Fluorescence Behaviors of Fluorescent Probes in Aqueous Solutions of a Nonionic Surfactant

Abstract

The fluorescence properties of the fluorescent probes, 4-methyl-7-anilinocoumarin (MAC) and 4-methyl-7-diethylaminocoumarin (MDC), have been found to be highly sensitive to environment change in the previous study of the solvent effect on their fluorescence spectra. In the present study the fluorescence spectra of the fluorescent probes in aqueous solutions of nonyl-C61-1, -(-CH₂CH₂0-)₁₀H (NP-10) were measured in order to develop a simple method for the determination of CMC of nonionic surfactants and to examine the solubillization mechanism of small molecules into micelle of the surfactant.
From results of the measurements, the CMC of the surfactant was determined accurately from the change of the fluorescence intensity. The solubillization curves of MAC, MDC, and dansyl amino acids (DNS-amino acid) into NP-10 micelle were also obtained from the change of the fluorescence intensity. In addition, the partition coefficients of the fluorescent probes between micellar phase and bulk aqueous phase could be obtained. The partition coefficients increase in the order: DNS-alanine DNS-leucine NDS-phenylalanine MDC MAC.
Each of the fluorescent probes solubilized in different regions of the micelle depending on their hydrophobicities. By comparison of the emission maximum of the probes in the micellewith that in various organic solvents, it was recognized that MAC solubilized in the region of the micelle whose polarity corresponds to 10 in units of dielectric constant, that is, hydrocarbon phase and that MDC solubilized in the region whose polarity corresponds to 28, that is, the region between hydrocarbon and polyoxyethylene phases and that DNS-leucine and DNS- phenylalanine solubilized in the region whose polarity corresponds to 38, that is, polyoxyethylene phase and that DNS-alanine lied at the surface of the micelle.