The influence of nonionic surfactant structure on the thermodynamics of anionic dye–cationic surfactant interactions in ternary mixtures

Anionic dye–cationic surfactant interaction was measured in a solution of a nonionic surfactant with a concentration greater than the critical micelle concentration using potentiometry. Two azo dyes namely, C.I. Acid Red 88 and C.I. Acid Orange 7, the cationic surfactant dodecyltrimethylammonium bromide and seven nonionic surfactants were studied. The use of ion-selective membrane electrodes for potentiometric titrations and the theoretical model chosen enabled two constants to be established: (i) the association constant, K1, for the first association step in the formation of a complex between the anionic dye and cationic surfactant in the solution of the nonionic surfactant and (ii) the association constant, κ1, for the first association step in the formation of the cationic surfactant–micelle of the nonionic surfactant complex. The thermodynamic parameters ΔG10, ΔH10, and ΔS10 for these two steps were determined. The tendency to form dye–cationic surfactant complexes in a micellar solution of the nonionic surfactant decreases as the hydrophile–lipophile balance of the nonionic surfactant increases. This finding is in contrast to that obtained for the interactions between cationic and nonionic surfactants under the same conditions. An increase in the hydrophile–lipophile balance of the nonionic surfactant also reduced the ΔH10 and ΔS10 values for complex formation; the formation of the dye–cationic surfactant complexes in ternary mixtures is an entropy-driven process, regardless of nonionic surfactant structure.