Binding of phenol red to cetylpyridinium chloride at air–solution and micelle–solution interfaces in aqueous ethylene glycol media

Abstract

Knowledge about the fundamental aspects of dye–surfactant interactions is important for developing better formulations required in textile and other coloring industries, and also for improving the efficacy of dye separation processes. In this paper, the interaction between cetylpyridinium chloride (CPC) surfactant and phenol red (PR) dye has been investigated in water and water+ethylene glycol (EG) media using surface tension and spectrophotometric methods. The synergetic effect of PR on the surface tension reducing ability of CPC decreases on adding EG. The modified Corrin–Harkins equation has been derived using the Gibbs–Duhem approach, and by applying this equation CP (cetylpyridinium) micelles are found to be bound predominantly by PR− which is shown to be in accordance with the Collins’ ion-specificity rule. From the absorbance data, using an equation that accounts for the stoichiometry of dye–micelle interaction, the values of binding constant were determined and in water+EG media the electrostatic interactions are found to control the binding of PR− to CP micelles. A spectrophotometric method to determine the counterion binding constant is illustrated. A simple expression that can predict the value of the binding constant of dye–micelle interaction at the cmc is proposed. Dynamic light scattering measurements showed that the amount of dye required to increase the micellar size is dependent on the solvophobicity of the medium. Surface excess values indicate that at the air–solution interface the monolayer consists of both CP+ and PR− when the medium is water, whereas in water+EG the monolayer has CP+ alone and PR− resides in the subsurface.