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Interactions across nanoconfined ionic liquids
Ionic liquids are pure salts that are liquid under room temperature-pressure conditions. They exhibit exceptional stability (low volatility, nonflammability, wide electrochemical windows, etc.), which makes them very promising systems for a range of applications. These applications include energy storage (batteries, supercapacitors) and lubrication, in which the electrolyte is confined at the nanometric scale. That is why the behaviour of ionic liquids in nanoconfinement has been the focus of intense study during the last decade.
Recent studies reported on the observation of anomalously long-ranged electrostatic interactions in ionic liquids, with a range of typically 10 nanometers, i.e. two orders of magnitude larger than the Debye length and one order of magnitude larger than the ion size. However, these studies did not consider the effect of viscosity, which could be substantial given ionic liquids are typically 100 times more viscous than water. In the first part of the talk, I will present systematic drainage measurements that we performed with a Surface Force Balance, in order to fully disentangle and characterize the equilibrium and dynamical contributions.
When further confined at the scale of a few nanometers, ionic liquids are squeezed-out by discrete steps, because they are arranged in ordered layers of alternating cations and anions. An even more exotic phenomenon, reported as “quantized friction”, is the fact that the friction coefficient is indexed by the number of ion layers forming the liquid film. In the second part of the talk, I will present the new method we designed, which allows for an unprecedently detailed investigation of the molecular mechanisms of friction.