Ionic liquid formation with deep eutectic forces at an atypical ratio (2:1) of naproxen to lidocaine in the solid-state, thermal characterization and FTIR investigation

This study aimed to determine if an ionic liquid resulted from the combination of naproxen and lidocaine and the impact that varying the molar percentage of each component had on thermal behavior. Samples were prepared by mixing naproxen and lidocaine in tetrahydrofuran (THF), followed by solvent removal using a vacuum oven. Thermal events were investigated using differential scanning calorimetry (DSC) and the extent of proton transfer was probed using Fourier-Transform Infrared Spectroscopy (FTIR). The binary samples that had a higher molar percentage of naproxen formed a 2:1 naproxen to lidocaine complex, with a melting temperature in the range of 82-85 °C. This complex was determined to be a peritectic composition of naproxen and lidocaine and spectroscopic analysis confirmed the complex had ionic liquid character as well as hydrogen bonding interactions. Binary samples that contained a greater molar percentage of lidocaine formed high-order complexes of varying stoichiometry with melting temperatures below 50 °C. Spectroscopic analysis determined that a greater molar concentration of lidocaine drove more eutectic behavior, although some ionization was also present. The conclusion from this study was that naproxen and lidocaine form an ionic liquid material at a peritectic point corresponding to 2 mol of naproxen to 1 mol of lidocaine. At molar concentrations of greater lidocaine than naproxen, higher order complexes form which are largely driven by eutectic behavior.