Self-association and thermodynamic behavior of etilefrine hydrochloride in aqueous electrolyte solution

Abstract

The self-association (micellization) behavior of etilefrine HCl, an amphiphilic drug, in aqueous electrolyte solution has been investigated as a function of temperature and sodium chloride (NaCl) concentration by conductivity and 1H NMR measurements. The critical micelle concentration (CMC) was calculated from the inflection in the data obtained from both techniques. The CMC and the degree of ionization (α) values were determined over the temperature range (298.15 to 313.15)K in water and in presence of different concentrations of NaCl. The thermodynamic parameters of micellization for etilefrine HCl i.e. the standard Gibbs free energy change ΔG°m, the standard enthalpy change, ΔH°m, and the standard entropy change, ΔS°m, were evaluated according to the pseudo-phase model. The obtained CMC values, in presence and absence of electrolyte, showed an inverted U-shaped behavior. While the degree of micelle ionization (α) showed a linear response to the increase in temperature in absence of electrolyte, addition of NaCl did not cause a specific response.