Influence of Cation Size on the Ionicity, Fluidity, and Physiochemical Properties of 1,2,4-Triazolium Based Ionic Liquids Dharmendra Singh and Ramesh L. Gardas* Indian Institute of Technology Madras, Chennai 600036, India * S Supporting Information ABSTRACT: Interpreting the physiochemical properties and structure-property correlations of ionic liquids (ILs) is a key to the enlargement of their optimized structures for specific applications. In this work, a series of ILs based on 1-alkyl-1,2,4- triazolium cation with trifluoromethanesulfonate anion were synthesized and the effect of cation and temperature on physiochemical properties such as density, viscosity, speed of sound, conductivity, and rheology was studied. Temperature dependence densities were correlated with the densities estimated by the Gardas and Coutinho model, whereas viscosity and molar conductivity have been found to satisfy the Vogel- Tammann-Fulcher (VTF) equation over the studied temperature range 293.15-343.15 K. Further, to explore the wide range of applications, ionicity has been tested by correlating the fluidity with molar conductivity and it was found that synthesized ILs can be referred to as “good ILs”. Furthermore, the fluidity behavior describing the interactions between the cation and anion of ILs was investigated through their rheological properties, and the Newtonian behavior of ILs has been examined by varying the effect of shear rate on viscosity. Finally, the impact of structure variants in terms of the N-1 functionalized 1,2,4-triazole ring has been analyzed over the studied properties. ■ INTRODUCTION Ionic liquids (ILs) are novel solvents of greater interest as an alternative for conventional organic solvents which have aimed at facilitating sustainable chemistry through their tunable properties. 1-3 Among various classes of ILs, the potential utilization and applications of nitrogen-rich ILs have been rapidly increased over conventional energetic materials, mainly due to low carbon and hydrogen content and thus resulting in good oxygen balance. 4,5 Generally, nitrogen-rich ILs exhibit a high heat of formation and high density, with dinitrogen as a decomposition product. Because of these properties, such classes of ILs are considered as highly accomplished energetic materials for industrial and military applications. 6 Shreeve and co-workers 7 explored the application of nitrogen-rich ILs consisting of various families, e.g., imidazolium, picrate, urotropinium, triazolium, tetrazolium, tetrazine, etc., as “energetic salts”. A combination of theoretical and empirical calculations conveyed that 5-aminotetrazolatate based salts have desirable properties to be consigned as energetic materials. 8 The energetic behavior of tetrazolium based ILs was investigated by Dong and co-workers 9 who found that, on increasing the alkyl chain length, density decreases and consequently the energetic behavior of ILs was found to be reduced. Triazolium based ILs have gained increased interest due to their potential use in energy-rich applications. Salts consisting of the triazolium moiety have been found to exhibit a large heat of formation (ΔH f = 109 kJ/mol), compared to similar salts having an imidazolium moiety (ΔH f = 58.5 kJ/mol), and they also possess the combination of unique properties, such as higher densities, enhanced thermal stabilities, and reduced vapor pressure, thus making them more advantageous as compared to conventional energetic materials. 6,10-12 1,2,4- Triazolium salts can be used as energetic materials and efficient CO 2 absorbents with low reaction enthalpy. 13,14 Strassner et al. 15 explored the mesomeric effect to modify the alkylated or arylated 1,2,4-triazolium ILs for desired properties. 1-Alkyl- 1,2,4-triazolium methanesulfonate based Brønsted acidic ILs can exhibit good catalytic activity for the Mannich base reaction with appreciable efficiency. 16 Additionally, -SO 3 H function- alized ILs have been reported offering a new possibility for developing environmentally friendly acidic catalysts because they are flexible, nonvolatile, noncorrosive, and immiscible with many organic solvents and could be used as dual solvents and catalysts. 17 In spite of a wide range of applications, very few reports are available 18,19 on systematic study of structure variants and the effect of temperature on the physiochemical properties of 1,2,4- triazolium based ILs. Study on physiochemical properties is essential (i) to understand the geometrical configurations and intermolecular interactions of these ILs, (ii) to develop the structure-property correlations and predictive models, and (iii) to design processes and products involving these com- pounds. 20-22 By considering these factors, Narita et al. had shown how alkyl chain length can improve the performance of zwitterions as ion conductive matrixes. 23 In the present work, Received: April 11, 2016 Revised: May 5, 2016 Published: May 9, 2016 Article pubs.acs.org/JPCB © 2016 American Chemical Society 4834 DOI: 10.1021/acs.jpcb.6b03669 J. Phys. Chem. B 2016, 120, 4834-4842