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 \(mol kg^{ - 1}\) ) and CAF (0.025, 0.050, 0.075 \(mol kg^{ - 1}\) ) 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 \(\left( {B_{J} } \right)\) 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 \(B_{J}\) \(\left( {\frac{{(\partial B_{J} }}{\partial T)}_{P} } \right)\) 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.
