Study of complex formation processes in the system “fluorescent probe– surfactant” in aqueous solutions at surfactant concentration before and above CMC

Abstract

Using optical spectroscopy technique, the interaction of cationic carbocyanine dye 3,3'-diethyloxacarbocyanine bromide (DiOC₂) with anionic surfactant sodium dodecyl sulfate (SDS) has been investigated in aqueous solutions at SDS concentrations before and above the critical micelle concentration (CMC). It has been shown that depending on the surfactant concentration in the solution, different types of associates can be revealed, such as dye-surfactant D⁺S-ion pairs, submicelles that contain dye aggregates, or the normal SDS micelles which solubilize dye monomers. Using the Benassi-Hildebrand equation, the binding constant of the dye to the micelles and the fraction of the dye bound to the micelles have been calculated. The obtained values are relatively small that allows us to conclude that the Coulomb interactions are not sufficient for the effective binding of the positively charged dye to the negatively charged micelle.