Using the techniques of fluorescence recovery after fringe-pattern photobleaching (FRAP), we measured the self-diffusion coefficient Ds of elongated micelles of potassium oleate and sodium oleate as a function of the concentration of the amphiphiles for various salinity. In the dilute regime of the amphiphile concentration we evaluated the weight-averaged aggregation number ⟨n⟩ from measured Ds and demonstrated that for each salinity ⟨n⟩ increases in proportion to the square root of the amphiphile concentration in agreement with the theoretical growth law. The dependence of the proportionality constant on the salinity was also explained well (at least for high salinity) by a theory based on the Evans-Mitchell-Ninham electrostatic free energy of spherical and rodlike colloidal particles (J. Phys. Chem. 88 (1984) 6344). In the semi-dilute regime we confirmed the power-law dependence of Ds on the amphiphile concentration. The exponent was dependent on the salinity in accordance with the reported behavior of other amphiphiles.
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