Engineering encapsulated ionic liquids for next-generation applications

Yan, Jiusheng; Mangolini, Filippo

doi:10.1039/d1ra05034f

Cited by 25 publications

(19 citation statements)

References 154 publications

(196 reference statements)

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“…Several studies highlighted the good lubricating properties of neat ILs [41][42][43][44][45][46][47], which, in some cases, even outperform the one of commercially used fully-formulated oils [43,44,48]. Despite these promising results, the implementation of ILs in oil formulations has been constrained by four main issues: (i) their high cost; (ii) the corrosivity of most ILs, which derives from the sensitivity of halogenated ILs to moisture that can result in the release of toxic and corrosive halogen halides [49][50][51][52]; (iii) the limited solubility of the vast majority of ILs in hydrocarbon uids; and (iv) our limited understanding of the IL lubrication mechanism(s), which has hampered our ability of rationally designing task-speci c ILs [53]. The rst three issues have progressively been solved over the last decade with: (i) the recent synthesis of airstable, eco-friendly, protic ILs (PILs), whose ease of preparation can signi cantly lower costs [54,55]; (ii) the transition towards halogen-free ILs, which has drastically decreased corrosion problems [56-65]; and (iii) the synthesis of oil-soluble ILs [31,61,[66][67][68][69][70][71][72] and the development of polymer-encapsulated ILs [53].…”

Section: Introductionmentioning

confidence: 99%

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

Yan

Lien

Mangolini

2023

Preprint

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While ionic liquids (ILs) have gained wide interest as potential alternative lubricants able to meet the requirements of next-generation tribological systems owing to their unique physico-chemical properties and promising lubricating behavior, our understanding of the mechanisms by which ILs reduce friction and/or wear is still elusive. Here, we combine macroscale tribological experiments with surface-analytical measurements to shed light on the lubrication mechanisms of a class of halogen-free ILs, namely tetraalkylammonium orthoborate ILs, at steel/steel sliding contacts. The tribological results indicate an improvement of the friction-reducing properties of these ILs as the length of the alkyl chains attached to ammonium cations increases. Ex situ X-ray photoelectron spectroscopy analyses provide further evidence for the dependence of the lubrication mechanism of tetraalkylammonium orthoborate ILs on the IL structure. In the case of tetraalkylammonium orthoborate ILs with asymmetric ammonium cations containing a long alkyl chain, no sacrificial tribofilms were formed on steel surfaces, thus suggesting that the friction-reducing ability of these ILs originates from their propensity to undergo a pressure-induced morphological change at the sliding interface that leads to the generation of a lubricious, solid-like layered structure. Conversely, the higher friction response observed in tribological tests performed with tetraalkylammonium orthoborate ILs containing more symmetric ammonium cations and short alkyl chains is proposed to be due to the inability of this IL to create a transient interfacial layer owing to the reduced van der Waals interactions between the cationic alkyl chains. The resulting hard/hard contact between the sliding surfaces is proposed to lead to the cleavage of boron-oxygen bonds in the presence of water to form species that then adsorb onto the steel surface, including trivalent borate esters and oxalic acid from the decomposition of orthoborate anions, as well as tertiary amines from the degradation of alkylammonium cations induced by hydroxides released during the orthoborate decomposition reaction. The results of this work not only establish links between the molecular structure of a class of halogen-free ILs, their lubricating performance, and lubrication mechanism, but also provide evidence for the existence of multiple mechanisms underpinning the promising lubricating properties of ILs in general. Table of Content (Graphical Abstract)

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Section: Introductionmentioning

confidence: 99%

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

Yan

Lien

Mangolini

2023

Preprint

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“…On the other hand, the main approach to electric power storage is the formation of chemical batteries and electric double layer capacitors, but there are still few studies on microencapsulation. Recently, however, some research on microencapsulation of ionic liquids as electrolytes with a conductive or porous shell material has been reported [ 10 , 11 , 12 ], and the possibility of the formation of electric power storage devices in micron size has been mentioned. In this case, the shell for microencapsulation is required to be a conductive or porous material to make electrical contact with the outside.…”

