Experimental ultrasonic investigation of speed of sound and molecular interactions in [Bmim][PF₆] + tetrahydrofuran binary mixtures.

An experimental ultrasonic investigation of the speed of sound in binary mixtures of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) and tetrahydrofuran (THF) has been carried out over the entire mole fraction range at temperatures from 298.15 to 328.15 K. Ultrasonic velocity measurements were performed using a multifrequency ultrasonic interferometer operating at 2 MHz under strictly controlled thermal conditions. The experimental data were employed to evaluate adiabatic compressibility and to analyze the nature of molecular interactions in the system. A pronounced non-linear increase in speed of sound with increasing ionic liquid concentration was observed, particularly at intermediate compositions, indicating strong ion-dipole interactions and enhanced molecular packing. Increasing temperature resulted in a systematic decrease in ultrasonic velocity due to thermal expansion and weakening of intermolecular forces. The results demonstrate that molecular interactions governed by polarity contrast and structural compatibility dominate the acoustic behavior of the [Bmim][PF₆] + THF system. The present work provides reliable experimental acoustic data and insight into ionic liquid-solvent interactions relevant to thermodynamic and industrial applications.