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In this work, interactions occurring among solute, solvent, and cosolute in aqueous solutions containing caffeine (CAF) as the solute and protic ionic liquid (ethanolammonium butyrate; [EABu]) as the cosolute was studied experimentally (viz. volumetric properties and spectroscopic analysis) and theoretically (using density functional theory). Density, ρ, and speed of sound, u, of CAF in mixed aqueous solutions of protic ionic liquid (PIL) with concentration (0.05, 0.10, 0.15, and 0.20) mol·kg–1 were measured at temperatures (298.15–313.15 K) and at 0.1 MPa. The data was used to calculate apparent molar volume, apparent molar isentropic compressibility, and transfer volumes to understand the type of interactions present between CAF and PIL in water. Further, the interactions were studied by density functional theory and spectroscopic analysis (UV–visible and 1H NMR). Hyperchromic shift in the UV–visible absorption spectra of ternary solutions signifies the presence of hydrogen bonding interactions. Such hydrophilic types of interactions were also observed from chemical shift (Δδ) in 1H NMR spectra of CAF in the ternary system. The structure of CAF and ionic liquid were optimized in the gas and solvent phase by employing density functional theory, indicating hydrogen bond interactions between these molecules. For drug delivery, physicochemical studies of such PIL-CAF systems are pivotal in understanding the kind of interactions possible in liquid mixtures.

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