The Effect of Water and Confinement on Self-Assembly of Imidazolium Based Ionic Liquids at Mica Interfaces

Abstract

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to tailor performance of ILs in applications such as super-capacitors, catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement, surface charging and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, at high surface charging and/or in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and dominated by bulk structuring. Our results demonstrate that boundary conditions such as charging, confinement and doping by impurities have decisive influence on structure formation of ILs at interfaces. As such, these results have important implications for understanding the behavior of solid/IL interfaces as they significantly extend previous interpretations.

Interfacial Structure and Boundary Lubrication of a Dicationic Ionic Liquid

“…The wavelength of oscillations is found to be 0.8±0.1 nm, which is very similar to that previously reported for imidazolium-based 1:1 ionic liquids with the same [NTf 2 ] ion and short alkyl-chains: For [C n C 1 (Im)][NTf 2 ] with n = 2 or 4, wavelengths or repeat-distances for the structural force have been reported in the range 0.71-0.8 nm. 5,10,34,35 These values are similar to the dimension of an ion pair (or, equivalently, to the ++ or --correlation length in the bulk fluid), and so have been interpreted as the repeat-distance for a layered film consisting of (on average) alternating layers of cations and anions which are expelled as ion-pair-layers when the surfaces approach. 5 As long as the alkyl chain is sufficiently small, the value of n was found not to alter the wavelength, indicating that the alkyl chain is oriented in the plane of the film.…”

“…Similar behaviour has been reported in the past with 1:1 ionic liquids. 34 The strong enhancement of ionic liquid nano-structure due to small fractions of water (often at a level undetectable by analytical methods such as Karl-Fischer titration) is well known. In this case of enhancement of the force between mica surfaces, there are two possible contributions: (i) increase of the bulk nano-structure due to water (solvophobic self-assembly), which is detected by way of the structural forces, and (ii) increase of the mica surface charge in the presence of water, which leads to increase of the surface-charge-induced structuring (overscreening).…”

Long range electrostatic forces in ionic liquids

“…These results show that the presence of water in ionic liquids can result in complex, and at times unexpected, changes to surface forces. 14,46,47 While the presence of long range surface forces is increasingly measured for numerous combinations of ionic liquids and charged surfaces, long range interaction forces are not measured in every experiment on ionic liquids. In 1988, Horn and colleagues 38 observed solely short range ion binding forces in an SFA study of the protic ionic liquid EAN between mica surfaces.…”

Structural interactions in ionic liquids linked to higher-order Poisson-Boltzmann equations

“…This phenomenon has been observed in numerous experiments and simulations, see e.g. [3][4][5][6][7]; for a review, see [8].…”