Transport properties for aqueous sodium sulfonate surfactants - 2. Intradiffusion measurements: Influence of the obstruction effect on the monomer and micelle mobilities

Intradiffusion coefficients of sodium alkylsulfonates [CH3-(CH2)(n-1)SO3- Na+, CnSNa] (n = 5-9, 11) in mixtures with heavy water were measured by the PGSE-NMR technique at 25 degrees C. A slope change in the experimental trends permits the determination of the critical micelle concentration (CMC). In the micellar composition range, solubilized TMS molecules were used to determine the micelle intradiffusion coefficient, from which the micelle radii were obtained. Both the monomer surfactant and the micelle intradiffusion coefficients 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 experimental coefficients has been proposed and the results are briefly discussed. (C) 1999 Academic Press.