Transport Properties for Aqueous Sodium Sulfonate Surfactants. 2. Intradiffusion measurements:influence of the obstruction effect on the monomer and micelle mobilities.

Intradiffusion coeffs. of sodium alkylsulfonates [CH3-(CH2)n-1SO-3 Na+, CnSNa] (n = 5-9, 11) in mixts. with heavy water were measured by the PGSE-NMR technique at 25°C. A slope change in the exptl. trends permits the detn. of the crit. micelle concn. (CMC). In the micellar compn. range, solubilized TMS mols. were used to det. the micelle intradiffusion coeff., from which the micelle radii were obtained. Both the monomer surfactant and the micelle intradiffusion coeffs. show a sharp decrease above the CMC. These results can be interpreted in terms of the obstruction effect due to the micelles. The electrostatic repulsion among charged particles strongly enhances this effect. A simple approach that permits the computation of the Gouy-Chapman layer thickness from the exptl. coeffs. has been proposed and the results are briefly discussed.