Temperature and Alkyl Chain Dependence of Physicochemical Properties of Pyrrolidinium‑ and Imidazolium-Based Ionic Liquids

At temperatures ranging from 293.15 to 343.15 K and atmospheric pressure, the densities, refractive indices, dynamic viscosities, surface tensions, and sound speeds for [Cnmpyr][[NTf₂] (n = 3,4,6,8), [Cnmpyr][[N(CN)₂] (n = 3,4), [Cnmim][[N(CN)₂] (n = 2,4) and [Bnzmim][[N(CN)₂] were measured and discussed. As expected, all properties decrease as the temperature rises, with dynamic viscosity being the property most affected by the temperature change. Using linear correlation as a function of temperature, the density, speed of sound, surface tension, and refractive index are calculated, whereas the dynamic viscosity is correlated using the well-known Vogel–Fulcher–Tamman (VFT) equation. Using the experimental density data, the corresponding coefficients of thermal expansion were calculated. Furthermore, the Laplace–Newton equation was utilized to determine the isentropic compressibility. Results unveiled that the type of anions has the higher influence on the physical properties of the studied ionic liquids, while the elongation of the alkyl chain is less significant.