STSM to Trieste: Simulation of surface charge effect on the sliding friction of a nanoconfined ionic liquid

The interest towards using ionic liquids as lubricants has been increasing since ca. 2012 [1]. It was demonstrated that the interfacial structures in ionic liquids control the nanoscale friction. Experimental (see refs in [1]) and several computational studies [2–5] revealed that lubricity varies with the number and lateral structure of confined ion layers, which in turn are dependent on the applied potential. This opens a possibility of electrochemical control of friction, in other words a “Tantalizing prospect of tunning friction on small dimensions without changing surfaces with a self-replenish­ing layer, and could be easily integrated into niche situations, … because ionic liquids are cheaper than existing nonconducting molecular lubricants” (see refs in [1]). At the same time it was shown that the restructuring of the potential dependent interfacial structure happens in ionic liquids on a regular manner [6,7]. Therefore, there is a direct relation between various interfacial properties through the potential dependent interfacial structure [8]. For instance, it has not been marked in the literature that the potential dependent friction force [9–11] is proportional to the potential dependent capacitance [12–14]. The understanding of this structure-based relationship can help in controlling of the nanoscale friction in ionic liquids by a potential-tuned ionic lubricant layer.

During this STSM visit, we have initiated a comprehensive study: formulated a hypothesis, prepared a research plan, and obtained preliminary results. The later reveal a proportionality between potential dependent capacitance and friction force. Such structure-based relationship can be of use in controlling of the nanoscale friction in ionic liquids by an applied potential. Further work should verify the generic mechanism of the electrotunable lubricity and capacity. In the future study we plan to perform a detailed atomistic simulation to enable comparison between specific computational and experimental data.

This research was supported by a short term scientific mission funded by COST action MP1303.

References:

[1] R. Hayes, G.G. Warr, R. Atkin, Structure and Nanostructure in Ionic Liquids, Chem. Rev. 115 (2015) 6357–6426.

[2] R. Capozza, A. Vanossi, A. Benassi, E. Tosatti, Squeezout phenomena and boundary layer formation of a model ionic liquid under confinement and charging, J. Chem. Phys. 142 (2015) 64707.

[3] R. Capozza, A. Benassi, A. Vanossi, E. Tosatti, Electrical charging effects on the sliding friction of a model nano-confined ionic liquid, The Journal of Chemical Physics. 143 (2015) 144703.

[4] O.Y. Fajardo, F. Bresme, A.A. Kornyshev, M. Urbakh, Electrotunable Lubricity with Ionic Liquid Nanoscale Films, Sci. Rep. 5 (2015) 7698.

[5] O.Y. Fajardo, F. Bresme, A.A. Kornyshev, M. Urbakh, Electrotunable Friction with Ionic Liquid Lubricants: How Important Is the Molecular Structure of the Ions?, The Journal of Physical Chemistry Letters. 6 (2015) 3998–4004.

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[7] V. Ivaništšev, S. O’Connor, M.V. Fedorov, Poly(a)morphic portrait of the electrical double layer in ionic liquids, Electrochem. Commun. 48 (2014) 61–64.

[8] M.V. Fedorov, A.A. Kornyshev, Ionic liquids at electrified interfaces, Chem. Rev. 114 (2014) 2978–3036.

[9] J. Sweeney, F. Hausen, R. Hayes, G.B. Webber, F. Endres, M.W. Rutland, R. Bennewitz, R. Atkin, Control of Nanoscale Friction on Gold in an Ionic Liquid by a Potential-Dependent Ionic Lubricant Layer, Phys. Rev. Lett. 109 (2012) 155502.

[10] H. Li, R.J. Wood, M.W. Rutland, R. Atkin, An ionic liquid lubricant enables su­perlubricity to be “switched on” in situ using an electrical potential, Chem. Com­mun. 50 (2014) 4368–4370.

[11] H. Li, M.W. Rutland, R. Atkin, Ionic Liquid Lubrication: Influence of Ion Structure, Surface Potential and Sliding Velocity, Phys. Chem. Chem. Phys. 15 (2013) 14616–14623.

[12] M. Drüschler, N. Borisenko, J. Wallauer, C. Winter, B. Huber, F. Endres, B. Roling, New insights into the interface between a single-crystalline metal elec­trode and an extremely pure ionic liquid: slow interfacial processes and the influ­ence of temperature on interfacial dynamics, Phys. Chem. Chem. Phys. 14 (2012) 5090–5099.

[13] R. Atkin, N. Borisenko, M. Drüschler, S.Z. El Abedin, F. Endres, R. Hayes, B. Huber, B. Roling, An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophos­phate/Au(111) interface: potential dependent solvation layers and the herringbone reconstruction, Phys. Chem. Chem. Phys. 13 (2011) 6849.

[14] R. Costa, C.M. Pereira, A.F. Silva, Charge Storage on Ionic Liquid Electric Double Layer: The Role of the Electrode Material, Electrochim. Acta. 167 (2015) 421–428.