Differential scanning calorimetric study on the Krafft phenomenon of local anesthetics

The Krafft phenomenon of six local anesthetics--tetracaine hydrochloride (TC . HCl) and hydrobromide (TC . HBr); dibucaine hydrochloride (DC . HCl), hydrobromide (DC . HBr), and hydroiodide (DC . HI); and bupivacaine hydrochloride (BC . HCl)--was investigated by differential scanning calorimetry (DSC) of the aqueous anesthetic solutions. The aqueous solutions of TC . HCl, BC . HCl, TC . HBr, and DC . HI showed the Krafft phenomenon, while those of DC . HCl and DC . HBr did not. The enthalpy change of phase transition from the coagel phase (or the so-called hydrated crystal) to the micellar solution (Delta H-t) for the anesthetics was determined directly from the DSC thermograms of local anesthetic solutions at different concentrations, The Krafft temperature and the critical micelle concentration (CMC) for these anesthetics were determined from the solubility vs temperature curves. The Krafft temperatures of these anesthetics were greatly dependent on the chemical structures of hydrophobic groups in the molecules and on the negative halide counterions. It was also found that the local anesthetics have larger CMC values and smaller Delta H-t ones than straight-chain surfactants such as alkyltrimethyammonium bromide. This is attributable to the weak hydrophobicity of local anesthetic molecules compared with the surfactant molecules. The difference in the solution behavior between TC . HCl and BC . HCl in water was considered using the enthalpy change of the transition among the three states of anesthetic assembly: coagel phase,single dispersion,and micellar solution. (C) 1996 Academic Press, Inc.