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.

Does water have a high viscosity