![]() In order to resolve these difficulties, we separate the change in viscosity into two steps. At present, there is considerable dispute about which of the approaches is preferable for ionic liquids, but clearly the ultimate goal is to develop physically sound models which also provide reliable quantitative predictions. Other models aim to provide predictive tools – which are numerically accurate but often without physical basis – for engineering purposes. 17–33 Some models aim to further our understanding of the underlying physics from a basic scientific viewpoint with often limited predictive value. 16 If this were the case, it would imply that the high density itself was the cause of the high viscosity.Ī wide variety of models with considerable conceptual overlap have been developed to describe viscous flow of ionic liquids. It is conceivable that the two systems would then also have similar viscosities. In other words, the molecular mimic must be subjected to pressure high enough so that its density becomes equal to that of the ionic liquid under ambient pressure at the same temperature. 12–15 Consequently, the comparison between a molecular mimic and an ionic liquid should be made under isodensity conditions. 10,11 Indeed, the viscosity of both molecular and ionic liquids can often be expressed as a function of ρ γ/ T, where ρ is the density, T the temperature and γ a material parameter. pressure) generally leads to an increase in viscosity. The differences in density and viscosity lead to an important question: what is the degree to which coulombic compaction causes the high viscosity of ionic liquids? It is well known that an increase in density ( i.e. 1 The ionic liquid (left) and molecular mimic (right) investigated by Shirota and Castner. strong (attractive) coulombic interactions which reduce the volume of the liquid phase. The higher density is the result of coulombic compaction, i.e. However, the ionic liquid also has a higher density than the molecular mimic, which still constitutes a bias. One might interpret this factor of 30 as the difference between conventional molecular solvent and ionic liquid, i.e. 9 Crucially, the viscosity of the ionic liquid is almost 30 times that of the molecular mimic, despite the similar molecular structures. 1 shows the molecular mimic used by Shirota and Castner, together with the viscosity and density values at room temperature and ambient pressure. 6–8 To ensure similarity, the molecular mimic and the corresponding ionic liquid should ideally be isoelectronic and isostructural to each other. The neutral system has been called the ‘molecular mimic’ 6,7 and is a mixture of neutral analogues of the anionic and cationic molecular constituents. In order to optimise the viscosity, it is necessary to develop a mechanistic understanding of the difference between how viscosity arises in ionic liquids and conventional molecular solvents.Ī fair, unbiased comparison between ionic liquids and conventional molecular solvents necessitates two systems which are as similar as possible one charged, and one neutral. This is a key aspect for applications such as batteries, gas separation or biomass processing. 1–5 However, the practical applicability of most ionic liquids is limited by their high viscosity compared with conventional molecular solvents. Introduction In recent years, ionic liquids have transformed from a scientific curiosity to extensively used functional fluids, both in academia and industry. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach. ![]() We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries.
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