Unusual location of the pyrene probe solubilized in the micellar solutions of tetraalkylammonium perfluorooctanoates

Abstract

The aqueous solutions of tetraalkylammonium perfluorooctanoates (C₇F₁₅COON(C_nH_2n+1)₄, n = 1, 2, 3, 4, abbr. TMAPFO, TEAPFO, TPAPFO, TBAPFO) were studied by steady-state and time-resolved fluorescence with pyrene as a probe, and the ammonium perfluorooctanoate (APFO) was also studied as a comparison. Unusual response of pyrene fluorescence was observed. The intensity ratio of the first and third vibronic peaks of pyrene (I₁/I₃) greatly varied with the counterions for these fluorinated surfactants after their critical micelle concentration (cmc): for TEAPFO, the I₁/I₃ abnormally increased to a value higher than that of water, while for TBAPFO the I₁/I₃ value exhibited a small extent of decease after cmc. However, no significant change of I₁/I₃ was observed during the micellization of TMAPFO and TPAPFO. The lifetime of pyrene fluorescence (τ₀) among the systems of tetraalkylammonium perfluorooctanoates (TAAPFOs) was observed to be increased with the hydrophobicity of counterions. Moreover, the time-resolved fluorescence decay curves of pyrene for all micellar solutions exhibited single exponential decay, suggesting a single environment for pyrene, i.e. each surfactant had a single location of pyrene solubilized in its micelle. The performances of the pyrene probe in concentrated solutions of neat ammonium/tetraalkylammonium chlorides (NH₄Cl/TAACls) were also investigated and an unusually high I₁/I₃ could be found for tetraethylammonium chloride. It reached a conclusion that pyrene located in the counterion layer of the TAAPFO micelles, which could be ascribed to the hydrophobic interaction and cation–π interaction between pyrene and tetraalkylammonium ions (TAA⁺) and the immiscibility between pyrene and the fluorocarbon core.