Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids

Abstract

Ionic liquids can be intricately nanostructured in the bulk and at interfaces resulting from a delicate interplay between interionic and surface forces. Here we report the structuring of a series of dialkylpyrrolidinium-based ionic liquids induced by confinement. The ionic liquids containing cations with shorter alkyl chain substituents form alternating cation-anion monolayer structures on confinement to a thin film, whereas a cation with a longer alkyl chain substituent leads to bilayer formation. The crossover from monolayer to bilayer structure occurs between chain lengths of n = 8 and 10 for these pyrrolidinium-based ionic liquids. The bilayer structure for n = 10 involves full interdigitation of the alkyl chains; this is in contrast with previous observations for imidazolium-based ionic liquids. The results are pertinent to these liquids' application as electrolytes, where the electrolyte is confined inside the pores of a nanoporous electrode, for example, in devices such as supercapacitors or batteries.

Computer simulations of ionic liquids at electrochemical interfaces

“…69 Negative temperat dence was observed for similar ILs and an Hg elec well as for molten salt electrolytes. 71 While Mo simulations, [72][73][74] modified MSA theory, 75 and DF have revealed that the DC can have both negative a temperature dependence for the restricted primitiv negative temperature dependence is expected for t and temperature of our simulations. that the use of single-frequency measurements could lead to misleading results [98][99][100] .…”

Are Ionic Liquids Good Boundary Lubricants? A Molecular Perspective

“…[12][13][14][15]. However, ionic liquid structure consists of alternating layers of excess cation density and excess anion density, and one squeeze-out event corresponds in fact to the squeeze-out of an electroneutral "slab" which can be considered, approximately, as two layers (one cation-and one anion-excess) together [9,[16][17][18][19][20][21][22]. In several cases of long-chain and geometrically irregular ionic liquids the squeeze-out behaviour is found to become continuous, rather than discontinuous.…”

“…The influence of this structure on the lubrication properties was illuminated by comparisons made between measurements with dry and wet ionic liquids. Figure 2d,e shows measurements made with [C 10 C 1 Pyrr][NTf 2 ], which is organized in bilayers [22,24], in dry or wet conditions. A key finding of this study is that the friction coefficient for a single bilayer is independent of the water content, whereas it is enhanced by more than an order of magnitude for two bilayers when the liquid is wet.…”

Interfaces between Charged Surfaces and Ionic Liquids: Insights from Molecular Simulations

“…42-44 a "monolayer" to "bilayer" restructuring induced by confinement has been observed by surface forces apparatus 45 and solid-like multilayers in ionic liquids have been recently reported. 22,46 Conclusions MD simulations of ionic liquid-electrode interfaces show that a transition from a multilayer ion structure to an ordered monolayer structure can be observed at certain values of the surface charge density.…”