Comparative Study on the Interaction Between Micelles of Gemini Quaternary Ammonium Surfactants with Different Counterions and Curcumin

Curcumin is a natural polyphenol with diverse pharmacological activities, but its poor aqueous solubility and rapid alkaline degradation severely limit its application. In this study, a series of gemini quaternary ammonium surfactants with different counterions, 12-2-12·2Y (Y = CH3CO3−, CH3COO−, CH3CH(OH)COO−, CH3(CH2)3COO−, C6H5COO−, and Br−, abbreviated as Mc, Ac, Lac, Va, Bz and Br), were used to prepare micellar systems for curcumin solubilization and stabilization. The interaction between the trianionic form of curcumin (Cur3−) and the micelles were investigated. It was demonstrated that Cur3− was preferentially distributed in the palisade layer via electrostatic attraction to the headgroups of 12-2-12·2Y. The type of counterion significantly affected the micropolarity of the palisade layer and the fluorescence quantum yield of Cur3−. Cur3− exhibited the highest quantum yield (1.70 × 10−3) in the 12-2-12·2 Va micelles, which may be due to their highly compact and hydrophobic palisade layer, whereas the lowest yield (1.79 × 10−4) was obtained in the 12-2-12·2Br micelles, possibly due to the strong hydration of Br− and its tight adsorption onto the headgroups. After 24 h at pH 13, Cur3− retention was 54.30–82.29% in the micellar systems, compared with only 4.21% in water. DPPH radical scavenging assays demonstrated that micellar solubilization/stabilization can enhance the antioxidant activity of curcumin. Curcumin exhibited the highest DPPH scavenging activity in the 12-2-12·2Bz system and the lowest in the 12-2-12·2Mc system. These results revealed that counterions may regulate the positioning, fluorescence behavior, chemical stability and antioxidant activity of Cur3− in the palisade layer via multiple mechanisms, including electrostatic attraction/repulsion, the balance between hydrophilic and hydrophobic forces, and steric hindrance. Practical translation will require pH-compatible, food-grade surfactant systems and dedicated toxicological evaluation.