Volumetric, Compressibility, Spectroscopic, and Theoretical Evaluation of Interactions between Caffeine and Ethanolammonium Butyrate in Aqueous Solutions

Abstract

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.

PEGylated Deep Eutectic Solvent–Caffeine Interactions in Aqueous Solutions Evaluated by Volumetric and Spectroscopic Studies

“…The supremacy of hydrophobic−hydrophobic or ion−hydrophobic interactions between CAF and DES components has been responsible for negative transfer volumes as interpreted utilizing the cosphere overlap model. 19,20 As can be seen from Figure 2, the transfer values continue to decline with the concentration of DES as well as temperature. This trend indicates that the hydrophobic interactions strengthen with temperature and DES concentration.…”

“…Otherwise, the electrostatic interaction causes an increase in absorbance for the ternary mixture containing CAF. 19,23 The addition of DES causing an observable decline in the absorption of CAF has been shown in the inset of Figure 5b. DES composed of citric acid and PEG may interact more strongly with water and less with the CAF molecules.…”

“…continued band in the range of 240−300 nm. 13,19 This absorption in CAF occurs due to the purine base. 19 After DES was added to aqueous CAF solutions, a hypochromic shift was observed at all studied temperatures.…”

“…13,19 This absorption in CAF occurs due to the purine base. 19 After DES was added to aqueous CAF solutions, a hypochromic shift was observed at all studied temperatures. Increase in temperature caused the absorption maximum to increase as shown in Figures 4 and 5.…”

“…The nature of interaction among components influences the shift in absorbance peaks. Due to electrostatic interactions, a hyperchromic shift was observed for CAF in the presence of ionic liquids, 19 and owing to enhanced CAF self-association in the presence of a cosolute (such as sucrose or cyclodextrin), a hypochromic shift was reported. 26,27 Similarly, in our studies, the supremacy of hydrophobic kinds of interactions may be responsible for the hypochromic shift.…”