Scientists from Novosibirsk State University, together with colleagues from the University of Rostock Germany, studied the molecular structure and molecular mobility of a new type of ionic liquid. This liquid adapts to environmental conditions due to changes in properties that result from changes in the structure. One example of the significance of this work is in the production of new generation batteries. The results of the study were published in The Journal of Physical Chemistry Letters.
Daniil Kolokolov, Head of the Research Team, Senior Researcher at the NSU Department of Physics Laboratory of the Structure and Functional Properties of Molecular Systems, and Ph.D. in Physics and Mathematics, reported that the team discovered unusual local clusters of singly charged cations linked by hydrogen bonds. The appearance of these clusters completely determines parameters for ionic liquids such as the type of phase transition and viscosity. It turned out that in the presence of cationic clusters, liquids do not crystallize upon cooling, but vitrify. At the same time, in the liquid state the local molecular mobility that is responsible for the mechanism of liquid viscosity also changes.
We have shown that when such cation-cation clusters are formed, the microscopic viscosity is determined by the presence of these cations. The temperature dependence of viscosity for ordinary ionic liquids is determined by the size of the ion pairs. In our research, an additional regime was obtained when, immediately after melting, the presence of singly charged cation clusters, larger formations than ion pairs, determined the viscosity of the substance. We demonstrated that with a certain structure and size of cations, the dispersion interaction stabilizes hydrogen bonds between singly charged cations. This contradicts the standard logic that "like charges repel". These clusters interact more strongly with the remaining molecules of the ionic liquid, which increases the microscopic viscosity since it determines the local dynamics of all liquid components. With further heating, these hydrogen bonds break up, and only ordinary cation-anion pairs remain as the viscosity drops.
The researchers consider their discoveries a breakthrough because earlier the very existence of non-trivial singly charged cation clusters was considered impossible, at least for ordinary inorganic salts. It turned out that in ionic liquids this qualitatively affects their phase behavior, structure, and ionic transport. These hypotheses existed earlier, but no attempts were made to synthesize them. A group of scientists have proven that it is possible to synthesize a whole class of similar substances. The results were considered so significant that they were presented by The Journal of Physical Chemistry Letters as their cover story.
In addition to Kolokolov, Candidate of Physical and Mathematical Sciences Alexander Khudozhitkov, Candidate of Chemical Sciences Sergey Arzumanov, and foreign scientists took part in the research. Under the leadership of Ralph Ludwig, Professor of Physical and Theoretical Chemistry at the University of Rostock (Germany), a team synthesized material for research, tested its purity, and conducted theoretical analysis using theoretical chemistry methods. The task of the Russian team was to study in detail the samples obtained by experimental methods of solid-state nuclear magnetic resonance spectroscopy.
Daniil Kolokolov noted that his Laboratory’s cooperation with their foreign colleagues will continue as they have agreements and common scientific interests. For example, the scientist and his team intend to study other types of phenomena in ionic liquids including changes in molecular mobility depending on the structure and types of hydrogen bonds and the process of melting and freezing at non room temperatures.