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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