Self diffusion and viscoelasticity of aqueous detergent solutions II. Effect of salt concentration (abstract)

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*.