Thermophysical Properties of the Binary Ionic Liquid Mixture [HMIM][NO3] + [BMIM][BF4] Using Laser Thermal Lens Spectroscopy
摘要
Thermophysical properties of binary ionic liquid mixtures are essential for designing heat transfer fluids, energy storage systems, and electrochemical devices. In this work, the thermal diffusivity of pure 1‑hexyl‑3‑methylimidazolium nitrate ([HMIM][NO₃]) and 1‑butyl‑3‑methylimidazolium tetrafluoroborate ([BMIM][BF₄]) as well as their binary mixtures over the entire composition range has been measured using a laser thermal lens spectroscopic (LTLS) setup. The experimental system was first validated by measuring thermal diffusivity of methanol and 1‑propanol at 298.15 K, and ethylene glycol at 358.15 K; the results agreed with literature data within 3.5 %, 2.9 %, and 2.1 %, respectively, confirming method reliability across the entire investigated temperature range (298.15–358.15 K). Thermal lens characteristic times and thermal diffusivities were then determined for the pure ionic liquids and their mixtures at temperatures from 298.15 K to 358.15 K in steps of 10 K. For the pure components, the obtained thermal diffusivities at 298.15 K are 0.805 × 10−7 m2 s⁻1 for [HMIM][NO₃] and 0.720 × 10−7 m2⋅s⁻1 for [BMIM][BF₄], corresponding to thermal conductivities of 0.162 W⋅m⁻1⋅K⁻1 and 0.157 W⋅m⁻1⋅K⁻1, respectively. For the binary mixtures, thermal diffusivity increases with temperature and decreases with increasing [BMIM][BF₄] mole fraction. The Vogel–Fulcher–Tammann (VFT) model was successfully applied to the entire experimental dataset (42 state points). The optimized parameters yield an average absolute relative deviation (AARD) of only 0.31 %, a root‑mean‑square error (RMSE) of 2.4 × 10−10 m2 s⁻1, and a coefficient of determination R2 = 0.9998, demonstrating excellent predictive capability.