The densities (ρ), viscosities (η), and 1 H nuclear magnetic resonance (NMR) studies for caffeine, theophylline, and theobromine in water and in aqueous solutions of 0.10, 0.25, 0.50, 0.75, and 1.00 mol•kg −1 sodium chloride over a temperature range T = 288.15−318.15 K and at p = 101.325 kPa have been carried out using vibrating-tube digital densimeter, micro-Ubbelohde type capillary viscometer, and Bruker (AVANCE-III, HD 500 MHz) NMR spectrometer, respectively. From the density and viscosity data, apparent molar volume (V 2,ϕ ), partial molar volume at infinite dilution (V 2,ϕ 0 ), viscosity B-coefficient, corresponding transfer (Δ tr V 2,ϕ 0 and Δ tr B) and other related parameters have been calculated. The trends in transfer parameters reveal the dominance of hydrophilic−ionic interactions at lower molalities of NaCl while hydrophobic−ionic interactions at higher molalities of NaCl. The expansibilities and dB/dT data show the structurebreaking behavior of theophylline and theobromine in water and in aqueous solutions of NaCl. However, behavior of caffeine is exceptional. The increase in chemical shift (δ) values with increasing molalities of NaCl also signifies the predominance of solute−cosolute interactions over the dehydration process. The results have further been discussed and rationalized in terms of various interactions.

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The density, ρ, and the speed of sound, u, of γ-aminobutyric acid (GABA) in water and in (0.005, 0.01, 0.02, 0.04, and 0.06) mol·kg–1 aqueous caffeine (CAF) solutions were measured at temperatures, T = (293.15–318.15) K and P = 0.1 MPa. The measured data have been used to calculate apparent molar volumes at infinitesimal concentration (V 2,ϕ o), apparent molar isentropic compressibility (K 2,ϕ o), the corresponding transfer parameters, and other derived properties. The negative transfer values suggest the dominance of hydrophobic interactions and the structure-breaking tendency of CAF molecules. UV–visible spectroscopic studies have been carried out, and the shifts in the absorption spectra signify the role of hydrogen-bonding interactions. The structures of GABA and CAF have also been optimized in gas phase and solution phase by employing density functional theory at B3LYP/6-31+G* theoretical level, and the H-bond interactions between the two molecules have been studied. It is observed that there are strong H-bond interactions between GABA and CAF.

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