Synthesis and thermophysical properties of imidazolate-based ionic liquids: Influences of different cations and anions

Six novel imidazolate-based room-temperature ionic liquids (ILs), 1-butyl-3-methylimidazolium imidazolate ([Bmim][Im]), 1-ethyl-3-methylimidazolium imidazolate ([Emim][Im]), 1-hydroxylethyl-3-methylimidazolium imidazolate ([HO-emim][Im]), 1-aminopropyl-3-methylimidazolium imidazolate ([NH2-pmim][Im]), 1,4-Bis(3-methylimidazolium-1-yl)butane imidazolate ([Bis(mim)C4][Im]2) and 1,2-Bis(3-methylimidazolium-1-yl)ethane imidazolate ([Bis(mim)C2][Im]2), were prepared with different kinds of cations, including conventional monocation, functionalized cation and dication. Their main physicochemical properties were measured, consisting of glass transition temperature, density, conductivity and viscosity. The influences of the cationic structure on each property were highly discussed. The results showed that the glass transition temperature increased with the decreasing alkyl chains length of cation and dication, whereas the density exhibited an opposite behavior. The viscosity of dicationic and functionalized ILs increased compared to conventional monocationic ILs. However, the conductivity was found to decrease after using dication and functionalized cation. In addition, the density and conductivity of imidazolate ILs were determined as the functions of temperature from T = (293.15 to 343.15) K. Vogel-Tammann-Fulcher and Arrhenius equations were utilized to study the temperature-dependence of conductivity. Moreover, the thermal expansion coefficients were calculated from the (density + temperature) dependence, the relationship between viscosity and conductivity was discussed using Walden plots.