A Pictorial View of Viscosity in Ionic Liquids and the Link to Nanostructural Heterogeneity

Abstract

Prototypical ionic liquids (ILs) are characterized by three structural motifs associated with (1) vicinal interactions, (2) the formation of positive–negative charge-alternating chains or networks, and (3) the alternation of these networks with apolar domains. In recent articles, we highlighted that the friction and mobility in these systems are nowhere close to being spatially homogeneous. This results in what one could call mechanical heterogeneity, where charge networks are intrinsically stiff and charge-depleted regions are softer, flexible, and mobile. This Letter attempts to provide a clear and visual connection between frictionassociated with the dynamics of the structural motifs (in particular, the charge network)and recent theoretical work by Yamaguchi linking the time-dependent viscosity of ILs to the decay of the charge alternation peak in the dynamic structure function. We propose that charge blurring associated with the loss of memory of where positive and negative charges are within networks is the key mechanism associated with viscosity in ILs. An IL will have low viscosity if a characteristic charge-blurring decorrelation time is low. With this in mind, engineering new low-viscosity ILs is reduced to understanding how to minimize this quantity.

A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids

“… 111 , 112 What this means is that from a computational perspective, guaranteed convergence of properties requires careful consideration in the case of ILs (simply put, simulations may require tens or hundreds of nanoseconds to converge depending on the property and temperature). 113 , 114 The issue of the high viscosity of ILs is compounded by the fact that force fields, particularly nonpolarizable force fields or those in which charges have not been scaled to account for some level of polarization or charge transfer, have actual model viscosities that are sometimes orders of magnitude larger than the experimental ones. 108 , 115 In other words, whereas an IL may be viscous but not in the glass regime, its fixed-charge force field model version may well be.…”

A review on machine learning algorithms for the ionic liquid chemical space

“…Purposeful post processing and visualisation is crucial to understand the behaviour of bulk ionic liquids by means of MD simulation. [245][246][247] MD simulations can thus serve as a bridge between molecular features and bulk properties. 248,249 The high dimensionality of an atomistic trajectory can in some cases be reduced to just a few dimensions which can be understood by a human.…”

Understanding the Photoexcitation of Room Temperature Ionic Liquids

“…The heterogeneity within RTILs can be further categorized into polar‐apolar and positive‐negative charged parts, whereas both parts are alternating through the RTIL (see Figure 3). Voids are detectable between the ion pairs [41] . This cavity seems to have an influence in charge transfer reactions by the ability to transport ions, especially protons, into this void [42] …”