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.

[6] V. Ivaništšev, K. Kirchner, T. Kirchner, M.V. Fedorov, Restructuring of the electrical double layer in ionic liquids upon charging, J. Phys.: Condens. Matter. 27 (2015) 102101.

[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.

STSM to Vienna: MD simulations of ionic liquid mixtures with polarizable force fi elds

Generic force fields are widely used in molecular level computations. However, in real physical systems, the magnitude and localization of the charges are variable due to the interatomic polarization and the molecular dynamics. There are two ways to take these into account: (i) to reduce the atomic charges to non-integer values or (ii) to include polarizableforces.

In this STSM the latter method was used adding Drude oscillators to the system to add the polarizability to the particles. The polarizable force fields (FFs) developed by the group of Prof. Schroder where used to simulate the mixtures of ionic liquids with classical molecular dynamics. Namely mixture of [EMIm][BF4] and [EMIm]I was used. For comparison, the same system with non-polarizable FFs and the system of pure [EMIm][BF4] using polarizable FFs were simulated.

The main problem with classical MD simulations of ions is that the viscosity is overestimated which comes from the fact of imprecise non-bonding interactions. The polarizable FFs estimate it better and taken from the simulations the diffusion constant for the same system with polarizable FFs was much higher than for non-polarizable (1.03 A2/ns and 0.48 A2/ns respectively for cations). The system of pure [EMIm][BF4] showed that iodine affects the movement of ions as the diffusion constant differs by 10% (1.15 A2/ns for cations in pure system). All in all, the polarizable FFs improves the system and adding the small amount of iodine does not change the diffusion nor the structure that much.

Come visit us in Tartu! Apply for scholarships!

Soon there will be an opportunity to apply money for one-to-ten months-long visits, study and research at the university of Tartu.

Scholarships is offered to foreign master and doctoral students. See more at: http://adm.archimedes.ee/stipendiumid/en/programm-dorapluss/dora-pluss-tegevus-2/#sthash.VPHdRrKc.dpuf

Let us know if you are interested to do some collaborations with us and we are happy to help!

Iuliia’s visit

from left Vladislav Ivanistsev, Iuliia Voroshilova, Isabel Lage, Meeri Lembinen, Samuel Coles

My Estonia experience has come to an end a few days ago when I returned to my habitual residence in Portugal after having spent one important month in the amazing Tartu, where I had such great time and made so many truly amazing friends with whom I shared so many fantastic moments!

I spend last March in The University of Tartu, with a Short Term Scientific Mission funded by COST action CM1206. The visit was meant to be a start of a new collaboration between our two research groups: the host group lead by Prof. Enn Lust (University of Tartu, Estonia) and the home group lead by Prof. Carlos M. Pereira and Prof. M. Natália D. S. Cordeiro (University of Porto, Portugal), and it definitely was a promising start! In the Analytical and Electroanalytical Chemistry Group (where my superviser in prof. Carlos M. Pereira) we study electrical double layer in solutions of ionic liquids experimentally, and in the Theoretical Chemistry Group (under the guidance of prof. M. Natália D. S. Cordeiro) we simulate bulk properties of ionic liquids and ionic liquids behavior on uncharged surfaces. The idea of this collaboration was to learn approaches to access electrical double layer phenomenon, observed experimentally, in computer simulation. The host group has an experience in simulation of charged electrode surfaces and in automating the computation work flow, as the leading developer of the NaRIBaS scripting framework, Dr. Vladislav Ivaništšev, works there. Thus, it was a perfect match! I could not find another group where I could learn all the “tricks” about simulation the charged interfaces and automate my calculations at the same time. During this mission I worked closely with Dr. Vladislav Ivaništšev on developing simulation setups and methods of charging the electrode’s surface. During my visit, besides workshops on working in Atomic Simulation Environment, I attended a very useful workshop on Scientific Writing, given by Djuddah A. J. Leijen in AVOK – Centre for Academic Writing and Communication. The ideas I learned there I implemented in my every day work, which made me more productive.

During my stay in Tartu University I had a chance to work in several university buildings. First two weeks, I worked on the Institute of Phisics, so called “Physicum”, and the last two weeks I staying in the Institute of Chemistry, the “Chemicum” building. Both building are modern, spacious and luninous, and I found it to be very inspiring to work there. But … that was only during the working day! The evenings and weekends were intense and highly saturated with cultural events. Time I spent in Tartu University was not only a great period of learning and collaboration, but also a great traveling experience. As it was my first time not only in Tartu, but in Estonia as well, each evening and weekend me and my new estonian friends and colleagues were exploring Estonia. Most of ours attention was dedicated the city of Tartu, of course. I have been to University of Tartu Natural History Museum, to AHHAA Science Center, I visited Orchid exhibition in Tartu University Botanical Garden, Historical building of Tartu University. Almost every day I was visiting Tartu Old City with its parks and gardens, ruins of Tartu Cathedral, the Kissing Students monument and lovely little cafes. I was invited to attend a Movie Night in the C!FP Sorority, where learned the history of this remarkable organization and of Estonia itself. Also I visited the capital of Estonia, Tallinn, and now I can say that it is known as “Pearl of the Baltic Sea” because of a good reason. The capital if very beautiful, and it looks like a town from a fairy tale. A small and lovely town of Viljandi, which is considered to be cultural capital of Estonia, partly due to the Viljandi Culture Academy being located there, impressed me with its landscapes and ruins of an ancient castle. In all my adventures I was accompanied by a PhD student of the host group, Meeri Lembinen who is a remarkably friendly and helpful person.

from left Vladislav Ivanistsev, Iuliia Voroshilova, Isabel Lage, Meeri Lembinen, Samuel Coles
From left to right: Vladislav Ivanistsev, Iuliia Voroshilova, Isabel Lage, Meeri Lembinen, Samuel Coles

Summarizing, I would like to say, that my visit to Tartu University was a beautiful, intense and influential time, full of impression and emotions, replete with studying, friendship, collaboration, and travels. I learned a lot not only about simulation (which was huge!), but also about Estonian culture and history. I met good people, which become my friends. I would like to thank for all the help and hospitatily to Meeri Lembinen and Vladislav Ivaništšev, who made my visit truly productive in all senses. It was my first visit to Estonia, but it will not be the last, definitely!

Iuliia Voroshylova from University of Porto