Determination of the critical micelle concentration of surfactants and amphiphilic block copolymers using coumarin 153

Abstract

We describe a method to determine the critical micelle concentration (CMC) of low molecular weight surfactants and amphiphilic diblock copolymers in water based on the use of the fluorescent dye coumarin 153 (C153). The method is based on the measurement of the fluorescence intensity, solvatochromic shift and fluorescence anisotropy of C153 and was tested with the low molecular weight surfactants SDS, CTAB and Triton X-405 in water, for which we obtained, within the experimental error, CMC values identical to those previously published. The method was further used to determine the CMC of a family of poly(N-decylacrylamide)-b-poly(N,N-diethylacrylamide) amphiphilic block copolymers synthesized by RAFT polymerization, with an equal poly(N-decylacrylamide) hydrophobic block length and increasing poly(N,N-diethylacrylamide) hydrophilic block lengths. By measuring the fluorescence anisotropy of C153 in block copolymer aqueous solutions, it is also possible to detect the formation of pre-micellar aggregates at concentrations below the CMC. The dye C153 is more appropriate than pyrene to study self-assembly in water, where pyrene forms ground and excited state aggregates at very low concentrations. Furthermore, C153 is an excellent anisotropy probe with a very high limiting anisotropy (0.375) that can be used to determine the CMC, detect the presence of pre-micellar aggregates, and evaluate the fluidity of the hydrophobic core of block copolymer micelles.