Effects of Cationic Ammonium Gemini Surfactant on Micellization of PEO-PPO-PEO Triblock Copolymers in Aqueous Solution

Effects of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) on the micellization of two triblock copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), F127 (EO97PO69EO97) and P123 (EO20PO70EO20), have been studied in aqueous solution by differential scanning calorimetry (DSC), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and NMR techniques. Compared with traditional single-chain ionic surfactants, 12-6-12 has a stronger ability of lowering the CMT of the copolymers, which should be attributed to the stronger aggregation ability and lower critical micelle concentration of 12-6-12. The critical micelle temperature (CMT) of the two copolymers decreases as the 12-6-12 concentration increases and the ability of 12-6-12 in lowering the CMT of F127 is slightly stronger than that of P123. Moreover, a combination of ITC and DLS has shown that 12-6-12 binds to the copolymers at the temperatures from 16 to 40 degrees C. At the temperatures below the CMT of the copolymers, 12-6-12 micelles bind on single copolymer chains and induce the copolymers to initiate aggregation at very low 12-6-12 concentration. At the temperatures above the CMT of the copolymers, the interaction of 12-6-12 with both monomeric and micellar copolymers leads to the formation of the mixed copolymer/12-6-12 micelles, then the mixed micelles break into smaller mixed micelles, and finally free 12-6-12 micelles form with the increase of the 12-6-12 concentration.