The thermophysical properties of choline salicylate ([Ch][Sal]), an active pharmaceutical ingredient ionic liquid (API-IL), in (0.1, 0.2, and 0.3) mol‧kg−1 aqueous amino acids (L-alanine, L-proline, L-arginine) solutions have been measured at 298.15 K. Density, speed of sound, viscosity, refractive index, and electrical conductivity of [Ch][Sal] in these solutions at varying concentrations were measured. The standard partial molar volumes, \(\mathop V\nolimits_{\varphi }^{0}\) , partial molar isentropic compressibility’s, \(\kappa_{\varphi }^{0}\) , viscosity B-coefficients, molar refractions, \(\mathop R\nolimits_{D}\) , ion association constants, KA, and limiting molar conductivities, Λ0, were derived. The values of \(\mathop V\nolimits_{\varphi }^{0}\) for [Ch][Sal] increased from 196.76 cm3‧mol⁻1 to 200.56 cm3‧mol⁻1 in L-alanine solutions but decreased to 192.03 cm3‧mol⁻1 in L-arginine solutions, indicating stronger ion-polar interactions with L-alanine. Similarly, \(\kappa_{\varphi }^{0}\) , shifted from negative in water to positive values for L-alanine and remained positive for L-proline and L-arginine, suggesting hydrophobic effects in longer alkyl chains. The viscosity B-coefficients showed an increasing trend in L-arginine solutions, indicating stronger solute–solvent interactions. To further investigating the interactions in the studied systems, COSMO-based analysis was employed. Results indicate the presence of strong ion-polar interactions between [Ch][Sal] and L-alanine. These results provide crucial insights into the physicochemical behavior of API-ILs in biologically relevant systems.
