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

Experimental determinations and analyses were conducted on the densities, refractive indices, viscosities, surface tensions, and speeds of sound of [C_neim][BF4] (n = 1, 3, 4) and [C_nmim][BF4] (n = 3 to 10) at atmospheric pressure and temperatures from 293.15 to 343.15 K. Among all the measured properties, viscosity was most significantly affected by temperature, as all properties decreased with increasing temperature. Density, speed of sound, surface tension, and refractive index were determined through linear correlations with temperature, while the relationship between density and viscosity was modeled by using the Vogel–Fulcher–Tammann (VFT) equation. Experimental density data were used to determine coefficients of thermal expansion, and isentropic compressibility was computed using the Laplace–Newton equation. The impact of the alkyl chain length and anion type on thermophysical properties was also investigated. Results indicate that the properties of ionic liquids are predominantly influenced by anion type, while alkyl chain length has a comparatively lesser effect. Increasing the alkyl chain length decreases the density, speed of sound, and surface tension, whereas the viscosity and refractive index exhibit increasing trends. Furthermore, theoretical models for density, surface tension, and speed of sound are evaluated against the experimentally measured data, providing insight into their predictive accuracy and applicability.