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Paradigm shift in understanding of the double layer in ionic liquids The rediscovery of room temperature ionic liquids (RTILs) some 15 years ago was a revolution in chemistry. In the past, a limited amount of solvents was available, but now we have thousands of RTILs. Their mixtures allow us to build ‘designer solvents’, targeted for each specific purpose. The applications are vast, but let us focus on those where RTILs are used as – solvent-free electrolytes. For this reason they can be used in supercapacitors and electroactuators, as well as they have interesting features as lubricants. In each of these categories spectacular new experimental discoveries have been reported, such as enhanced capacitance in nanopores and anomalously fast charging dynamics, ‘quantized’ friction, etc*. All of them are based on the behavior of RTILs in a confined geometry, and on the properties of the electrical double layer near a charged surface. For a theorist the area is fascinating, as it is based on strongly correlated Coulomb systems. This is a frontier line of the theory of condensed matter. One of the standard ways to probe the properties of an electrical double layer is to measure the double layer capacitance. While this was always problematic for high temperature ionic melts, it is rather straightforward for RTILs. However the progress in this area was impeded by the lack of theoretical understanding of the structure of the double layer in RTILs and the features of capacitance predicted by the theory. Some data where even not published, as not fitting any comprehensive theoretical concepts. The situation has changed dramatically after a Feature Article - Kornyshev AA, Double-layer in ionic liquids: paradigm change?, J Phys Chem B, 2007, Vol:111, Pages: 5545-5557, ISSN:1520-6106(doi). which has formulated the principles of the double layer and capacitance in RTILs, thereby triggerring a flux of works in this field. Further targeted computer simulations by Maxim Fedorov and his group at Max Planck Institute in Leipzig and Alexei Kornyshev - Fedorov MV, Kornyshev AA, Towards understanding the structure and capacitance of electrical double layer in ionic liquids, ELECTROCHIMICA ACTA. 2008, Vol: 53, Pages: 6835-6840 Fedorov MV, Kornyshev AA, Ionic liquid near a charged wall: Structure and capacitance of electrical double layer, J PHYS CHEM B, 2008, Vol: 112, Pages: 11868-11872, ISSN:1520-6106(doi) Fedorov MV, Georgi N, Kornyshev AA, Double layer in ionic liquids: The nature of the camel shape of capacitance, ELECTROCHEM COMMUN, 2010, Vol: 12, Pages: 296-299, ISSN:1388-2481(doi) have revealed the detailed structure of the double layer and the capacitance-voltage dependence, particularly the features that lie beyond simple mean-field theory. The door is now open for the study of the double layer structure, and dynamics, in nano-confinement. __________ * See e.g. - C.Largeot, C.Portet , J. Chmiola , P.L.Taberna , Yu.Gogotsi, P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor, JACS 130, 2730 (2008). S.Liu, W.J. Liu, Y.Liu , J.H.Lin , X.Zhou , M.J. Janik , R.H. Colby ,Q.M. Zhang, Influence of imidazolium-based ionic liquids on the performance of ionic polymer conductor network composite actuators, Polymer International, 321(2010) S. Perkin, T.Albrecht and J. Klein, Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces , Phys. Chem. Chem. Phys. 12, 1243 (2010).
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