FULL PAPER DOI: 10.1002/ejic.201200874 POSS–Tetraalkylammonium Salts: A New Class of Ionic Liquids [‡] Paola Cardiano, [a] Giuseppe Lazzara, [b] Sundar Manickam, [a] Placido Mineo, [c,d] Stefana Milioto, [b] and Sandra Lo Schiavo* [a] Keywords: Ionic liquids / Self-assembly / Structure–activity relationships / Extraction capability / Silsesquioxanes A series of new polyhedral oligomeric silsesquioxane ionic liquids (POSS-ILs) based on the trimethylpropylammonium hepta(isooctyl)octasilsesquioxane cation and a variety of anions have been synthesized and characterized. Their ther- mal behavior has been ascertained by thermogravimetric and differential thermal analyses and differential scanning calorimetry; room temperature conductivity, dielectric con- stants, and dynamic contact angles have been investigated as well. The obtained data show that the presence of the POSS moiety is responsible not only for their thermal proper- ties, but also for their low room-temperature conductivity and Introduction Ionic liquids (ILs), a modern way to define salts that melt below 100 °C, are very attractive materials due to their unique combination of properties (wide liquid range, excel- lent chemical and thermal stability, good electrochemical stability, high ionic conductivity, dispersant capabilities, tunable dielectric constants, biocompatibility), which can be conveniently tuned by “easy” synthetic means. Due to their ionic nature, the design of ILs with well-defined physico- chemical properties may be reached by an appropriate choice of the ionic constituents. Likewise, the role played by the cation and/or anion on determining their bulky properties should also be considered. [1,2] Thanks to their unique combination of low vapor pres- sure and solubilizing features (combined with their ability to be recycled in liquid–liquid processes), and the possibility of their recovery, ILs are currently classified as “green” sol- vents and have found applications in chemistry and indus- [‡] POSS = polyhedral oligomeric silsesquioxane. [a] Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, University of Messina, Viale Ferdinando Stagno D’Alcontres 31, S. Agata, 98166 Messina, Italy [b] Dipartimento di Chimica “S. Cannizzaro”, University of Palermo, Viale delle Scienze pad 17, 90128 Palermo, Italy [c] Dipartimento di Scienze Chimiche, University of Catania, Viale A. Doria 6, 95125 Catania, Italy [d] Istituto per i Processi Chimico Fisici – CNR, Viale Ferdinando Stagno D’Alcontres 37, S. Agata, 98158 Messina, Italy Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201200874. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2012, 5668–5676 5668 dielectric constants, which, conversely, display slight differ- ences originating from the nature of the anions. The long alkyl-chain substituents on the POSS core infer on this series of ILs an overall hydrophobic character and solution proper- ties comparable to those of classic cationic surfactants, re- sulting in an effective extractant capability of polyanions from aqueous solutions. Their solution diffusion properties, as investigated by room-temperature dynamic light scat- tering, solution conductivity, density, and osmometric mea- surements, do not provide evidence for the occurrence of any significant self-assembling processes. trial chemistry to replace volatile organic solvents in con- ventional and catalytic reactions and separation–purifica- tion processes. [3] They have been involved in a wide variety of engineering applications as absorbents in gas separations, in CO 2 storage, SO 2 and hydrofluorocarbon absorption, and reactive absorption of propylene, to name just a few. As a result of their high ionic conductivity and their wide electrochemical potential window, ILs have been employed as electrolytes for energy-storage devices, such as electric double-layer capacitors, fuel cells, and dye-sensitized solar cells. [4] A significant improvement in IL properties and functions comes from their combination with proper substrates (poly- mer networks, MOx, sol–gel silica-based matrices, and so on). [5–7] For example, we recently found that poly(ionic li- quid)s (pILs), i.e. polymers derived from IL monomers, ex- hibit, at room temp., a significant CO 2 reversible response absent in the parent IL monomer. [8,9] Likewise, ILs de- posited on a porous silicon matrix are useful in the con- struction of highly selective gas sensor arrays. [10] ILs/com- posites and nanocomposites, i.e. ILs incorporated into con- ventional matrices [biopolymers, cellulose, carbon nanot- ubes (CNTs), graphene, etc.], have been exploited in a variety of electrochemical applications (redox and nonredox de- vices, chemo- and electrochemical biosensors, etc.). [11–13] Enhancement of IL functional properties is even achieved by using nanoparticles (NPs), such as silica and other metal oxides, as IL-supporting substrates; these hybrids have been successfully applied in catalysis and, more recently, as solid electrolytes in the development of ion lithium batteries. [14,15]