Thermo-acoustic investigation on mixtures of Tetramethyl ammonium hydroxide and caffeine in aqueous medium with the goal to understand their mutual interactions

The present study explores, for the first time, the physicochemical and molecular interaction behaviour of tetramethylammonium hydroxide (TMAH)-caffeine (CAF) mixtures in aqueous medium. Caffeine (CAF) is a widely consumed central nervous system stimulant, and tetramethylammonium hydroxide (TMAH) is an industrially significant compound with known toxicological implications. Combination of these two solutes represents a novel system where ionic, hydrophobic, and hydrogen-bonding interactions coexist, providing unique insight into mixed solute behaviour and solvation dynamics in aqueous media. This work focuses on the physicochemical, acoustic and thermodynamic properties of aqueous TMAH solutions in the presence of varying concentrations of CAF over a range of temperatures (293.15-313.15 K). Acoustic measurements, including ultrasonic velocity, and density were utilized to compute key thermodynamic parameters such as compressibility, relaxation strength, specific heat capacity, acoustic impedance, internal pressure, and isobaric expansion coefficient. The results reveal complex ion-solvent and ion-cosolute interactions, highlighting significant structural reorganization within the solution matrix. Observations such as decreasing compressibility and relaxation strength with concentration and temperature suggest stronger intermolecular forces, while variations in partial molar compressibility and transfer parameters indicate hydration shell modification due to CAF's presence. The findings offer valuable insights into hydration dynamics and solvation behaviour, with practical relevance to drug delivery, material design, and toxicity evaluation. This work advances understanding of solute-cosolute interactions, supporting applications in pharmaceutical formulations and chemical process development.