Viscometric and UV–Visible spectroscopic characterization of L-Ascorbic Acid and Caffeine mixtures: insights into solute–solute and solute–solvent interactions

This study equips significant molecular intuition into the compatibility of L-Ascorbic Acid (LAA, solute) and caffeine (CAF, co-solute), two bio-actives widely co-formulated in nutraceutical and pharmaceutical products. Varying concentrations of LAA (0.02-0.14 [Formula: see text]) and CAF (0.025, 0.050, 0.075 [Formula: see text]) in aqueous medium are used in this present study. A cohesive approach combining viscometric measurements, thermodynamic analysis and UV-Visible spectroscopic analysis was employed to elucidate their intermolecular interactions in aqueous medium at varying working temperatures (293.15 K-313.15 K) and at atmospheric pressure. Dynamic viscosity data were scrutinized through the Jone-Dole equation to assess solute-solute and solute-solvent interactions, while transfer parameters, solvation numbers and thermodynamic functions were employed to explain hydration behaviour and viscous flow. Positive Jones-Dole coefficient [Formula: see text] and solvation number above 2.5 confirm predominant ion-solvent interactions and enhanced hydration structuring. Uprising trend of viscosity with solution concentration and downward slope of temperature derivative of [Formula: see text] [Formula: see text] indicate strengthened molecular associations across all systems. Thermodynamic parameters reveal higher activation free energies and contrasting enthalpies signatures for LAA-CAF systems, reflecting different molecular reorganization patterns. UV-Visible spectra exhibit hypsochromic, hyperchromic, hypochromic and bathochromic shifts upon mixing validating hydrogen bonding and dipole-dipole interactions. Collectively, these findings significantly deepen the molecular-level understanding of LAA-CAF compatibility and support the rational design of more stable, synergistic multi-component formulations.