Understanding the interactive behaviour of essential nutrients with ubiquitous stimulants is pivotal for predicting their fate and functionality in biological systems, pharmaceutical formulations and aqueous chemical environments. In this context, the study examines the molecular interactions landscape of l-ascorbic acid (LAA) in the presence of Caffeine (CAF) in aqueous medium, emplyoying an integrated volumetric-spectroscopic framework over the temperature range 293.15–313.15 K. Density derived thermodynamical parameters, including limiting partial molar volume ( \(\:{V}_{\phi\:}^{0}\) ), partial molar expansibility ( \(\:{E}_{\phi\:}^{0}\) ), Helper’s constants \(\:{{(\partial\:E}_{\phi\:}^{0}/\partial\:T)}_{\text{P}}\) , transfer volumes and ion-pair volumetric interaction coefficients, were systematically evaluated to elucidate the nature, strength and temperature dependence of solute-solute and solute-solvent interactions. The consistently positive \(\:{V}_{\phi\:}^{0}\) and \(\:{E}_{\phi\:}^{0}\) values, together with Hepler’s criterion, enhanced solvent organization through strong hydrogen bonding and hydrophilic-hydrophobic interactions. Negative transfer volume and ion-pair volumetric interaction coefficient, however, indicate restricted binary association but dominate triplet interactions at higher concentrations, emphasizing cooperative solvation effects. These thermodynamic interpretative results are strongly reinforced by FTIR spectroscopic evidence, wherein characteristic shifts in \(\:-\text{O}-\text{H}\) and \(\:>\text{C}=\text{O}\) vibrational bands confirm specific hydrogen-bond-mediated interactions of both solutes with the aqueous matrix. Collectively, this study delivers a unified thermodynamic and spectroscopic narrative of nutrient-stimulant coexistence, offering molecular implications relevant to the rational design of functional beverages, nutraceutical formulations and predictive models of nutrient–drug interactions in complex aqueous systems.
