Comprehensive Physicochemical Characterization of Tetramethylammonium Hydroxide and Caffeine in an Aqueous Medium Using Viscometric, HPLC and UV Spectroscopic Techniques

This work investigates the systematic physicochemical interactions between tetramethylammonium hydroxide (TMAH) and caffeine (CAF) in aqueous solution, a system that has not been studied in detail and is scarce in the literature. TMAH is an ionic quaternary ammonium compound consisting of (CH3)4+ and OH– ions, whereas CAF is a neutral bioactive xanthine alkaloid. The combination of these two compounds in aqueous solution creates a system in which ion–neutral interactions are very likely to occur. The present study combines viscometry, UV–visible spectroscopy, and HPLC techniques to provide a clear understanding of the molecular interactions occurring in this system, which helps to interpret solute–solute and solute–solvent behavior. The values obtained from the viscosity measurements of binary (TMAH + water) and ternary (TMAH + CAF + water) systems were used to determine Jones–Dole coefficients (AF and BJ), solvation numbers (Sn), and activation parameters for viscous flow. The UV–visible spectra show a gradual decrease in CAF absorbance with the increase in concentration of TMAH solutions at a wavelength of 273 nm. This change arises due to changes in the surrounding solvent environment, such as changes in hydration or hydrogen bonding, rather than from any direct chemical interaction like complex formation. Chromatograms obtained from HPLC analysis indicate consistent retention times for CAF with no detectable signs of degradation, confirming that CAF remains chemically stable under strong alkaline conditions. Temperature-dependent measurements over the range of 293.15–313.15 K reveal how the thermal effects influence interaction strength and solution structure. The findings of this research enable deeper insight into the nature of ion–dipole interactions, hydrogen bonding, and hydrophobic interactions in such systems. The generated data contribute toward the better understanding of quaternary ammonium bases and neutral bioactive molecules, with potential implications for analytical chemistry, pharmaceutical formulation, and solution thermodynamics.