Modulation of Micellization Behavior of Cetyltrimethylammonium Bromide (CTAB) by Organic Anions in Low Concentration Regime

Abstract

Micellization of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in dilute aqueous solutions of sodium butyrate (Na-Bu), sodium benzoate (Na-Ben) and sodium salicylate (Na-Sal) at different concentrations, viz. 0.005, 0.01, and 0.05 mol·L−1, has been investigated. Conductivity, steady-state fluorescence, dynamic light scattering (DLS), atomic force microscopy (AFM), and 1H NMR techniques shed light on different aspects of micellization. Various micellization parameters such as critical micelle concentration (cmc), degree of ionization of micelle (α), and standard Gibbs energy of micellization ( $$ \Delta G_{\text{m}}^{\text{o}} $$ ) have been obtained from conductivity measurements. Steady-state fluorescence has provided useful information about the cmc, polarity of cybotactic region and aggregation number of micelles of CTAB using pyrene as fluorescent probe in various aqueous salt solutions. The data have been analyzed in terms of hydrophobicity/hydrophilicity and polarizability of the anions of the added electrolytes. The content and nature of organic anions governs the shape and size of the micelles as revealed by DLS and AFM measurements. In the dilute concentration regime of added electrolytes, contrary to most of the reported studies pertaining to very high concentration of added electrolyte, no worm like micelles forming a gel like structure have been observed. 1H NMR experiments provided information about the sites of interaction of anions of added electrolytes with the surfactant head group.