Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces

Abstract

We report high-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than approximately 2 nm oscillatory structural forces are observed as the surface separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1-2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining surfaces lead to a robust film which is maintained between the shearing surfaces when pressure is applied across the film.

Computer simulations of ionic liquids at electrochemical interfaces

“…Nevertheless the main picture of the structure has remained essentially the same since the pioneering work from Heyes and Clarke 17 : The ionic liquid adopts a layered structure, and the structure of the bulk is recovered after several layers. This particular pattern has since then been confirmed by several experimental techniques such as high-energy X-ray reflectivity 68 , AFM 60,69,70 or surface force apparatus (SFA) 61,[71][72][73] . Figure 3 illustrates this layering as it is observed in AFM 60 (top) and SFA 61 (middle) experiments or in MD simulations 62 (bottom).…”

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].…”

Controlling turbulent drag across electrolytes using electric fields

“…As such, it is natural to wonder whether the ionic nature of ionic liquids or electrolytes can be exploited to control friction using electric fields. For ionic liquids, seminal works by Perkin et al show that nanoconfined ionic liquids near a charged surface arrange themselves in a layered structure of alternating cation-rich and anion-rich layers [13,14]. The number of ion layers confined between the surfaces is an integer dependent on the surface separation, and thus the friction coefficient in the low velocity regime also shows a "quantized" behaviour as a function of surfaces separation [15].…”