[Show abstract][Hide abstract] ABSTRACT: In these lecture notes we will briefly overview current progress in theory and simulations of (room temperature) ionic liquids (ILs) at charged interfaces (CIs). ILs are important highly concentrated electrolyte media for many applications - from energy storage to friction. When a certain IL is to be chosen for a given application, a suitable one has to be selected from, practically, an infinite number of possible combinations of cations and anions. Therefore, the main role or modelling and theoretical techniques in this area is to understand the main trends in the IL behaviour at CIs and reveal the most important guiding factors that determine it. The notes are focussed on computational and theoretical analysis of the fundamental properties of the structure of the electrical double layer (EDL) in ILs that is dramatically different from the one in diluted electrolytes. Several methodological aspects that are specific to IL systems are discussed.
Computational Trends in Solvation and Transport in Liquids, Edited by Godehard Sutmann, Johannes Grotendorst, Gerhard Gompper, Dominik Marx, 03/2015: pages 107-127; Forschungszentrum Jülich GmbH.., ISBN: 186808489
[Show abstract][Hide abstract] ABSTRACT: We have investigated the electrical double layer (EDL) structure at an interface between ionic liquid (IL) and charged surface using molecular dynamics simulations. We show that for three different models of ILs the EDL restructuring, driven by surface charging, can be rationalized by the use of two parameters-renormalized surface charge (κ) and charge excess in the interfacial layers (λ). Analysis of the relationship between the λ and κ parameters provides new insights into mechanisms of over-screening and charge-driven structural transitions in the EDL in ionic liquids. We show that the restructuring of the EDL upon charging in all three studied systems has two characteristic regimes: (1) transition from the bulk-like (κIon = 0) to the multilayer structure (κIon ≈ 0.5) through the formation of an ionic bilayer of counter- and co-ions; and (2) transition from the multilayer (κIon ≈ 0.5) to the crowded (κIon > 1) structure through the formation of a monolayer of counter-ions at κIon = 1.
[Show abstract][Hide abstract] ABSTRACT: The simulated systems represent model ionic liquid (IL) ions confined between two model electrodes [1–3]. The models represented IL ions as charged Lennard-Jones spheres [1, 3] with the cation-to-anion diameter (d LJ) ratios were chosen to be 1 : 1 (large anion – LA), 1 : 0.8 (medium anion – MA) and 1 : 0.5 (small an-ion – SA) with d LJ (Cation) = 1.0 nm (the SA model was previously used in] [1, 3]). The electrodes consist of 2500 Lennard-Jones spheres with a diameter (d LJ) of 0.22 nm that are arranged on a square lattice with a size of 11 nm × 11 nm, in x and y directions. Distance between the electrodes was chosen to be 54 nm, 36 nm and 24 nm for the LA, MA and SA systems, respectively. The electrodes were charged with surface charge densities (σ) ranging from 0 to ±24 µC/cm 2 for the LA model, from 0 to ±40 µC/cm 2 for the MA model, and from 0 to ±76 µC/cm 2 for the SA model. The ion pair number was fixed in all simulations to be equal 1050. The force field parameters of the models are summarized in Table I. TABLE I. Force field parameters of the model ionic liquids and the electrodes. Ions and electrode particles are modelled as charged Lennard-Jones spheres with only repulsive part of the short-range potential U ij LJ = 4εLJ [(di + dj)/r] 12 , where εLJ = 1.8698 kj mol −1. The arithmetic combination rule was used for both dLJ and εLJ.
[Show abstract][Hide abstract] ABSTRACT: We report a molecular dynamics simulation of liquid water using the SWM4-NDP polarizable water model that belongs to the class of Drude oscillator type models. Generally, parameterisation protocols currently used for this type of models implicitly assume the validity of point-dipole approximation for the induced dipole (where the response of the dipole to the external field should depend only on the value of polarizability but not on the actual values of the charge or spring constant of the Drude oscillator). Therefore, possible dependency of the model results from the Drude charge/spring constant is often neglected.
[Show abstract][Hide abstract] ABSTRACT: We have studied structural transitions in the electrical double layer of ionic liquids by molecular dynamics simulations. A model coarse grained room temperature ionic liquid (RTIL) with asymmetric sized ions confined between two oppositely charged walls has been used. The simulations have been performed at different temperatures and electrode charge density values. We found that for the studied charge densities the electrical double layer has a multilayered structure with multiple alternating layers of counter- and co-ions at the electrode–RTIL interface; however, at certain charge densities the alternating multilayer structure of the electrical double layer undergoes a structural transition to a surface-frozen monolayer of densely packed counter-ions (Moiré-like structure). At this point the dense ordered monolayer of counter-ions close to the electrode surface coexists with apparently non-structured RTIL further from the electrode. These findings might bring possible explanations to experimental observations of formation of Moiré-like structures in ionic liquids at electrified interfaces. Moreover, we report the formation of herring-bone interfacial structures at high surface charge densities, that appear as a result of superposition of two ordered monolayers of RTIL ions at the electrode–RTIL interface. Similar structures were observed experimentally; however, to the best of our knowledge they have not been modelled by simulations. We discuss the dependence of the electrical double layer structure in RTILs on the ion size and the surface charge density at the electrodes.
[Show abstract][Hide abstract] ABSTRACT: By means of fully atomistic molecular simulations we study basic mechanisms of carbon nanotube interactions with several different room temperature ionic liquids (RTILs) in their mixtures with acetonitrile. To understand the effects of the cation molecular geometry on the properties of the interface structure in the RTIL systems, we investigate a set of three RTILs with the same TFSI (bis (trifluoromethylsulfonyl)imide) anion but with different cations, namely, EMIm (1-ethyl-3-methylimidazolium), BMIm (1-butyl-3-methylimidazolium) and OMIm (1-octyl-3-methylimidazolium) ions. The cations have identical charged methylimidazolium 'heads' but different nonpolar alkyl 'tails' where the length of the tail increases from EMIm to OMIm. The analysis of the simulation data results in the following conclusions: There is an enrichment of all molecular components of ionic liquids under study at the CNT surface with formation of several distinct layers even at the non-charged CNT surface. Mixing RTIL with acetonitrile decreases ion-counterion correlations in the electric double layer. Increase of the length of the non-polar 'tail' of cations increases the propensity of imidazolium-based cations to lay parallel to the CNT surface. At the CNT cathode TFSI anions and molecular cations are preferentially oriented parallel to the surface. At the CNT anode the TFSI anions are oriented parallel to the surface, however the preferred orientations of cations depend on the length of non-polar tail: EMIm cations are oriented perpendicular to the surface, BMIm cations can be in both parallel as well as perpendicular orientations, OMIm cations are oriented parallel to the surface. As a result, by applying an electric potential on the CNT electrode and/or varying the structure of molecular ions it is possible to change molecular ion orientations at the surface and, consequently, the structure of the electrical double layer at the CNT-RTIL interface.