Influence of Temperature, Anions, and Alkyl Chain Length on the Density, Viscosity, Speed of Sound, Surface Tension, and Refractive Index of Imidazolium Hexafluorophosphate-Based Ionic Liquids

The densities, refractive indices, viscosities, surface tensions, and speed of sound of [Cneim][PF6] (n = 3, 4) and [Cnmim][PF6] (n = 4, 5, 6, 7, 8, 9) were experimentally determined and analyzed at temperatures ranging from 293.15 to 343.15 K, under atmospheric pressure conditions. All of the properties decrease as the temperature increases, as expected, with viscosity being the most influenced by the temperature change. The density, speed of sound, surface tension, and refractive index are estimated by using linear correlation as a function of temperature, whereas viscosity is correlated by using the well-known Vogel–Fulcher–Tammann (VFT) equation. The corresponding coefficients of thermal expansion were determined by using the experimental density data. Moreover, the Laplace–Newton equation was used to calculate the isentropic compressibility. Furthermore, the influence of anion type and alkyl chains on the thermophysical properties of the studied ionic liquids is studied. Based on the findings, the physical properties of the investigated ionic liquids are greatly influenced by the nature of the anion, while the alkyl chain has less significance. As the alkyl chain length increases, the density, speed of sound, and surface tension all decrease. Viscosity and refractive index, on the other hand, exhibit diametrically opposed behavior. Furthermore, a comparison between theoretical models for density, surface tension, speed of sound, and experimental values obtained from this work is discussed