Comparative study of the physical and electrochemical behavior of direct N-SO3H functionalized 1, 3-disulfo-2-alkyl-imidazolium trifluoroacetate ionic liquids in molecular solvents

A series of N-SO3H functionalized imidazolium [RSIM][CF3COO] ionic liquids, where R = H-,H- CH3 , C2H5 and n-C4H9 were synthesized and fully characterized with spectroscopic techniques. A detailed study on Bronsted acidity, thermal stability, conductivity, density and electrochemical window of the ionic liquids (ILs) was carried out to analyze the effects of the C-2 alkyl substituent of imidazolium cation. Increasing +I effect of the C-2 substituent decreased the BrOnsted acidity of the ILs. The ILs were found to be thermally stable up to 250-260 degrees C with more loss of physisorbed water below 100 degrees C as the size of C-2 alkyl group increased. The conductivity of these ILs in molecular solvents (MeOH and MeCN) slowly decreased with the increasing temperature (278.15 K to 333.15 K) at a particular mole fraction (X-IL = 0.0255) of the ILs, showing temperature dependent self-aggregation behavior. At a given temperature (298.15 K), the conductivity variation of the ILs in molecular solvents with rising concentration of the IL also evidenced their self-aggregation tendency by displaying a minimum value with its neat condition. The variations of conductivities were also correlated with Kamlet-Taft solvatochromic parameters (E-N(T), alpha, beta) of the solvents. Cyclic Voltammetry study expressed maximum electrochemical window for the acetonitrile solution of bulkier C-2 alkyl groups (R = n-Bu & Et) containing ILs. (C) 2019 Elsevier B.V. All rights reserved.