Thermophysical Properties and PC-SAFT Modeling of 1-Alkyl-3-methylimidazolium Hexafluorophosphate Ionic Liquids and Tributyl Phosphate Mixture at (288.15–318.15) K

Ultrasonic-assisted separation with ionic liquid membranes (ILMs) exhibits notable enhancements in processing. The volumetric and compressibility properties of ILMs are critical to their performance under ultrasonic conditions. This study provides the thermophysical behavior of ILMs composed of 1-alkyl-3-methylimidazolium hexafluorophosphate and tributyl phosphate (TBP), focusing on alkyl chain variations (butyl, hexyl, and octyl) across temperatures from (288.15 to 318.15) K with 10 K intervals. Density (ρ) and speed of sound (u) data were measured for the full concentration range, and excess molar volume (VE) and isentropic compressibility deviation (ΔκS) were calculated and fitted to the fifth-order Redlich–Kister equation. Apparent molar properties were also determined and correlated with the Redlich-Rosenfeld-Mayer model. A DFT calculation was conducted using Dmol3 containing structure and energy optimization to obtain COSMO results and the σ-profiles of the ionic liquids and TBP, with cavity volume and area employed for PC-SAFT parameter calculation and density prediction. Results highlight significant alkyl chain effects on intermolecular interactions, which intensify with increased temperature. The butyl chain demonstrates stronger structure-breaking behavior compared with hexyl and octyl chains. The PC-SAFT model effectively predicts thermodynamic properties, validating its use in the process simulation for these systems. These results have a considerable effect on the design and optimization of the ILM in modern separation technology.