Abstract
Diffusion of rod‐like micelles of cetyltrimethylammonium bromide (CTAB) in aqueous sodium salicylate (NaSal) solutions were studied by varying CTAB concentration CD and NaSal concentration CS. For solutions with CD=0.01 and 0.1 M, a ratio CS/CD ranged from 1.0 to 41 and from 0.70 to 6.0, respectively. For measurement of the diffusion coefficient D, the forced Rayleigh scattering technique was employed at 25 °C. Plots of D against CS/CD gave very complicated behavior such that D took a maximum and afterward took a minimum with increasing CS/CD. CS/CD values of the maximum and minimum for solutions with CD=0.01 M was very different from that for CD=0.1 M. But the magnitude of D was of the same order of 10−9 cm2 s−1 for both solutions. Diffusion data could be reduced to one master curve by plotting D against the concentration CS* of free salicylate ion Sal− in the low CS* region. The CS* dependence of D at low CS* was found to roughly correspond to that of the relaxation time τ obtained in earlier investigations from a fit of the complex shear modulus data of the same solutions to a Maxwell model with one relaxation time. At high CS*, however, D appeared to increase more rapidly for solutions with CD=0.01 M than those with CD=0.1 M. The diffusion mechanism of rod‐like micelles was interpreted in terms of a theory of Brownian motion of a rod in the semidilute region at low CS* and the breaking and reforming process of network strands at high CS*.
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