Abstract
The electrical conductivity technique was used to investigate the critical micelle concentration (CMC) of cationic surfactants N-alkyltrimethylammonium chloride (CnTAC; n = 12, 14, 16, and 18). Meanwhile, the interaction between CnTAC and indigo carmine (IC, an anionic dye) was studied using spectrophotometry. Employing the Benesi-Hildebrand and Scott equations, the binding constant (Kb) of IC to the cationic micelles was determined. Calculation of the Gibbs free energy (ΔG) revealed the tendency of IC to bind to micelles. The association between IC and CnTAC clearly depends on the CnTAC concentration: a lower surfactant concentration disrupts the IC-CnTAC complex and decreases IC adsorption, while a higher surfactant concentration has the opposite effect. The alkyl chain length of CnTAC was also found to affect the CMC, Kb, and thermodynamic parameters. A longer alkyl chain reduces the CMC and ΔG and increases Kb, leading to increased solubilization and high spontaneity of the dye. The binding of IC to micelles is ranked as C12TAC < C14TAC < C16TAC < C18TAC.