Tracer diffusion of ionic micelles: effects of size and interactions

Abstract

Using the Taylor dispersion technique, diffusion coefficients for pyrene solubilized in micelles of decyl-, dodecyl-, tetradecyl- and hexadecyl-trimethylammonium bromides (C₁₀TAB, C₁₂TAB, C₁₄TAB and C₁₆TAB) have been measured in water and aqueous NaBr solutions at 298.2 and 308.2 K. The values obtained can be regarded as tracer diffusion coefficients for the micelles because essentially all pyrene molecules are solubilized in the micelles. In all cases the diffusion coefficients decrease with increasing concentration of the surfactants. The results were fitted by D=D_c.m.c.[1 –k_D(c– c.m.c.)] or D=D_c.m.c.[1 –k^′_Dϕ], where D_c.m.c. is the diffusion coefficient at the critical micelle concentration (c.m.c.) and ϕ is the volume fraction of the micelles. In cases of C₁₀TAB and C₁₂TAB micelles, the D_c.m.c. values are close to those expected from the hydrated radii of the micelles. In the cases of C₁₄TAB and C₁₆TAB micelles in water, the D_c.m.c. values are substantially smaller than those expected from the radii of the hydrated micelles. In NaBr solutions, D_c.m.c. values increase with increasing concentration of NaBr and reach maxima at 0.01–0.03 mol dm^–3 NaBr for C₁₄TAB and C₁₆TAB micelles. The effects of electrostatic interactions on the tracer diffusion of the micelles are discussed.