Kinetics of indomethacin degradation II: Presence of alkali plus surfactant.

The kinetics of indomethacin were studied in the presence of the surfactants ethoxylated lanolin, polysorbate 80, and cetrimonium bromide under alkaline conditions at 30.3 degrees. The degradation followed apparent first-order kinetics. Plots of kobs versus surfactants; concentrations were curved with negative slopes for nonionic surfactants; but with the ionic surfactant, the plots showed a marked positive change in kobs as the surfactant concentration passed through the critical micelle concentration. Literature model systems adequately explained the data for nonionic surfactants but not for the ionic surfactant. A new set of equations was derived for each case using electrochemical potentials. The experimental data for all three surfactants fit the derived equations quite well.