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- N a+, 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 tren ds permits the determination of the critical micelle concentration (CMC). I n the micellar composition range, solubilized TMS molecules were used to de termine the micelle intradiffusion coefficient, from which the micelle radi i were obtained. Both the monomer surfactant and the micelle intradiffusion coefficients show a sharp decrease above the CMC. These results can be int erpreted in terms of the obstruction effect due to the micelles. The electr ostatic repulsion among charged particles strongly enhances this effect. A simple approach that permits the computation of the Gouy-Chapman layer thic kness from the experimental coefficients has been proposed and the results are briefly discussed. (C) 1999 Academic Press.