Effect of the nature of counterion on the micellar properties of cationic surfactants: a conductometric study.

Interaction of cationic surfactants, tetradecyltrimethylammium bromide (TTAB) and tetradecyltrimethylammonium chloride (TTAC) with anionic dye, tartrazine in aqueous solution has been investigated employing conductometric technique. The free energy of transfer of hydrophobic chain, ΔGHPo, surface contribution, ΔGSo and their transfer values, ΔGHP;tro and ΔGS;tro, free energy, ΔGmo, enthalpy, ΔHmo, and entropy, ΔSmo of micellisation have been calculated. A marked decrease in the critical micelle concentration for TTAB compared to TTAC indicates dramatic role of the smaller counterion Br- over the bigger counterion Cl-. Formation of dye-surfactant ion-pairs which behave as nonionic surfactant has been proposed. Higher negative ΔGHPo and positive ΔGSo values for TTAB than TTAC are attributed to difference in the size and nature of the counterions Br- and Cl-. Negative ΔGmo and ΔHmo values clearly suggest that micellisation of TTAB and TTAC is thermodynamically spontaneous and exothermic in nature. As (-TΔSmo) » ΔHmo), it suggests that the process of micellisation is entropy dominated for the two surfactants. [ABSTRACT FROM AUTHOR]