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

Diffusion of rod‐like micelles of cetyltrimethylammonium bromide (CTAB) in aqueous sodium salicylate (NaSal) solutions were studied by varying CTAB concentration C_D and NaSal concentration C_S. For solutions with C_D=0.01 and 0.1 M, a ratio C_S/C_D 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 C_S/C_D gave very complicated behavior such that D took a maximum and afterward took a minimum with increasing C_S/C_D. C_S/C_D values of the maximum and minimum for solutions with C_D=0.01 M was very different from that for C_D=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 C_S^* of free salicylate ion Sal⁻ in the low C_S^* region. The C_S^* dependence of D at low C_S^* 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 C_S^*, however, D appeared to increase more rapidly for solutions with C_D=0.01 M than those with C_D=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 C_S^* and the breaking and reforming process of network strands at high C_S^*.