Thermodynamic properties of new ether-functionalized [Co(NCS)4]2− paramagnetic ionic liquids

A series of ether-functionalized paramagnetic ionic liquids, 1-(2-methoxyethyl)-3-alkylimidazolium tetra thiocyanate cobalt [C_n2O1IM]₂[Co(NCS)₄] (n = 1, 2, 3), was synthesized and characterized. The density, surface tension, refractive index, and electrical conductivity of these ionic liquids were measured at 293.15 to 343.15 K at intervals of 5 K, and their thermal expansion coefficients α were calculated. The molecular volume was obtained by measuring the pore volume and porosity. Based on Glasser theory, the standard entropy S₀ (298 K), lattice energy U_POT, surface entropy S_a, and surface enthalpy, H of the ionic liquids were calculated, and the reason why the ionic liquid is in a molten state at room temperature was explained from the perspective of lattice energy. The molar surface Gibbs energy was introduced to improve the traditional Eötvös equation, which was combined with the refractive index to estimate the surface tension, obtaining a fitting index exceeding 0.99. Finally, the relationship between the electrical conductivity of the ionic liquids and temperature was investigated, and the activation energy, molar electrical conductivity, and electrical conductivity diffusion coefficient of the ionic liquids were obtained. The relationship between their properties was summarized. Compared with previously reported ionic liquids using 1-(2-methoxyethyl)-3-methylimidazolium as a cation, this type of ionic liquid has higher density and a smaller coefficient of thermal expansion.