High-Frequency Dielectric Relaxation of Linear Micelles in Aqueous Solutions of Cetyltrimethylammonium Bromide with Sodium Salicylate

Abstract

It is well-known that ionic surfactants in aqueous solutions with added salts aggregate to form a linear (rodlike or wormlike) micelle with electric charges. 1-3 This kind of linear micelle can be called a quasi-polyion because it has a charge distribution and linear conformation similar to the polyion. It is noted, however, that the linear micelle is different from the polyion in that the number of surfactant molecules (i.e., "monomers") constituting the micelle is changeable with environmental conditions. For example, when salts of usual kinds are added to the ionic surfactant solutions, the aggregation of surfactants proceeds and the contour length L of a linear micelle becomes longer. An exceptional salt is sodium salicylate (NaSal) 4-6 added to aqueous solutions of cetyltrimethylammonium bromide (CTAB): when the ratio of the NaSal concentration C S to the surfactant concentration C CTAB is less than unity, the viscoelastic measurement indicates that the relaxation time gets longer with increasing C S , which implies that the aggregation of surfactant proceeds. When the ratio of C S to the C CTAB is larger than unity, the relaxation time decreases with increasing C S , which is peculiar to NaSal.In a recent study we have investigated the influence of NaSal concentration C S on the dynamics of linear micelles in CTAB solutions by using the frequency-domain electric birefringence relaxation (FEB) spectroscopy. 7 The FEB spectroscopy gives the information on the relaxation time for the micelle rotation as well as the relaxation time of the low-frequency (LF) dielectric relaxation due to the induced dipole moment produced by Salcounterions which are tightly bound to the linear micelle and can fluctuate along the micelle contour. The dependence of the contour length L of linear micelles on the NaSal concentration C S has been obtained from the both relaxation times, which shows that Salions play an decisive role in the surfactant aggregation. We have also found that the diffusion constant of tightly bound Salions along the micelle contour is smaller than the diffusion constant of Salions in a free state. This hindered diffusion of Salions along the micelle contour is consistent with an NMR result 5 which suggests that Salions put their aromatic rings half into the micelle. The fluctuation of Salions along the micelle contour causes the fluctuation of charge density along the contour and produces positively and negatively charged local areas on the micelle, even if the micelle is neutral in the average. This situation can be treated by Oosawa's theory 8 for the polarizability of polyelectrolytes due to the fluctuation of counterions along the polyions. In the present case, the other counterions, Brand Na + ions, are expected to be loosely bound to the local areas with positive and negative charges, respectively, and fluctuate in the direction perpendicular to the micelle contour much faster than the charge fluctuation due to Salions. This faster fluctuation of loosely bound counterions is expected to produce the h...

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