Effect of the spacer length on the electrostatic interactions of cationic gemini surfactant micelles with trianionic curcumin

The absorption and fluorescence measurements have been used to study the interactions between cationic gemini surfactant micelles of alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) (C12CsC12Br2, where s = 2, 3, 4, 6, and 12, indicating the number of carbons in the spacer) with trianionic curcumin (Cur3−) at pH 13. With increasing spacer length of gemini surfactant, the maximum intensities of absorption and fluorescence peaks of Cur3− give the highest values at s = 6, reflecting that the spacer length of gemini surfactant significantly regulates the electrostatic attractive interactions between C12CsC12Br2 micelles and Cur3−. The maximum fluorescence intensity of Cur3− in C12C6C12Br2 micelle shows increasing tendency at 0–100 mM sodium bromide concentration (CNaBr), but decreases above CNaBr = 120 mM. This result is explained in terms of the enhanced electrostatic attraction of Cur3− with C12C6C12Br2 micelle at low salt concentrations and the reduced electrostatic attraction at high salt amounts. The determined pKa1 values of curcumin in C12CsC12Br2 micelles support the strongest electrostatic association between C12C6C12Br2 micelle and anionic Cur−. The interaction mechanisms of C12CsC12Br2 micelles with Cur3− have been further proposed at s = 2–4, s = 6, and s = 12, which is related to the matching of the positive charges in C12CsC12Br2 micelles to the negative charges in Cur3−.