Effect of hydrophobicity of cationic carbocyanine dyes DiOCn on their binding to anionic surfactant micelles

The interaction of a series of cationic dialkyloxacarbocyanine perchlorate (DiOCn) dyes of different degrees of hydrophobicity with micelles of an anionic surfactant, sodium dodecylsulfate (SDS), has been studied spectrophotometrically in aqueous solutions. The Benesi–Hildebrand equation was used to calculate binding constants (Kb) of the dyes to surfactant micelles, the fraction of dye bound to the micelles (fmic), and the standard free-energy change (ΔG0) for the transfer of dye from the aqueous to micellar phase. It has been shown that the interaction of oppositely charged dye molecules and surfactant micelles is controlled by both electrostatic and hydrophobic interactions. A small increase in dye hydrophobicity due to lengthening of the hydrocarbon radical has been shown to cause an abrupt nonlinear increase of the fmic value. This points to a key role of hydrophobic interactions in the binding of dye molecules with the micelles.