Highly concentrated electrolytes are important in nature and in technology; most recently their importance has been highlighted in the design of next-generation electrolytes for energy storage devices. An extreme case of concentrated electrolyte is pure ionic liquid: solvent-free salt in liquid state under ambient conditions. Recent focus on the physical properties of ionic liquids and concentrated electrolytes has illuminated several important gaps in our understanding, together presenting fascinating puzzles for those interested in the nature of the ionic liquid state.<br/><br/>In our laboratory we study the structure of liquids at the nanoscale using the Surface Force Balance (SFB). In these experiments, the liquid of interest is placed between two atomically flat, uniformly charged mica surfaces, and the force between the surfaces is measured as a function of the film thickness. SFB measurements have shown that ionic liquids are arranged in layers when in confinement between two charged interfaces. The layering structure gives rise to an oscillatory force profile, where the force switches between repulsion and attraction as a function of the surface separation, and where the period of oscillation corresponds to the ion pair size.<br/><br/>Recent results from our laboratory and from other leading groups show that besides the structural force, a long-ranged exponentially decaying repulsion force is present in ionic liquids. The electrostatic screening length derived from this force is up to an order of magnitude greater than the ion diameter, and strikingly larger than anticipated based on existing theories. Similar measurements with concentrated electrolytes, either aqueous solutions or mixtures of ionic liquids in polar solvents, show that the screening length increases with increasing volume fraction of ions, as well as with increasing Bjerrum length. The latter observations demonstrate that the unexpected screening in ionic liquids is, in fact, a general phenomenon pertinent to electrolytes at high ionic strength and irrespective of their chemical makeup. We will discuss the SFB technique, show the recent experimental results, and outline a simple scaling theory to explain these observations.

Structure and screening behaviour of ionic liquids under nanoconfinement

255th National Meeting (2018)

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