Temperature-Dependent Ordering Phenomena of a Polyiodide System in a Redox-Active Ionic Liquid

• Published in: Journal of physical chemistry. C Vol. 116; no. 14; pp. 7989 - 7992
• Main Authors: Thorsmølle, Verner K, Brauer, Jan C, Zakeeruddin, Shaik M, Grätzel, Michael, Moser, Jacques-E
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 12-04-2012
• Subjects: Chemical thermodynamics; Chemistry; Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; General and physical chemistry; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Physics; Specific phase transitions; Structural transitions in nanoscale materials; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thermal properties of condensed matter; Thermal properties of small particles, nanocrystals, nanotubes; Thermodynamic properties; Solar cells; Methyl iodide; Temperature dependence; Crystalline phase; Order-disorder transformations; Phase diagrams; Nucleation; Iodides; Molten salts; Physical properties; Technological application; Quantity ratio; Nematic state; Phonon mode; Ordering; Methyl compounds; Redox system; Thermodynamic properties
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp300105h

Iodine added to iodide-based ionic liquids leads to dramatic changes of their physical properties which may have implications for technological applications. Here we study the phase diagram of 1-methyl-3-propylimidazolium iodide, the temperature versus iodine (I2) concentration. Above a threshold I2 concentration of 3.9 M, polyiodides are found to be the major determinant of the thermodynamic properties, where nucleation occurs at reduced temperatures leading to a crystalline phase followed by a nematic phase. At the highest concentrations and for increasing temperatures a phonon mode develops which gives indication of mesophases with both improved orientational order of the polyiodide chains and a degree of positional order close to melting. These novel results are important for the fundamental understanding of the physical properties in molten salts and for applications where ionic liquids are used as charge-transporting media such as in batteries and dye-sensitized solar cells.