Comparison of interaction patterns of a triblock copolymer micelle with zwitterionic vs. cationic surfactant: An excited-state proton transfer dynamics investigation

Abstract

This investigation explored the interaction between a triblock copolymer F127 (poly-(ethylene oxide)101 (PEO101)–poly(propylene oxide)56 (PPO56)–PEO101)) micelle and a zwitterionic sulfobetaine surfactant N-dodecyl-N, N-dimethyl-3-ammoniopropane sulfonate (SB3–12 or SB12), primarily utilizing the excited-state proton transfer (ESPT) dynamics of a photoacid, 8-hydroxy pyrene-1,3,6-trisulfonate (HPTS). We compared the obtained interaction pattern to that of the cationic surfactant, dodecyl trimethylammonium bromide (DTAB or C12TAB), with the same alkyl chains. The two surfactants, irrespective of charge differences, grossly follow the same pattern as manifested by the change in the intensity ratio of the protonated and deprotonated emission bands. For both cases, the ratio first increases above a specific surfactant concentration, attaining a maximum at a particular concentration. The ratio decreases at a high concentration and finally levels off at a very high concentration. Interestingly, the ratio attains maximum for a post-micellar (30–35 mM) concentration (~ 10 times higher than critical micellar concentration or CMC) for SB12 whereas, the maximum occurs at a sub-micellar (6–8 mM) concentration (significantly lower than its CMC) for DTAB. Detailed analysis of the time-dependent emission intensity ratio reveals that ESPT dynamics is slower in the optimum F127-SB12 assembly than in the case of the cationic surfactant. The fluorescence anisotropy decay of 8-methoxypyrene-1,3,6-trisulfonate (MPTS), a methoxy analog of HPTS, also corroborates well with the anomalous pattern; the rotational dynamics become more retarded inside the F127-SB12 assembly than in the case of the cationic surfactant. Thus, although SB12 requires a higher surfactant concentration to acquire the optimum state, it results in a more organized and less hydrated state than the cationic surfactant.