A robust method for critical micelle concentration determination using coumarin-6 as a fluorescent probe

The critical micelle concentration (CMC) is the point at which micelles are self-assembled from surfactants added to the system. Determination of the CMC is commonly based on surface tension and conductivity measurements since these are relatively simple and well described. The dye micellization method performed using pyrene as a fluorescent probe, is limited like other methods by inaccuracy and the lack of a standard procedure. We established a simple, robust, and straightforward method based on dye micellization, using coumarin-6 as a fluorescent probe for CMC determination. We first optimized the method and demonstrated the problematic pyrene detection by a plate reader. We compared the novel method to the established surface tension method using anionic, cationic and non-ionic surfactants, and compared to reported values in the literature. We selected to measure sodium dodecyl sulfate using the conductivity method, followed by visualization by fluorescence microscopy. The values obtained by our method were reproducible and in accordance with the values reported in the literature, regardless of the ionic charge of the surfactant. Adopting such a robust protocol would be beneficial for pharmaceutical research and industry and has the potential to replace the more laborious and less accurate commonly available methods.