Volumetric and\nSpectroscopic Analysis of Betaine Hydrochloride–Protic\nIonic Liquid Interaction in Aqueous Solution

The apparent molar volume and isentropic compression\nof betaine\nhydrochloride (B·HCl) in aqueous solutions of protic ionic liquid\n(PIL) were determined using measurements of the solution densities\nand speed of sound at a pressure of 101 kPa and a temperature range\nof (298.15–313.15) K. In order to comprehend solute–solute,\nsolute–solvent, and solute–cosolute interactions, the\npartial molar volumes at infinite dilution (<i>V</i><sub>2</sub>° and <i>K</i><sub>s,2</sub>°) were estimated\nfrom the Masson equation. As the PIL concentration increased from\n0.05 to 0.20 mol·kg<sup>–1</sup>, the transfer volumes\nbecame positive, indicating that B·HCl–PIL hydrophilic\ninteractions have been enhanced. Also, structure-making or -breaking\nbehavior of B·HCl was examined utilizing Hepler’s equation.\nFurthermore, UV–visible spectroscopy was employed to assess\nthe nature of interactions with incrementing PIL concentration. The\nhyperchromic shift observed during the aqueous titration of B·HCl\nwith PIL solutions indicated the presence of hydrogen bonding interactions.\nA strong and attractive interaction between B·HCl and PILs was\nconfirmed by calculated volumetric parameters, which offer comprehensive\nunderstandings into the various interactions occurred in aqueous mixtures.