Anomalous Variation of Excited-State Proton Transfer Dynamics inside a Triblock Copolymer-Cationic Surfactant Mixed...

Mixed micelles formed by triblock copolymers [poly(ethylene oxide) (EO)-poly(propylene oxide) (PO)-EO] with various surfactants have widespread applications. Molecular-level understanding of the composition, interfacial organization, and hydration of the copolymer-surfactant mixed micelle is greatly necessary from application perspectives. Here, we applied 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) to probe the mixed micelle of a triblock copolymer F127 (EO₁₀₁-PO₅₆-EO₁₀₁) and a cationic surfactant dodecyltrimethylammonium bromide (DTAB) at various compositions. The emission spectrum of HPTS modulates anomalously with the variation of DTAB concentration, displaying at least four regimes. The ratio of the emission intensities of the two bands (protonated/deprotonated) (1) first increases steeply in the low-concentration range (0.1-6 mM), (2) remains almost steady at the intermediate concentration (8-20 mM), (3) decreases at high concentration (20-80 mM), and (4) finally, remains almost constant at a very high concentration (100-400 mM) of DTAB. Time-resolved measurements confirm that excited-state proton transfer dynamics varies unusually with the concentration of DTAB in the mixed micelle; substantial retardation is observed up to ∼12 mM, but after that, the dynamics becomes somewhat faster upon further addition. The rotational dynamics of a methoxy analogue of HPTS, 8-methoxypyrene-1,3,6-trisulfonate, becomes slower up to ∼12 mM DTAB and after that becomes faster at higher concentration. Moreover, dynamic light scattering measurements showed that the size of the mixed micelle decreases sharply in the low-concentration region (<20 mM DTAB) but decreases moderately at high concentration. Thus, the nature of the mixed micelle is very different at low and high concentrations of DTAB. At low concentration, the incorporation of DTAB results in a more compact and less hydrated mixed micelle, whereas a more hydrated and less organized assembly is formed at high concentration of DTAB.