Physicochemical and Acoustic Investigations of Tetrabutylammonium Chloride in Halogenated Solvents: Insights from Excess Thermodynamic Parameters

• First systematic study of binary mixtures of DCM and TCE with tetrabutylammonium chloride (TBAC) using acetone as a constant co-solvent at multiple temperatures and 5 MHz ultrasonic frequency. • Comprehensive evaluation of standard and novel excess parameters, including excess molar sound velocity, excess available volume, and excess coefficient of thermal expansion. • Results reveal dominant structure-breaking effects of bulky TBAC ions, evidenced by positive excess properties and increased free volume in all studied mixtures. • Findings provide deeper insight into ion–solvent interactions and molecular packing efficiency in halogenated solvent systems. • The new excess parameters introduced here offer broader applicability for the characterization and design of advanced solvent systems in chemistry and related fields. This study investigates the molecular interactions in mixtures of tetrabutylammonium chloride (TBAC) with halogenated solvents, specifically dichloromethane (DCM) and trichloroethylene (TCE). To overcome the solubility challenges of the bulky TBAC salt, acetone was employed as a constant-proportion co-solvent, ensuring a controlled environment where the observed thermodynamic variations are uniquely attributed to ion–solvent interactions. Precision measurements of density (ρ) and speed of sound (u) were conducted at 5 MHz across a temperature range of 301.15 K to 313.15 K. Beyond standard acoustic parameters, this research introduces and evaluates three additional excess parameters: excess molar sound velocity (uₘᴱ), excess available volume ( V E A ), and excess coefficient of thermal expansion ( αᴱ ₚ). These parameters, together with other excess properties, indicate a dominant structure-breaking effect of TBAC ions within the halogenated matrices. These findings provide critical insights into the packing efficiency and weakened molecular interactions in ionic-liquid-based systems, offering new diagnostic tools for characterizing complex solvent environments.