness of the tail layer increases with increasing alkyl chain length, while the headgroup layer shows little change. Lowering the C12E4 concentration from 1 to 0.1 wt % decreases the adsorbed amount, and the headgroup layer becomes thinner and less solvated, whereas C14E4 and C16E4 adsorbed layers are unaffected by dilution over the same concentration range. The C16E4 layer thickness increases and area per molecule decreases on warming to 60 degrees C, but the adsorbed layer structures of C12E4 and C14E4 are unchanged. Both effects are attributed to surfactant solubility.
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| 27. |
- Wakeham, Deborah, et al.
(author)
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Surface structure of a "non-amphiphilic" protic ionic liquid
- 2012
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 14:15, s. 5106-5114
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Journal article (peer-reviewed)abstract
- The nanostructure of the ethanolammonium nitrate (EtAN)-air surface has been investigated using X-ray reflectometry (XRR), vibrational sum frequency spectroscopy (VSFS) and neutral impact collision ion scattering spectroscopy (NICISS). The XRR data decays more rapidly than expected for a perfectly sharp interface, indicating a diffuse electron (scattering length) density profile. Modelling of the XRR data using three different fitting routines produced consistent interfacial profiles that suggest the formation of interfacial EtAN clusters. Consistent with this, VSFS reveals that the EtAN surface is predominantly covered by -CH2- moieties, with the -NH3+ and -OH groups of the cation buried slightly deeper in the interface. The elemental profiles determined using NICISS also show enrichment of carbon relative to nitrogen and oxygen in the outermost surface layer, which is consistent with the surface cation orientation deduced from VSFS, and with the presence of EtAN aggregates at the liquid surface.
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| 28. |
- Zhang, Yunxiao, et al.
(author)
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Potential-Dependent Superlubricity of Ionic Liquids on a Graphite Surface
- 2021
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In: The Journal of Physical Chemistry C. - : American Chemical Society. - 1932-7447 .- 1932-7455. ; 125:7, s. 3940-3947
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Journal article (peer-reviewed)abstract
- The lubricities of four quaternary phosphonium ionic liquids-trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P6,6,6,14][(iC8)2PO2]), trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate ([P6,6,6,14][BEHP]), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P6,6,6,14][TFSI]), and tributylmethylphosphonium bis(trifluoromenthylsulfonyl)imide ([P4,4,4,1][TFSI])-were measured as a function of potential on highly oriented pyrolytic graphite (HOPG) by using atomic force microscopy (AFM). The shear strength and surface contact radius values fitted from JKR model indicate the AFM probe slides on an IL boundary layer rather than bare HOPG. Frictions change as the compositions of the boundary layers switch from cation enriched to anion enriched when the potential changes from negative to positive. Superlubricity, which refers to near zero increase in friction with load, is achieved for the [P6,6,6,14]+ cation at-1.0 V and the [TFSI]- anion at +1.0 V. The lubricities of ILs are mainly influenced by three factors: The alkyl chain length, chemical composition, and ion sizes.
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