Quinolone Derivative Physicochemical Studies: Phase Equilibria in Water and Alcoholic Solutions, Ionization Constants, and Calorimetric Analyses

The studies were conducted on commonly used or withdrawn from the market active ingredients of drugs used in the treatment of many bacterial infections. In this research, solid–liquid phase equilibria were determined in binary systems. The substances tested were the following quinolone derivatives: cinoxacin, ciprofloxacin, flumequine, lomefloxacin hydrochloride, norfloxacin, and ofloxacin. Water, ethanol, and 1-octanol were used as solvents. Phase equilibria were determined by using dynamic and spectrophotometric methods. The results were correlated using the Wilson, NRTL, and UNIQUAC equations. Calorimetric analyses were performed by using differential scanning calorimetry to determine the phase transitions of pure substances. Ionization constants were determined spectrophotometrically at 298.2 and 310.2 K using the Bates–Schwarzenbach method. Quinolone derivatives exhibited high melting points above 490 K. The substances showed the lowest solubility in water but were higher by an order of mole fraction in alcohols, in which solubility was comparable. These quantities were below 1000th of mole fraction. Ionization constants corresponding to the deprotonation of the quinolone carboxyl group and also their zwitterionic form were determined.