Aggregation of carboxylate gemini surfactant with p-Oxybenzene spacer in aqueous solution studied by intrinsic probe spectroscopy

The micellization of C11pPHCNa, a carboxylated gemini surfactant with an p-Oxybenzene residue in the spacer, in aqueous solution at pH 12 was characterized by the absorption and the fluorescence spectra of the p-Oxybenzene residue. The critical micelle concentration of C11pPHCNa was determined by the absorption spectra to be 0.0940 mmol L−1, which agreed well with that measured by surface tension technique. The fluorescent intensity of C11pPHCNa aqueous solution rapidly increased with increasing C11pPHCNa concentration (C) then reduced when C was beyond 0.45 mmol L−1, which meant the self-quenching occurring. At fixed C, the time-resolved and the steady-state fluorescence quenching with the p-Oxybenzene residue as the intrinsic probe and with methyl viologen (MV2+) as the extrinsic quencher could be used to measure the aggregation number (N) of C11pPHCNa micelle. The results showed the N for the micelle formed at C = 1.5 mmol L−1 was 14.5 and gradually increased with further increasing C. This indicated the more and more molecules packed within a micelle with increasing C, which explained well the fluorescence self-quenching when C was beyond 0.45 mmol L−1.