1242
Langmuir 1989, 5, 1242-1249
Polarographic Measurements of Micellar Diffusion Coefficients: New Results and Implications of Surfactant Adsorption at the Mercury-Solution Interface R. E. Verrall University of Saskatchewan, Department of Chemistry, Saskatoon, Saskatchewan, Canada
S. Milioto Universita degli Studi di Palermo, Istituto di Chimica-Fisica, Palermo, Italy
A. Giraudeau Laboratoire d'Electrochimie et de Chimie du Corps Solide, Universit6 L. Pasteur, 4 rue Blaise Pascal, Strasbourg, France
R. Zana* Institut Charles Sadron (CRM-EAHP), 6 rue Boussingault, Strasbourg, France Received May 5, 1988. I n Final Form: April 24, 1989 Polarographic studies of several probe molecules in aqueous cationic (tetradecyltrimethylammonium bromide, TTAB) and anionic (sodium dodecyl sulfate, SDS) surfactant solutions both in the presence and absence of additives (e.g., KBr in =AB; NaCl in SDS) are reported as a function of surfactant concentration and temperature. The probes studied were N-hexadecyl-4-cyano-and N-octadecyl-4-cyanoppidinium bromide, 1-nitropyrene, 1-pyrenecarboxaldehyde, 9-nitroanthracene, and 9-anthracenecarboxaldehyde. Estimates of self-diffusioncoefficients of micelles at the cmc, D,", derived from these polarographic studies in the absence of intermicellar interactions and the observed effects of additives on these estimates of D," are consistent with results from other techniques, most notably quasielastic light scattering (QELS). However, a large decrease of D, with increasing TTAB concentration in both the presence and absence of k d r is in disagreement with QELS results apd cannot be explained by micelle growth and/or micellar interactions, alone. Results of cyclic voltammetry, and the study of the effect of temperature, strongly suggest that cationic surfactant adsorption at the mercury-solution interface at high surfactant concentrations precludes obtaining accurate micelle self-diffusion coefficients under the experimental conditions used in this study. The adsorption interference at high concentration appears to be much less of a problem with SDS.
Introduction Interest in methods to characterize micelle sizes and interactions has led to the use of polarographic techniques to estimate micelle diffusion coefficients via the redox behavior of associated electroactive probe molecules.' These studies assume that the presence of a small number (> tdif owing to the (1 - e-R)/Rterm present in Cp,ef,whereas a linear plot indicates that trd
