Dynamic light scattering studies of rod-like micelles in dilute and semi-dilute regime

The effect of anionic hydrophobic salts, sodium p-toluenesulphonate (Na-PTS) and sodium salicylate (Na-Sal) on the growth behavior of cationic surfactant micelles, cetyltrimethyl ammonium bromide (CTAB) has been investigated in the presence as well as absence of sodium chloride (NaCl) using dynamic light scattering measurements. In the absence of NaCl, a progressive decrease in the diffusion coefficient of CTAB (50 mM) micelles was observed with increasing concentration of hydrophobic salts, Na-PTS and Na-Sal, suggesting an increase in the average dimension of the micelles. The micellar growth is explained in terms of a prolate ellipsoidal transition of the micelles. When the axial ratio of the micelles becomes long enough to drive the system into semi-dilute regime, a slow mode of long relaxation time begins to evolve and shows bimodal distribution of relaxation time. In the presence of NaCl, the micellar growth is enhanced and the appearance of slow mode is evident at lower hydrophobic salt concentration than that is observed in the absence of NaCl. The onset of semi-dilute regime is consistent with the modified Doi–Edwards model of rod-like polymers.