Curcumin degradation kinetics in micellar solutions: Enhanced stability in the presence of cationic surfactants

In this work, the rate of curcumin degradation in the presence of various surfactants (anionic, cationic, and nonionic) and salts (i.e., NaCl, NaHCO₃, Na₂CO₃, and NaOH) was investigated by photodiode-array UV–vis spectroscopy. The curcumin degradation at neutral pH or in the presence of NaCl during the first 16 h of the study was negligible (i.e., zero rate constant). Depending on the type and concentration of added alkaline salt, the color of the curcumin surfactant solution changed from yellow (λ_max. = 420−430 nm) to orange (λ_max. = 450−500 nm). The degradation rate constants and the final degradation percentage were obtained and reported for various conditions. The maximum curcumin degradation for the investigated surfactants was observed in the presence of 28.5 mM Na₂CO₃ at pH ∼10.8. The curcumin degradation rate was much lower in the presence of cationic surfactants (monomeric and gemini type). The gemini cationic surfactant (12-4-12) was even more efficient than the single chain cationic surfactant (DTAB) to suppress the curcumin degradation indicating the importance of cationic charge density of the surfactant head group on the electrostatic interaction with curcumin. Curcumin is deprotonated in alkaline conditions and carries negative charges; therefore, cationic surfactants can accommodate and protect it much better than anionic or nonionic surfactants against further degradation.