Shift of acid–base equilibrium of curcumin in its complexes with gemini surfactant hexamethylene-1,6-bis-(dodecyldimethyl ammonium bromide)

• Gemini surfactant 12-6-12 forms three kinds of complexes with curcumin.
• Cur0 is in equilibrium with Cur− in the palisade layer of 12-6-12 aggregates.
• Increase of surfactant concentration right shifts acid–base equilibrium of curcumin.
• Added salt is beneficial for curcumin to left shift acid–base equilibrium.
• Knowledge of acid–base equilibrium is important for curcumin encapsulation.

Surfactant micelles are suitable carriers for solubilizing and stabilizing curcumin that is a natural polyphenolic compound with many biological and pharmacological activities but suffers poor bioavailability. In this paper, the acid–base equilibrium of curcumin (Cur0 = Cur− + H+) has been studied in surfactant concentration dependent complexes of curcumin with gemini surfactant hexamethylene-1,6-bis-(dodecyldimethyl ammonium bromide) (12-6-12). The absorption and fluorescence spectra of curcumin show that Cur− electrostatically binds with cationic 12-6-12 monomer to form curcumin/monomer complexes, whereas Cur0 and Cur− are located in the palisade layer of 12-6-12 aggregates with different positions in curcumin/premicelle and curcumin/micelle complexes. The increase of surfactant concentration often leads to the right-shift of acid–base equilibrium of curcumin and higher amount of anionic Cur−, as shown in the fluorescence spectra, pKa1 and [Cur−]/[Cur0] of curcumin. By contrast, the added salt is found to be beneficial for curcumin to shift the acid–base equilibrium to the left side and keep in neutral Cur0.

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