Intermolecular interactions between a dye and cationic surfactants: Effects of alkyl chain, head group, and counterion

The interactions between an anionic dye and cationic surfactants were investigated using surface tension measurements, spectroscopy, conductometry, and pulsed field gradient NMR (PFG-NMR). Spectroscopic and surface property characterization of the solution as a function of surfactant concentration in the presence of the dye revealed formation of three species: a dye-surfactant ion pair, small mixed aggregates of the dye and surfactant (below the critical micelle concentration (CMC) of these surfactants), and micelles composed of the cationic surfactant. Above the CMC, the dye reverted to its monomeric state or bound to the surface of the micelle. Chemometric resolution analysis confirmed the formation of three species. The hydrodynamic radii of the micelles were determined by self-diffusion coefficient measurements. The average size of the micelles was larger in the presence than in the absence of the dye. Regular solution theory was used to describe the synergistically enhanced ability to form mixed aggregates of dyes and surfactants. (C) 2011 Elsevier B.V. All rights reserved.