Impact of confinement and polarizability on dynamics of ionic liquids

Abstract

Polarizability is a key factor when it comes to an accurate description of different ionic systems. The general importance of including polarizability into molecular dynamics simulations was shown in various recent studies for a wide range of materials, ranging from proteins to water to complex ionic liquids and for solid-liquid interfaces. While most previous studies focused on bulk properties or static structure factors, this study investigates in more detail the importance of polarizable surfaces on the dynamics of a confined ionic liquid in graphitic slit pores, as evident in modern electrochemical capacitors or in catalytic processes. A recently developed polarizable force field using Drude oscillators is modified in order to describe a particular room temperature ionic liquid accurately and in agreement with recently published experimental results. Using the modified parameters, various confinements are investigated and differences between non-polarizable and polarizable surfaces are discussed. Upon introduction of surface polarizability, changes in the dipole orientation and in the density distribution of the anions and cations at the interface are observed and are also accompanied with a dramatic increase in the molecular diffusivity in the contact layer. Our results thus clearly underline the importance of considering not only the polarizability of the ionic liquid but also that of the surface.

Theory and Simulations of Ionic Liquids in Nanoconfinement

“…In a very recent work, Gäding et al have investigated the effect of pore wall polarizability on ion diffusion for 1-butylpyridinium bis­(trifluoromethane) sulfon-imide [BuPy]­[NTf 2 ] ionic liquid. They found that for polarizable pore walls, the diffusion coefficient was about 20% lower for a 4.1 nm slit but more than 80% higher for a 1.65 nm slit compared to nonpolarizable (constant charge) pore walls.…”

“…For smooth walls (no atomistic structure), they found noticeable differences only for negative potentials, while for graphite-like walls, the differences were noticeable, albeit small, in the whole voltage range. In a very recent work, Gading et al 303 have investigated the effect of pore wall polarizability on ion diffusion for 1butylpyridinium bis(trifluoromethane) sulfon-imide [BuPy]-[NTf 2 ] ionic liquid. They found that for polarizable pore walls, the diffusion coefficient was about 20% lower for a 4.1 nm slit but more than 80% higher for a 1.65 nm slit compared to nonpolarizable (constant charge) pore walls.…”

Nanoionics from Biological to Artificial Systems: An Alternative Beyond Nanoelectronics

“…Several factors including pore size distribution, pore conductivity, pore connectivity, charging dynamics, and ion concentration will affect ionic diffusion or the flow of ions in and out of the pores. [ 57 , 58 , 59 , 60 , 61 , 62 ] Among them, pore size is the most important factor. In this section, ion transport in nanoscale (2–100 nm) and microporous (typical below 2 nm) will be introduced separately due to their different interaction forces.…”

Simulation studies on the dynamic heterogeneity of organic ionic plastic crystals

“…33,43,50,51,55,56,[76][77][78] However, previous studies showed that the polarizability of charged systems influences not only the phase transition but also the transport properties such as ionic conductivities. [79][80][81][82][83][84][85][86][87][88][89] For example, Borodin et al showed that the diffusion constants of ionic liquids were significantly enhanced by a factor of about two with polarizable force fields. 38,79,90 In this section, we present our recent results about the effect of ion polarization to the rotational dynamics of OIPCs.…”