Ultrasonic investigation of binary liquid mixture's excess thermodynamic characteristics at various temperatures

The ultrasonic velocity (U) and density (ρ) of pure components and binary liquid mixtures, including xylene and 1,4-dioxane with tetra butyl ammonium chloride, have been determined as functions of composition at different temperatures (T = 301.15 K, 305.15 K, 309.15 K, and 313.15 K) at a fixed frequency of 5 MHz for different concentration ranges. The values of adiabatic compressibility (β_s), intermolecular free length (L_f), acoustic impedance (Z), free volume (V_f), relaxation time (τ), coefficient of thermal expansion (α_P), excess adiabatic compressibility (β^E_s), excess intermolecular free length (L^E_f), excess acoustic impedance (Z^E), excess free volume (V^E_f), excess relaxation time (τ^E) and excess molar volume (V^E_m) were computed using experimental data. The increase or decrease of β_s, L_f, Z, V_f, τ and α_P with composition indicates the presence of interaction between the component molecules in the mixtures. While excess acoustic impedance and excess free volume consistently show negative variation, excess adiabatic compressibility (β^E_s), excess intermolecular free length (L^E_f), and excess relaxation time (τ^E) show mixed deviation. In the xylene + tetrabutylammonium chloride system, the observed trends suggest weak to moderate interactions, predominantly π–cation and dipole–induced dipole forces, which become more pronounced at higher concentrations. In contrast, the 1,4-dioxane + tetrabutylammonium chloride system exhibits comparatively stronger ion–dipole interactions, leading to greater structural rearrangement and compact molecular association across the entire composition range. These results emphasize the value of excess parameters in describing solute–solvent interactions by confirming that the degree and kind of departures from ideality are significantly impacted by the polarity and structural features of the constituents involved.