Section: Introductionmentioning

confidence: 99%

Transparent Gelation of Ionic Liquids Trapped in Silicone Microcup Structures under Scanning Electron Microscopy

Iwasaki

Okoshi

2023

Gels

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It is expected that ionic liquids will be used in the future as electrolytes for electric double layer capacitors, but currently microencapsulation with a conductive or porous shell is required for their fabrication. Here, we succeeded in fabricating a transparently gelled ionic liquid trapped in hemispherical silicone microcup structures just by observing with a scanning electron microscope (SEM), which allows the microencapsulation process to be eliminated and electrical contacts to be formed directly. To see the gelation, small amounts of ionic liquid were exposed to the SEM electron beam on flat aluminum, silicon, silica glass, and silicone rubber. The ionic liquid gelled on all the plates, and a color change to brown was observed on all the plates except for silicone rubber. This change might be caused by reflected and/or secondary electrons from the plates producing isolated carbon. Silicone rubber could remove the isolated carbon due to the large amount of oxygen inside it. Fourier transform infrared spectroscopy revealed that the gelled ionic liquid included a large amount of the original ionic liquid. Moreover, the transparent, flat gelled ionic liquid could also be made into three-layer structures on silicone rubber. Consequently, the present transparent gelation is suitable for silicone rubber-based microdevices.

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Section: Introductionmentioning

confidence: 99%

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

2023

Self Cite

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While ionic liquids (ILs) have gained wide interest as potential alternative lubricants able to meet the requirements of next-generation tribological systems owing to their unique physico-chemical properties and promising lubricating behavior, our understanding of the mechanisms by which ILs reduce friction and/or wear is still elusive. Here, we combine macroscale tribological experiments with surface-analytical measurements to shed light on the lubrication mechanisms of a class of halogen-free ILs, namely tetraalkylammonium orthoborate ILs, at steel/steel sliding contacts. The tribological results indicate an improvement of the friction-reducing properties of these ILs as the length of the alkyl chains attached to ammonium cations increases. Ex situ X-ray photoelectron spectroscopy analyses provide further evidence for the dependence of the lubrication mechanism of tetraalkylammonium orthoborate ILs on the IL structure. In the case of tetraalkylammonium orthoborate ILs with asymmetric ammonium cations containing a long alkyl chain, no sacri cial tribo lms were formed on steel surfaces, thus suggesting that the friction-reducing ability of these ILs originates from their propensity to undergo a pressure-induced morphological change at the sliding interface that leads to the generation of a lubricious, solid-like layered structure. Conversely, the higher friction response observed in tribological tests performed with tetraalkylammonium orthoborate ILs containing more symmetric ammonium cations and short alkyl chains is proposed to be due to the inability of this IL to create a transient interfacial layer owing to the reduced van der Waals interactions between the cationic alkyl chains. The resulting hard/hard contact between the sliding surfaces is proposed to lead to the cleavage of boron-oxygen bonds in the presence of water to form species that then adsorb onto the steel surface, including trivalent borate esters and oxalic acid from the decomposition of orthoborate anions, as well as tertiary amines from the degradation of alkylammonium cations induced by hydroxides released during the orthoborate decomposition reaction. The results of this work not only establish links between the molecular structure of a class of halogen-free ILs, their lubricating performance, and lubrication mechanism, but also provide evidence for the existence of multiple mechanisms underpinning the promising lubricating properties of ILs in general.

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Engineering encapsulated ionic liquids for next-generation applications

Abstract: Encapsulated ionic liquids (ILs) are candidate materials for several applications owing to the attractive properties of ILs combined with the enhanced mass transfer rate obtained through the discretization of ILs in small capsules.

Cited by 25 publications

References 154 publications

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

Transparent Gelation of Ionic Liquids Trapped in Silicone Microcup Structures under Scanning Electron Microscopy

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

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